Defra, UK - The Department of Environment, Food and Rural...

Development of a UK Integrated Plankton

Monitoring Programme

A final report of the Lifeform and State Space project

Prepared for

The Department of Environment, Food and Rural Affairs

Nobel House, 17 Smith Square. London SW1 P 3JR

May 2015

i

Prepared by

C. Scherer and R.J. Gowen

Fisheries and Aquatic Ecosystems Branch, Agri-food and Biosciences

Institute, Newforge Lane, Belfast, BT9 5PX.

P. Tett

Scottish Association for Marine Science, Scottish Marine Institute, Oban,

PA37 1QA.

A. Atkinson

Plymouth Marine Laboratory, Prospect Place, The Hoe Plymouth PL1

3DH

M. Baptie

Scottish Environment Protection, Agency Angus Smith Building, 6

Parklands Avenue, Eurocentral, Holytown, North Lanarkshire, ML1

4WQ

M. Best

Environment Agency, Kingfisher House Orton Goldhay, Peterborough,

PE2 5ZR

E. Bresnan and K. Cook

Marine Scotland, Marine Laboratory, P.O. Box 101, 375 Victoria Road,

Aberdeen, AB11 9DB.

R. Forster

Cefas, Pakefield Rd Lowestoft, Suffolk NR330HT

S. Keeble

Blue Lobster IT Ltd, Sheffield Technology Parks, Cooper Buildings,

Arundel Street, Sheffield, S1 2NS.

A. McQuatters-Gollop

Sir Alister Hardy Foundation for Ocean Science, The Laboratory, Citadel

Hill, Plymouth, PL1 2PB.

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Summary

The lifeform and state space project (Defra ME5312) was part of work undertaken to

support the implementation by the UK of the Marine Strategy Framework Directive

(MSFD). It was carried out by a consortium of nine institutional partners, led by the

Agri-Food and Biosciences Institute (AFBI), between August 2013 and March 2015.

The MSFD requires the establishment of a series of environmental targets and

associated indicators, and the establishment and implementation of a monitoring

programme for ongoing assessment of the marine regions for which member states

are responsible. In the case of the UK, the relevant sub-regions are the Greater North

Sea and the Celtic Seas, within the North-east Atlantic Region.

During earlier workshops, the UK had identified, in principle, indicators, criteria

and targets for the 'pelagic habitat' in these sub-regions. For MSFD purposes, 'pelagic

habitat' means the plankton - the small drifting animals and the tiny micro-algae of

the water column. These form the basis of the marine food web and in this way

provide the habitat for fish and higher marine vertebrates. The MSFD lists 11

'Qualitative Descriptors' (QDs) that provide broad-brush environmental targets. Of

these, QD 1 (Biodiversity), 4 (Marine Food Webs), 5 (Eutrophication) and 6 (Sea-

Floor Integrity) are sensitive to the state of the plankton.

The earlier workshops had recommended the use of a lifeform and state space

approach for tracking changes in the state of the plankton in UK waters and

suggested that data integration and reporting could be achieved through the web-

based EMECO data tool. The main objectives of the present project were to

operationalise this approach, and to provide costed options for a UK plankton

monitoring programme mainly but not exclusively for the biodiversity element of

the MSFD.

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Operationalisation required work on three main components: (1) Arranging for the

results of plankton monitoring - typically as lists of species and their abundances

from regular samples - to be uploaded to the EMECO data tool website and the

species data aggregated into a small set of 'lifeforms' (Table 3.2) defined by the

consortium in relation to MSFD QDs; (2) Plotting the aggregated data into a set of

state spaces - where in each case the axes were the abundances of a pair of lifeforms

- and using these plots to generate time-series of annual values of a 'Plankton Index'

or PI (Figures 2.3d and 2.4); (3) relating the time-series of PI to time-series of

pressures and to the agreed target that there should be no significant trend in a PI

series that was correlated with a pressure time-series.

The design of the monitoring programme took into account recent advances in

understanding the physical dynamics of UK waters and aimed to take observations

from the main ecohydrodynamic regimes within each of the regions used in

'Charting Progress 2'. A map of regimes in UK waters, based on a 50-year model

hind cast by Cefas, showed mixed, indeterminate, `regions of freshwater influence',

seasonally-stratified and near-permanently-stratified waters. The map was over-

plotted with existing fixed-site monitoring stations and existing routes of the

Continuous Plankton recorder (CPR) (Figure 3.2). A gap analysis was performed to

identify the options for taking data from these sites and routes into EMECO and for

upgrading sampling to meet requirements of the MSFD monitoring scheme

proposed in this report.

The minimum recommended biodiversity monitoring programme consists of regular

sampling of the phytoplankton and zooplankton at each fixed point sampling station

(total 13) and CPR route (total 10) in the main ecohydrodynamic regimes of each CP2

region (Table 5.1). This programme is the minimum needed to deliver an integrated

assessment of changes in the condition of plankton community. Other options are

given.

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The integrated system for uploading plankton data and establishing and reporting

outputs is now operational in EMECO. Graphical outputs (graphs, plots, and maps)

of reference conditions and comparisons are available and time-series plots to track

changes can be illustrated (www.emecodata.net). The reporting tool is also in place.

It was agreed that the reference period for the PI would be 2008-10 for all sites and

routes. The lifeform and state space method will begin to provide objective

assessment of 'plankton habitat' state, in relation to pressures, as years of new data

become available to extend the PI time-series. Meanwhile, assessments of the current

state of the plankton at the monitoring sites are in hand using expert judgement and

a template devised by AFBI. All of the five sites assessed so far were determined as

being representative in 2008-10 of GES and so for the plankton habitat, this allows

the reference conditions to be interpreted as GES. It is hoped that papers can be

published in peer-reviewed scientific journals to substantiate these semi-subjective

assessments.

http://www.emecodata.net/

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Contents

Summary ................................................................................................................................. ii

1. General introduction .......................................................................................................... 1

1.1 Background .................................................................................................................... 1

1.2 The Marine Strategy Framework Directive ............................................................... 2

1.3 UK approach .................................................................................................................. 3

2. The scientific basis for the monitoring programme ....................................................... 6

2.1 Ecosystem Health and Good Environmental Status ................................................ 7

2.2 Lifeforms ...................................................................................................................... 10

2.3 A regional approach based on ecohydrodynamic conditions .............................. 13

2.4 Detecting change in the plankton ............................................................................. 18

2.4.1 Introduction ............................................................................................................ 18

2.4.2 The state space approach ......................................................................................... 19

2.4.3 The Plankton Index ................................................................................................. 23

3. Key elements of the UK integrated plankton monitoring programme ..................... 27

3.1 Targets, baseline conditions and indicators ............................................................ 27

3.1.1 Establishing a target for the plankton indicator ..................................................... 27

3.1.2 Reference conditions ............................................................................................... 30

3.1.3 Indicators ................................................................................................................ 32

3.2 Sampling strategy ....................................................................................................... 37

3.3 Quality Assurance ....................................................................................................... 43

3.4 Reporting ...................................................................................................................... 43

3.4.1 EMECO – General background .............................................................................. 43

3.4.2 Data processing ....................................................................................................... 44

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3.4.3 Outputs ................................................................................................................... 47

3.4.4 Reporting with the EMECO reporting tool............................................................ 49

3.4.5 EMECO Reporting Tool ......................................................................................... 52

4. Operational readiness ....................................................................................................... 52

4.1 State of readiness ......................................................................................................... 52

4.2 Outstanding matters ................................................................................................... 54

4.2.1 Minor elements to be completed at the March 2015 workshop .............................. 54

4.2.2 Elements that require additional funding ............................................................... 55

5. Recommendations to Defra ............................................................................................. 55

5.1 Gap analysis ................................................................................................................. 56

5.2 Additional monitoring sites ...................................................................................... 62

5.3 Options ......................................................................................................................... 62

Option 1 – do nothing...................................................................................................... 63

Option 2 – upgrade existing fixed points but no new ones ............................................. 64

Option 3 - CPR based UK monitoring ............................................................................ 64

Option 4 – the minimum recommended monitoring programme ................................... 65

5.4 Options selection ......................................................................................................... 66

6. Future Research ................................................................................................................. 66

7. Reference ............................................................................................................................ 68

Annex A: Initial Assessments

Annex B: An example of an EMECO report

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1. General introduction

1.1 Background

As part of a programme to develop indicators and targets for the Marine Strategy

Framework Directive (MSFD), the Department for Environment, Food and Rural

Affairs (Defra) funded a consortium led by the Agri-Food and Biosciences Institute

(AFBI) to develop and operationalise an integrated plankton monitoring programme

to track changes in the condition or state of the plankton. The work formed part of a

suite of R&D projects undertaken to support Defra’s implementation of the first two

stages of the MSFD, including operationalising targets and indicators already

submitted to the European Commission of the European Communities, and

providing options on the monitoring programme for the second phase of the MSFD.

The Scottish Government funded participation of Marine Scotland staff in this

project.

The objectives of the Lifeform project were to:

Develop and operationalise planktonic indicators for the Biological Diversity

(D1), Food webs (D4), the relevant component of Eutrophication (D5.2.4) and

Seafloor integrity (D6) descriptors of the MSFD;

Assess the current state of the plankton in UK waters for which there are

suitable data by applying the ecohydrodynamic approach and use expert

judgement to determine if current state is representative of GES;

Provide written advice to Defra including: reporting the operational readiness

of the indicators; options for monitoring (including one that relies on existing

monitoring programmes) and include costing estimates for monitoring;

identification of gaps and issues; future work to expand the indicators to

cover all ecohydrodynamic regions within UK limits.

This is the final report of the R&D project ‘lifeforms and state spaces’ (Defra project

ME5312). The remainder of the report is divided into a further 6 sections. Section 2

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presents an overview of the scientific basis for the monitoring programme. Section 3

gives a detailed description of its key elements and Section 4 is an assessment of

operational readiness. Section 5 presents options for implementing the minimum

recommended monitoring programme. Recommendations for future research are

given in Section 6 and Section 7 provides details of the literature cited in the report.

1.2 The Marine Strategy Framework Directive

The Marine Strategy Framework Directive (DIRECTIVE 2008/56/EC) is the most

recent European Union directive which requires member states to assess the status of

marine ecosystems and monitor potential changes in the status of these ecosystems.

The directive is based on the Ecosystem Approach (Box 1) with the main goal to

achieve Good Environmental Status (GES) in EU marine coastal and shelf waters by

2020. GES is defined as:

“The environmental status of marine waters where these provide ecologically

diverse and dynamic oceans and seas which are clean, healthy and

productive” - Article 3

One important requirement of the Directive is that the services provided by marine

ecosystems are utilised at a sustainable level, ensuring their continuity for future

generations. In addition, GES means that:

Ecosystems, including their hydro-morphological (i.e. the structure and

evolution of the water resources), physical and chemical conditions, are fully

functioning and resilient to human-induced environmental change;

The decline of biodiversity caused by human activities is prevented and

biodiversity is protected;

Human activities introducing substances and energy into the marine

environment do not cause pollution effects. Noise from human activities is

compatible with the marine environment and its ecosystems.

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There are eleven quality descriptors that help member states in the interpretation of

what GES means in practice and describe what the ecosystem should look like when

GES has been achieved. The descriptors relevant to this project are detailed in Box 2.

1.3 UK approach

At the request of Defra, the UK Healthy and Biologically Diverse Seas Evidence

Group (HBDSEG), assisted by the Joint Nature Conservancy Committee (JNCC), was

asked to develop options for GES targets and indicators for quality descriptors 1-

Biodiversity, 4 – Food-Webs and 6- Seabed Integrity (collectively referred to as the

“biodiversity descriptors”).

To ensure that the process was integrated and manageable, the ecosystem was

divided into three species components (seabirds, marine mammals and non-

commercial fish) and three habitat components (pelagic, sedimentary benthos and

rocky and biogenic reefs). The development of indicators and targets across all three

biodiversity descriptors took place in subgroups of experts. The six subgroups were

tasked with developing indicators and targets for each relevant operational indicator

and/or indicator class. Where applicable and appropriate, the use of existing

Box 1: The Ecosystem Approach

The aim of the Ecosystem Approach (EA) is the holistic management of human

pressures on marine ecosystems protecting the structure and functioning of the

systems ensuring the long-term sustainability of the services they provide

(Mace, 1997). There is an expectation that the EA will: maintain ecosystem

structure and functioning; avoid negative interactions between different

human activities; reduce the risk of failing to identify the cumulative effects of

different pressures. The International Council for the Exploration of the Sea

(ICES, 2005) defines the EA as:

“A comprehensive integrated management of human activities based on the

best available scientific knowledge about the ecosystem and its dynamics, in

order to identify and take action on influences which are critical to the

health of the marine ecosystems, thereby achieving sustainable use of

ecosystem goods and services and maintenance of ecosystem integrity.”

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indicators and targets (e.g. OSPAR strategy to combat eutrophication) was

encouraged. A drafting team consisting of all six subgroup leads and supported by

representatives from Defra and JNCC ensured that the work undertaken was

consistent across subgroups.

The pelagic subgroup met at a Defra-funded workshop (Birmingham 29-30th March

2011) to consider the establishment of indicators, criteria and targets that would be

relevant to the pelagic habitat: the plankton. Consideration was given to whether

indicators, criteria and targets could be established for each of the biodiversity group

of descriptors and for the plankton as a whole (to integrate across the three

descriptors). Descriptor 5, eutrophication also has an element that relates to the

Box 2: Definitions of GES for the relevant MSFD quality descriptors

Descriptor 1 Biodiversity: “GES will be achieved given no further loss of the

diversity of genes, species and habitats/communities at ecologically relevant scales and

when deteriorated components, where intrinsic environmental conditions allow, are

restored to target levels”

Descriptor 4 Food Webs: “GES will be achieved when the indicators describing the

various attributes of the descriptor reach the thresholds set for them. These should

ensure that populations of selected food web components occur at levels that are within

acceptable ranges that will secure their long-term viability.“

Descriptor 5 Eutrophication: “GES with regard to eutrophication has been achieved

when the biological community remains well-balanced and retains all necessary

functions in the absence of undesirable disturbance associated with eutrophication (e.g.

excessive algal blooms, low dissolved oxygen, declines in sea grasses, kills of benthic

organisms and/or fish) and/or where there are no nutrient-related impacts on

sustainable use of ecosystem good and services”

Descriptor 6 Sea Floor Integrity: “For the purposes of good environmental status of

the seafloor, uses can be considered sustainable if the pressures associated with those

uses do not hinder the ecosystem components to retain their natural diversity,

productivity and dynamic ecological processes. If ecosystem components of the sea floor

are perturbed, recover needs to be rapid and secure.”

http://ec.europa.eu/environment/marine/good-environmental-status/descriptor-1/index_en.htm




5

floristic composition of pelagic micro-algae (the phytoplankton) and it was agreed

that this element of the eutrophication descriptor should be treated in the same way.

The subgroup recommended a lifeform and state space approach that had been

developed during the Defra-funded Cefas project Undesirable Disturbance:

development of a UK Phytoplankton Trophic Index, CSA 6754/ME2204. However,

the subgroup pointed out that further work was required to consider how the

lifeform and state space method could be used to track changes in the condition of

the plankton and recommended that Defra fund an expert group for this purpose.

Defra accepted the recommendation and funded a group of experts who met at AFBI

in Belfast in June 2011. The Belfast workshop was requested to: (i) review the life-

form and state space approach for quantifying the state (or condition) of the pelagic

community of organisms, called the plankton; (ii) devise a target that was indicative

of the plankton being in Good Environmental Status (GES); (iii) recommend how

change in the state of the plankton could be monitored; (iv) recommend an

integrated UK monitoring and assessment procedure.

The workshop report (Gowen et al., 2011) presented details of the lifeform and state

space approach, proposed a target and indicators for the plankton and a procedure

for determining whether the target had been met. It was also suggested that data

integration and reporting could be achieved through the EMECO data tool but

identified a need for further work to operationalise EMECO for plankton and build a

UK integrated plankton monitoring programme. The workshop therefore

recommended that Defra fund an R&D project to ‘operationalise’ the lifeform and

state space method and provide costed options for implementing the optimum

monitoring programme.

The lifeform and state space method for tracking changes in the condition of the

plankton in UK was reviewed as part of Defra’s public consultation exercise in 2012

and was accepted as the method which would be adopted for use by the UK. Defra

6

also accepted the recommendation for an R&D project to operationalise the method

and develop a UK integrated plankton monitoring programme. Following a pre-

project workshop to ‘scope’ the project (Gowen et al., 2013) the project which was led

by AFBI began in July 2013.

2. The scientific basis for the monitoring programme

As mentioned above, the MSFD is a departure from previous legislation in that it

requires marine ecosystems to be fully functioning, resilient and diverse. This

requires a holistic view of ecosystems and their state or health and holistic indicators

(to detect change) of condition (or state) that are grounded in a theory of ecosystem

functioning. Such a theory has been developed through several Defra R&D funded

projects to Napier University: (i) ‘Understanding of Undesirable Disturbance in the

Context of Eutrophication and Development of UK Assessment Methodology for

Coastal and Marine Waters’ (2004; a consortium led by Napier University with

involvement of Heriot –Watt, Liverpool, Cefas and DARD); (ii) ‘Research Supporting

the Development of Eutrophication Monitoring and Assessment’ (2004 - 2009)

(Subcontract ME 2202 from Cefas to Napier University); ‘Development of A

Phytoplankton Trophic Index’ (2004 - 2006) (Cefas, CSA 6754 subcontract ME2004 to

Napier University); (iii) ‘Research to improve understanding and assessment of

ecosystem health’ (2010 - 2013) (E5302 subcontract to SAMS).

The understanding gained from these studies was the starting point for the

development of the monitoring programme recommended by Gowen et al. (2011).

The programme is therefore underpinned by four important scientific concepts:

ecosystem health; functional groups or lifeforms; ecohydrodynamics; and state space

theory (to track change in state).

These concepts have been the subject of recent peer review publication (Tett et al.

2007, 2008, 2013; Gowen et al., 2012; Gowen et al., 2015a) and included in reports to

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Defra (Anon, 2004; Gowen et al., 2011; 2013; Scherer et al., 2014). This section presents

an overview of these concepts and provides an update on earlier reports.

2.1 Ecosystem Health and Good Environmental Status

According to Costanza (1992), a healthy ecosystem, like a healthy human body, is a

system that functions well and is able to resist or recover from disturbance. In their

review, Tett et al. (2013) defined a healthy ecosystem as:

“the condition of a system that is self-maintaining, vigorous, resilient to

externally imposed pressures, and able to sustain services to humans. It contains

healthy organisms and populations, and adequate functional diversity and

functional response diversity. All expected trophic levels are present and well

interconnected, and there is good spatial connectivity amongst subsystems.”

They suggested that if Good Environmental Status (GES) requires ecosystems to be

fully functioning and resilient, then a healthy ecosystem is one which is in GES. An

important first step in developing the plankton monitoring programme was

therefore to consider how the plankton contribute to a healthy ecosystem and what

plankton data would be of value in supporting assessments of GES.

Mageau et al. (1995) argued that the health of an ecosystem had quantifiable

components of vigour, organization, resistance to disturbance, and resilience. Tett et

al. (2013) took this a step further by suggesting that based on general systems theory,

one conceptualisation of ecosystem health was that structure (organisation) and

function (vigour) contributed to ecosystem resilience which they argued was a

property of the ecosystem as a whole (emergent property) and a key to ecosystem

health. Resilience1 maintains ecosystem state against pressures and also buffers the

services provided by an ecosystem against human pressures (Figure 2.1) thereby

helping to maintain the sustainability of resources.

1 Defined as ‘the capacity of a system to absorb disturbance and reorganise while undergoing changes so as to

maintain essentially the same functions, structure, identity and feedbacks’ (Folke et al., 2004).

8

Figure 2.1 A conceptual diagram illustrating the relationship between human

pressures, ecosystem state (structure and functioning) and resilience. (Redrawn from

Tett et al., 2013).

Understanding of resilience and how it might be quantified for the purpose of

supporting the management of human pressures on marine ecosystems is at an early

stage (see Tett et al., 2013 and references cited therein) and our focus was therefore

on the contribution that the plankton makes to ecosystem structure and function.

Structure includes the trophic and biogeochemical connections or networks amongst

functional groups.2 Function or vigour includes fluxes of energy and materials, such

as primary production and nutrient cycling. The plankton contribute to structure by

contributing to trophic networks and we consider the annual succession of lifeforms

(see below), to be an important aspect of the pelagic ecosystem and forms part of its

structure.

In waters 10-15 m deeper than the low water mark (and hence most of the sea area

around the UK covered by the MSFD, see Figure 2.2), the species that make up the

phytoplankton are the dominant primary producers. The productivity of these

waters and the resources they provide by way of fisheries are therefore ultimately

2 the sets of species (or components of biodiversity) responsible for ecosystem functions.

9

dependant on phytoplankton production. Zooplankton is the main conduit through

which energy is transferred from phytoplankton to higher trophic levels.

Good Environmental Status (GES) requires ecosystems to be fully functioning

and resilient.

An important first step in developing the plankton monitoring programme

was therefore to consider how the plankton contribute to a healthy

ecosystem and what plankton data would be of value in supporting

assessments of GES.

The plankton has a major role in the functioning and structure of coastal and

shelf sea ecosystems.

Figure 2.2 A map showing the charting progress 2 (CP2) assessment regions and the

main ecohydrodynamic areas in UK coastal and shelf seas (provided by the Cefas

modelling team). The blue numbers refer to fixed point monitoring stations

(indicated by a pink filled circle) which will contribute to the UK integrated

plankton monitoring programme.

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2.2 Lifeforms

The plankton comprises many species. Sournia et al. (1991) estimated that: ‘towards

the end of the 1980s, living plankton flora of the world ocean amounted to between

474 and 504 genera and between 3,444 and 4,375 species’. An analysis of species

present at a particular location can reveal the occurrence of several hundred species

of phytoplankton during the growing season in UK waters. Accompanying these

might be a hundred or more species of microzooplankton, loosely named as

‘protozoa’ and a similar number of larger zooplankton.

The microplankton (protictistan micro-organsims such as ciliates and tintinnids) can

be distinguished from the remainder of the zooplankton the embryozoa. The

plankton exhibits variability on a range of spatial and temporal scales and the

assemblage of species and populations of individual species are not fixed in time

and space but are dynamic. Overlaying this variability there are higher-order

constancies in the plankton (see Section 3.1.3). Therefore, any method for detecting

change in the plankton must be capable of discriminating and quantifying long-term

change against this natural dynamic variability and at the same time incorporate

information on these higher order consistencies. According to Gowen et al. (2012a)

understanding the phytoplankton (and plankton in general) as a dynamic system

suggests its status should be diagnosed from perturbations of ecosystem structure

and function rather than from changes in fixed assemblages of species and

thresholds of abundance. Therefore, an approach using multiple characteristics of

the plankton is needed to assess the condition of the plankton community and track

changes in its status.

There does not seem to be any single species of plankton that can be used as a

universal indicator of the condition of the plankton. There are several reasons for this:

(i) No single species of the plankton has a controlling influence on the

plankton as a whole;

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(ii) Plankton is spatially heterogeneous, so that species important in one

region, or under one set of conditions, may be rare in another region.

(iii) Relationships between particular organisms and specific pressures have

often been based on limited or over-interpreted evidence. For example, the

contention that the occurrence of harmful species of phytoplankton, or of

blooms of these phytoplankters, is on its own, sufficient diagnose

eutrophication (see Gowen et al., 2012b and references cited therein).

Given the richness of the plankton in UK waters, it is doubtful that simple lists of

species abundance would adequately discriminate between natural variability and

human pressure driven change. This is because any list is likely to comprise

hundreds of species, with different numbers and biomasses depending on when and

where samples are collected. In addition, a difficulty would arise when trying to

relate thresholds for species abundance or biomasses to ecosystem structure and

functioning.

An alternative would be to use one or more established diversity indices such as:

counts of the number of species; measures of species number in relation to total

abundance of organisms; quantifications based on amount of information contained

in a list of taxa and their abundances (e.g. Margalef, 1958). Such indices, however,

disregard information about the particular contributions of each species to the

functioning of the pelagic community.

From an MSFD perspective the condition of an ecosystem is shown by the services it

provides. Measurements of the magnitude of primary production, for example, have

been used to establish a trophic basis for fisheries (Ryther, 1969). It could therefore

be argued that statistics derived from fisheries, sea-bird breeding success, and

primary production data might well suffice to indicate the condition of the plankton.

Such information is clearly important, and an indicator for production of

phytoplankton has now been submitted for consideration by the ‘Food-webs’

working group of OSPAR (with UK contribution). Estimates of primary production

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can be better interpreted when we have sufficient insight into what is happening in

the structure of the planktonic component of the pelagic ecosystem. For example,

sea-bird breeding success might change because of changes in water column

stratification regimes, abundance of suitable prey fish, or changes in fisheries

discarding practices, and so an indicator based on this alone would not provide

sufficient information on changes in the condition of the plankton and how this in

turn might influence the structure and functioning of marine ecosystems.

Multivariate statistical analysis (MVA) provides tools for processing and analysing

large amounts of information and reducing large data sets to a small number of

principal axes or components. These could be used for the indicators that are needed

but there is a difficulty in interpreting what the axes represent. An axis might point

to the importance of a particular group of plankton or to the influence of a certain set

of environmental variables but the interpretation is only valid for the data set

analysed and is subject to the ‘problem of induction’. That is, how do we know that the

associations of species or their response to pressures will remain valid in the future,

given potential changes in the wide range of environmental pressures that act on

marine ecosystems? Thus, although MVA is a powerful tool in analysing historical

data sets (Fromentin and Planque, 1996), and in suggesting testable hypotheses

about the nature and causes of change in marine ecosystems (Tett et al., 2013), it

would not seem to provide a sufficiently firm foundation for indicators that will be

used to track future change (Kenny, et al., 2009).

An alternative to MVA is to use lifeforms or functional groups, the concept of which

is more theoretically based. The definition of a healthy ecosystem proposed by Tett

et al. (2013) includes a requirement for the ecosystem to have ‘adequate functional

diversity’3. This is because recent scientific opinion (e.g. Folke et al., 2004; Hooper et

3 Functional-group diversity: the sets of species (or components of biodiversity) responsible for ecosystem

functions; the sets of species correspond to benthic guilds, or pelagic lifeforms (from Tett et al., 2013).

13

al., 2005) has argued that the maintenance of functional diversity rather than species

diversity is important in understanding ecosystem resilience to pressure. Lifeforms

are, in principle, units of ecosystem functional diversity and a lifeform can be

defined as: a group of species (not necessarily taxonomically related) that carry out

the same important functional role in the marine ecosystem. To provide a simple

example, the absence of spring blooming phytoplankton would deprive the food

web of a major input of organic matter. The Diatom lifeform is the one that fulfils

this function in typical marine temperate ecosystems. For this reason the UK will

apply a plankton lifeform approach to monitoring the status of the plankton.

An approach using multiple characteristics of the plankton that provides

an insight into ecosystem structure functioning is needed to assess the

status of the plankton community and track change in state.

The UK plankton monitoring programme will be based on plankton

lifeforms to track changes in the status of the plankton.

2.3 A regional approach based on ecohydrodynamic conditions

According to Harris (1980) the earliest view of the planktonic environment was of

an: “isotropic homogeneous environment at equilibrium over large scales”. However, this

has proven not to be the case. Plankton experiences an inherently variable

environment as a result of physical variability driven by meteorology and climate,

interacting with tidal and density-driven flows. Consequently, the plankton exhibit

variability on a range of spatial and temporal scales. Nevertheless, in temperate seas

such as those around the UK, there are higher-order constancies. In the case of

phytoplankton, these include the recurrent annual cycle of growth (e.g. Tett and

Wallis, 1978; Smayda, 1998; Gowen et al., 2008) and the succession of species in

seasonally stratifying temperate shelf seas (Margalef, 1978). Thus, the onset and

duration of the phytoplankton production season is determined by the sub-surface

light climate as a function of turbulent mixing and the annual cycle of solar radiation

14

(Sverdrup, 1953; Smetacek and Passow, 1990; Tett, 1990). The influence of regional

differences in hydrodynamic conditions on the seasonal cycle of phytoplankton

production is illustrated by the study of Gowen et al. (1995) who found that in the

seasonally stratifying region of the western Irish Sea, the production season lasted ~4

months but that there was a short (~2 months) and late season in the deep waters of

the North Channel which is only weakly stratified for a few months each year.

Hypotheses about lifeforms and the succession of phytoplankton species in coastal

and shelf seas stem from the work of Margalef (1978) who suggested that variations

in external energy in the form of nutrients and turbulence was the main factor

controlling the temporal succession of phytoplankton. A number of studies have

shown that Margalef’s general model is broadly applicable to the succession of

phytoplankton species in shelf seas (Pingree et al., 1976; Holligan and Harbour 1977;

Bowman et al. 1981; Jones et al., 1984). Jones and Gowen (1990) investigated the

distribution of lifeforms in relation to turbulent mixing and irradiance regimes in

shelf seas around the British Isles and found that: (i) diatoms were generally more

abundant in waters of low vertical stability and steep irradiance gradients; (ii) a mix

of dinoflagellates dominated stable water columns where irradiance gradients were

small. However, this succession is not fixed and variations in mixing can retard and

alter the pattern of succession (Smayda, 1980).

There are also pronounced seasonal cycles in the abundance of species which

comprise the zooplankton. These exhibit regional differences which result in part

from the seasonal patterns of phytoplankton production and hydrodynamic

conditions (Backhause et al., 1994; Fromentin and Planque, 1996; Dickey-Collas et al.,

1996; Gowen et al., 1998a). For example, in the English Channel (Bautista and Harris,

1992, Eloire et al., 2010) and western Irish Sea (Gowen et al., 1999) the seasonal

abundance of planktonic copepods is coupled to the spring phytoplankton bloom.

Here, elevated zooplankton abundances are maintained for an extended period from

spring to autumn, sometimes with spring and autumn maxima reminiscent of spring

15

and autumn phytoplankton blooms (Atkinson et al., 2013). However, in the cooler

coastal waters off Northumberland (Roff et al., 1988) and the Stonehaven site off

Scotland (Cook, 2013) copepod production is delayed and peak abundance does not

occur until mid-summer.

Spatial differences in the plankton community in relation to hydrographic conditions

were also observed on the Malin Shelf by Gowen et al. (1998b) and it has been

suggested that there are distinct differences in the zooplankton communities of

seasonally stratifying and mixed waters (Williams et al., 1994) which influence

ecosystem functioning (Cushing, 1989). The tidally mixed inshore waters tend to be

characterised by an assemblage dominated by smaller copepod species with an

important contribution from meroplankton. In contrast, the mesozooplankton

biomass of stratifying regimes is often dominated by large copepods of the genus

Calanus (Williams et al., 1994).

Superimposed on this primary delineation between mixed and stratifying regimes,

further sub-delineations can apply. First, some European shelf regimes receive

variable incursions of oceanic waters and their distinct fauna. For example, the Celtic

and Irish Seas and western Scottish shelf receive these inputs, with further periodic

influxes of mixed oceanic and shelf assemblages that penetrate into the North Sea

(Falkenhaug et al., 2013). Secondly, primary and secondary production tends to be

elevated at physical transition zones such as at tidal or shelf-break fronts. These

important habitats have sharp spatial/temporal variations in the strength of mixing

(for example due to the spring-neap cycle). This allows some of the advantages of

highly mixed systems (nutrient injection to surface waters) to be shared with those of

stratified regions (high light environment), boosting primary and secondary

production and sometimes tertiary production as well (Kiørboe, 2008).

The recurring but varying cycles described above can be seen in terms of ’basins of

attraction’ within ecosystem state space (Holling, 1973). Hydrodynamic conditions

16

such as turbulent mixing, advection and buoyancy inputs influence these basins of

attractions and Nihoul (1981) introduced the term ecohydrodynamics to convey the

important influence that hydrodynamic processes have on marine ecosystems. This

is especially the case for the planktonic component of pelagic ecosystems. Emerging

from these studies and concepts is the idea of ecohydrodynamic regions:

A water body with distinct hydrographic and hydrodynamic characteristics to

which the species that make up the plankton are adapted

Such regions could be characterised on the basis of their: (i) physical conditions; (ii)

typical primary producers (in the absence of anthropogenic interference); and (iii)

significant ecosystem features emerging from such primary producer dominance

and from biogeography (Tett et al., 2007). However, these regions should be large

enough for structure and function to be controlled more by internal processes than

by outside forcing. Tett et al. (2007) used the duration of stratification and bio-

optical characteristics to identify 5 distinct ecohydrodynamic types of water body in

UK waters.

Recently, work at Cefas involving S. van Leeuwen and J. van der Molen, has

involved analysis of a 50-year hind cast using the GETM physical model of North

Sea hydrodynamics [and, at lower resolution, elsewhere on the N-W European

continental shelf]. The results showed much interannual variation. We have drawn

on them to classify each shelf-sea grid-point as showing the following predominant

type and seasonal pattern of water-column structure:

1. Mainly mixed through the year

2. ROFIs - regions of freshwater influence, mixing andwhere haline stratification

alternate every few days

3. Seasonally thermally stratified (for about half the year, including Summer)

4. Near-permanent, predominantly haline, stratification

17

5. Indeterminate regions, which may show one of the above in one year and a

different one in another year

These ecohydrodynamic (EHD) regimes are mapped in Figure 2.2 in relation to the 8

sea areas around the UK that were used to assess the state of the marine

environment (Charting progress 2 (CP2): http://chartingprogress.defra.gov.uk).

Overlaying the Charting Progress 2 (CP2) regions onto the maps of

ecohydrodynamic water bodies shows that there are several different water bodies

in each CP2 region (Figure 2.2). It was agreed amongst partners and with Defra that

the UK integrated sampling programme should sample the three main EHD areas in

each CP2 region.

The concept of ecohydrodynamic regions and their associated plankton communities

argues against the need for large scale spatial monitoring of the plankton in UK

waters and provides a method of scaling up from well-studied, relatively small

water bodies to CP2 regions and the MSFD reporting regions.

However, existing spatial plankton surveys such as those which are done using

established, repetitive survey routes (e.g. International Bottom Trawl Surveys, 1974-

present) provide an opportunity to obtain quantitative plankton data combined with

high-quality oceanographic data at low cost for validation of the ecohydrodynamic

approach.

The UK integrated plankton monitoring programme should therefore be based on a

network of quality sampling sites and CPR routes with sampling in each of the three

main ecohydrodynamic water bodies in each CP2 region, and with validation from

spatial surveys as appropriate to each region. For CPR routes, only data from within

the boundaries of those ecohydrodynamic regions being sampled will be used. This

will avoid combining CPR data collected from different ecohydrodynamic water

bodies.

18

Ecohydrodynamic regions are water bodies which have distinct

hydrographic and hydrodynamic characteristics to which the species that

make up the plankton are adapted.

Monitoring changes in the condition of the plankton will be based on

ecohydrodynamic water bodies using fixed point sampling stations and

relevant sections of CPR routes.

The three main ecohydrodynamic water bodies in each Charting Progress

2 assessment region should be monitored.

2.4 Detecting change in the plankton

2.4.1 Introduction

One approach to detecting changes in the plankton would be, simply, to plot

lifeform abundances, or the ratio of lifeforms or the proportion (percentage) that

each lifeform contributes to total plankton biomass, against time4. Additional data

could be added as they become available. Simple statistics for example, annual mean

or median abundance or percentage of a particular lifeform, could then be extracted

from the data. However, there is a difficulty in establishing targets for such

statistics, or for simple ratios of lifeform abundances, that provide meaningful

information on changes in the structure of plankton communities and the

functioning of the planktonic component of the pelagic ecosystem. Furthermore,

these simple statistics discard too much important information. As noted earlier, the

higher order consistencies observed in the plankton can be seen in terms of systems

theory: as movement around an attractor or within a basin of attraction (Holling,

1973) which corresponds to a particular regime or ecosystem state. It therefore

seems more appropriate to adopt a method of tracking change in the condition of the

plankton that is based on a generalised theory of systems (that of von Bertalanffy,

4 The indicators being developed by the OSPAR regional seas convention Committee on Biodiversity

Assessment and Monitoring (COBAM) were not reviewed as part of this project but were reviewed

by project partners during a post-project workshop held at AFBI in Belfast in March 2015 (see Gowen

et al., 2015a).

19

1968).

2.4.2 The state space approach

Tett et al. (2008) proposed such an approach, suggesting that changes in the state of

the phytoplankton community could be tracked by means of plots in a state space

where the axes of the space are the abundances of lifeforms of pelagic micro-algae.

Building on this approach and plotting plankton lifeform abundances in a multi-

dimensional state space provides a means of monitoring changes in the organization

of plankton communities. A state can be defined as a single point in state space, with

co-ordinates provided by the values of the set of state variables, in our case lifeform

abundances. In the example illustrated in Figure 2.3a, the axes of the 2 dimensional

(2D) space are the abundances5 of the diatom and dinoflagellate lifeforms. The total

abundance of all the diatoms and all the dinoflagellates in a water sample gives two

numbers, which are the co-ordinates of a point that can be plotted into this state

space. This point represents the state of the ecosystem in terms of diatoms and

dinoflagellates at the time the water sample was collected. Subsequent samples yield

additional pairs of diatom and dinoflagellate abundances that can be mapped onto

the diatom-dinoflagellate state space (Figure 2.3a).

5 Logarithmic ((Log10 (x+1)) transformations of the data allow more reliable statistical analysis and

interpretation, and also allow change at low abundance to be seen as clearly as change at high abundance. In

essence, a given amount of change on a logarithmic axis shows the same proportionate increase or decrease,

irrespective of abundance. Such a transformation is also desirable because it ensures commensurability of axes

in state space plots.

20

Figure 2.3a Mapping diatom and dinoflagellate abundance in state space.

We refer to these state space diagrams as ‘maps’, and to the lines that link points as

‘trajectories’ rather than ‘graphs’. In normal scientific usage, a graph implies a

functional relationship between the values on the x (horizontal) and y (vertical) axes.

That is, a change in x causes a change in y. In the case of state space diagrams, there

is no implication that changes in one state variable causes change in another,

although change in both might be linked in some way. Just as in the case of a map of

the Earth’s surface, it makes no sense to say that changes in latitude cause changes in

longitude: instead, latitude and longitude are the two co-ordinates that define a

location. Thus, when referring in a general way to the axes of a 2D state space plot,

they are labelled as ‘Y1’ and ‘Y2’ in contrast to the ‘x and y’ labels used in a graph

that implies a functional relationship. Furthermore, just as latitude and longitude

cannot be considered a ratio, the abundance of two lifeforms used to map a location

in 2D state space is not a ratio.

The path between the two states is called a trajectory, and the condition of the

phytoplankton is defined by the trajectory drawn in the state space by a set of points.

Such trajectories reflect: (i) cyclic and medium-term variability (the higher order

consistencies in the plankton that result from seasonal cycles, species succession and

inter-annual variability); (ii) long term variability that might result from

environmental pressure. The seasonal nature of plankton production and the

21

succession of species in seasonally stratifying seas results in this trajectory tending in

a certain direction and (as plankton growth increases in the spring and declines

during autumn), such that the trajectory tends towards its starting point (Figure

2.3b). Given roughly constant external pressures, the data collected from a particular

location over a period of years forms a cloud of points in state space that can be

referred to as a regime. Long term variability may show a persistent trend of

movement away from a starting point in state space.

Figure 2.3b A cloud of points forming a regime in state space.

To define a regime, an envelope can be drawn about this group of points, using a

convex hull method (Sunday, 2004; Weisstein, 2006). Because of theoretical

arguments (see Tett & Mills, 2009) that the envelope should be doughnut shaped

with a central hole, bounding curves can be fitted outside and inside the cloud of

points (Figure 2.3c).

22

Figure 2.3c An example of a regime defined by the envelope drawn by the convex

hull method (Sunday, 2004; Weisstein, 2006). The plot is displayed on a logarithmic

scale because this is a common method of showing the full extent of seasonal

variability (Barnes, 1952). It allows more reliable statistical analysis and

interpretation and displays changes in low abundance as clearly as change in high

abundance.The data are from the Marine Scotland, Marine Science ecosystem

observatory off Stonehaven.

Tett et al. (2008) found that the size and shape of the envelope was sensitive to

sampling frequency and the total numbers of samples. Envelopes were made larger

by including extreme outer or inner points, and the larger the envelope, the less

sensitive it will be to change in the distribution of points in state space and therefore

to detect a change in condition. Conversely, if too many points are excluded the

envelope will be small and even minor changes will result in a statistically

significant difference. It is therefore desirable to exclude a proportion (p) of points, to

eliminate these extremes and it was decided to follow the approach adopted by

OSPAR and use the 90th percentile (OSPAR, 2005). Envelopes are therefore drawn

around the cloud of points to include a proportion (p = 0.9) of the points: with 5% of

23

points that were most distant from the cloud's centre, and 5% of points that were

closest to the centre excluded.

This state space approach, although initially appearing complex, has several

advantages. The first is that of potential conceptual consistency across the variety of

animals and micro-organisms that contribute to the ecological status of the plankton

community. The second is that this consistency leads to a very simple method (that

of counting points) for measuring change. The third is that the state space approach

and time-series graphs lend themselves to simple visualization: experience suggests

that most people find pictures (geometry) easier to understand than complicated

numbers (algebra).

One objection to state space as opposed to time-series graphs might be that a state

space plot results in a loss of information, about the time-dependency of changes in

abundance. The main justification is that system state is not defined by time but by

the instantaneous values of state variables; two systems that have the same pair of

values for Y1and Y2 are said to be in the same state. A practical advantage is that

compared to statistics based on time-series graphs, state space plots are less sensitive

to defects in sampling regimes. Nevertheless, it is important to sample throughout

the year so that the plankton is fully characterised.

2.4.3 The Plankton Index

In order for a Plankton Index (PI) to be calculated, it is necessary to establish a

reference condition as the basis for making comparisons. In this report the term

reference is used simply to denote the data set against which comparisons will be

made and does not imply GES or pristine conditions (see section 3.2.2). Data

collected from a location over a number of years can be used to create an envelope. It

is desirable to include 3 years of data in drawing the envelope, in order to take

account of natural inter-annual variability: but not too many years (no more than 5),

24

because Plankton Indices are tools to examine change in time, i.e. if there is a

temporal trend, a reference envelope based on many years will be bigger than one

based on few years and the resulting PI will be less sensitive to the trend. In

addition, it is important that data are collected throughout the year because seasonal

variation is seen as an essential part of the structure of the plankton community. A

minimum of 36 points should be used to calculate the reference envelope (Scherer,

2012). The envelope, thus drawn (Figure 2.3c) defines a domain in state space that

contains a set of trajectories of the diatom-dinoflagellate component on the marine

pelagic ecosystem and thus represents the prevailing regime during the reference

period.

The next step is to map a new set of data into the reference state space and compare

these new data with the reference envelope (Figure 2.3d). Currently we think that it

is desirable to have at least a dozen points for comparison and as for the reference

data, these should represent samples taken throughout the year (Scherer, 2012). The

value of the PI is the proportion of new points that fall inside the envelope, or, to be

precise, between the inner and outer envelopes. In the example shown in Figure

2.3d, 53% or 8 of the 15 new points lie outside, and the PI is 0.47. A value of 1.0

would indicate no change, and a value of 0.0 would show complete change, with all

new points plotting outside the reference envelope. The envelope was made by

excluding 10% of points, so some new points are expected to fall outside: one and a

half, in the case of the example. Is 8 significantly more than 1.5? The exact

probability of getting 8 by chance alone when only 1.5 are expected, can be

calculated using a binomial series expansion, or by a chi-square calculation (with 1

df and a 1-tail test). The conclusion is that the value of 0.47, is significantly less than

the expected value of 0.9, and so the condition of the phytoplankton in the western

Irish Sea and in Liverpool Bay, as determined by diatoms and dinoflagellates, was

statistically significantly different.

25

Figure 2.3d An example for the comparison between two stations, one in the western

Irish Sea and one in Liverpool Bay. The PI value of 0.47 is significant with eight of

the new points lying outside the envelope.

What does this difference mean? It could be the result of natural inter-annual

variation, which might take the system outside the reference envelope. Attempting

to assess changes in the condition of the plankton by comparing the background

condition with data from individual years is therefore unlikely to give a clear picture

of persistent change over time. Furthermore, using such an approach would make it

difficult to relate change in the plankton to anthropogenic and climate pressures.

Therefore, the next step is to develop a time-series of the index to determine if there

has been a long-term trend.

The time-series of the index is produced by comparing the reference envelope with

data collected from each subsequent year. Figure 2.4 shows an example of a time-

series of the index using diatom and dinoflagellate data collected from the Thau

Lagoon in France (Gowen et al., 2015b). Once the time-series of the index has been

assessed it can be compared to time-series of pressures (see Section 3.2.1).

1 2 3 4 5 6 7

1

2

3

4

5

6

7min set at: 10 mf: 0 (ref & comp)

p: 0.90points: 135

months 1-3months 4-6months 7-9months 10-12

Reference: Western Irish Sea (2008 to 2009)

log10

(diatom abundance/L)

log

10(d

ino

flag

ell

ate

ab

un

dan

ce/L

)

1 2 3 4 5 6 7

1

2

3

4

5

6

7Comparison: Liverpool Bay (2008 to 2009)

log10


drawn by PCI1G on 02-Sep-2014MCI: 0.47new points: 15

binom p: 0.0000chi-sq: 28.2 (df=1)

26

Figure 2.4 A time-series of the Phytoplankton Index derived from diatom and

dinoflagellate abundance data collected from the Thau Lagoon in southern France

between 1988 and 2009. In this example there was no significant long-term trend in

the index. Redrawn from Gowen et al., 2015b).

It is unlikely that two lifeforms will be sufficient to describe all of the important

characteristics of the plankton. In principle, there is no constraint to adding more

lifeforms to the state space plots (see Tett et al., 2013). The rule is that each additional

lifeform has to be independent of those already used and the axis for each new

lifeform has to be drawn at right-angles to all existing axes. The state space map

therefore has to be drawn in as many dimensions as there are state variables

(lifeforms) but this becomes complicated when considering the number of lifeforms

that we might want to use to fully represent the plankton. The solution proposed by

Gowen et al. (2011) was to use sets of two dimensional state space diagrams. As long

as each axis in any plot is independent of all other axes in any plot, and all axes are

measureable by a common standard it is possible to combine the values of the index

from any number of 2D state space plots into a single Plankton Index.

Such a composite PI might include components for phytoplankton, heterotrophic

microplankton and zooplankton and provides a single holistic indicator of changes

in the condition of the planktonic component of the pelagic ecosystem. Following the

same combinatory rules, lesser compilations can be made to provide indices relevant

0.0

0.2

0.4

0.6

0.8

1.0

1988 1991 1994 1997 2000 2003 2006 2009

Index

Year

27

to particular MSFD descriptors. The label ‘PI‘ has been reserved for the holistic

indicator and the lesser compilations are referred to for example as , PI(D5) for the

eutrophication relevant PI. In addition, the notation PI(D5)[t1-t2] will be used where

t1 and t2 are the comparison years for the time-series. Details of the lifeform pairs

and the rationale for their selection are given in Section 3.

Tracking changes in the state of the plankton using the Plankton Index will

be an integral part of the UK monitoring programme to fulfil the

requirements of the MSFD.

Two dimensional state space plots of specific pairs of plankton lifeforms

can be used for particular MSFD descriptors and can be combined to

provide a holistic plankton indicator to track changes in the condition of

the planktonic component of the pelagic ecosystem.

Time-series of the index will be used to track persistent changes in the

condition of the plankton over time and relate any such trend to trends in

anthropogenic and climate pressures.

3. Key elements of the UK integrated plankton monitoring

programme

3.1 Targets, baseline conditions and indicators

3.1.1 Establishing a target for the plankton indicator

Article 10 of the marine strategy framework directive requires that:

“Member States shall, in respect of each marine region or sub-

region, establish a comprehensive set of environmental targets and

associated indicators for their marine waters so as to guide progress

towards achieving GES in the marine environment, taking into

account the indicative lists of pressures and impacts set out in Table

2 of Annex III, and of characteristics set out in Annex IV”.

Each criterion in the commission decision is accompanied by one or more related

indicators that will be used to monitor changes in the status of each criterion. An

28

indicator can be considered as a specific characteristic of a GES criterion that can be

assessed quantitatively or qualitatively. Typically, a target or threshold is established

for a particular indicator, to determine whether the criterion is representative of GES

or not. According to Article 3 of the MSFD a target is defined as:

"a qualitative or quantitative statement on the desired condition of the different

components of, and pressures and impacts on, marine waters in respect of each marine

region or sub-region".

In other words, environmental targets are specific requirements to be met in order to

demonstrate that GES has been achieved.

To allow for the influence of climate, which is one of the main drivers of variability

in the plankton, Gowen et al. (2011) proposed the following plankton target for GES:

“The plankton community is not significantly influenced by anthropogenic

pressures.”

To determine whether the target has been met the following three step procedure

will be adopted. The first step will determine whether there is a significant long-

term trend in an index using appropriate statistical tests. For example, Gowen et al.

(2015b) used the Mann-Kendal (M-K) test for monotonic trends and regression

analysis with year and year2 fitted as the explanatory variable and the phytoplankton

time series data as the response, to detect linear and non-linear trends, respectively.

The absence of a trend will be used as evidence that on the basis of the data

available, there has not been a long-term change in the status of the plankton and

that the target has been met.

The second step (examination of causal links between trends in plankton indices and

trend in anthropogenic and climate pressures) will be undertaken if there is a

statistically significant long-term trend in an index. For this step, time-series of an

index will be cross correlated (year to year) with time-series of pressures. If

correlations are not significant, it will be concluded that the significant trend in the

29

index (and change in the condition of the plankton) is not due to those

anthropogenic or climate (to identify its possible influence on changes in the status

of the plankton) pressures for which there are data available for analysis.

The third step is to avoid spurious cross correlations and because correlation is not

evidence of cause and effect, in cases where there are significant correlations, further

analysis will be undertaken to try to identify a cause and effect. If cause and effect

cannot be demonstrated it will be assumed that the correlation is spurious and that

the target has been met. In contrast, if cause and effect can be demonstrated, then it

will be concluded that anthropogenic pressure has brought about a change in the

condition of the plankton and there has been a failure to meet the target. The

decision path for determining whether GES has been met is shown in Figure 3.1.

Figure 3.1 The decision path for determining whether the target for GES has been

met. (Redrawn from Gowen et al., 2011 and as modified by Gowen et al., 2013.)

30

3.1.2 Reference conditions

For the lifeform and state space approach, the reference envelope is used to calculate

values of the index for each subsequent year for which there are data and build a

time-series. It was agreed by project partners that data from the years 2008 to 2010

would be used to establish reference envelopes. However, it is important to know

the state of the plankton that the reference envelope represents (Gowen et al., 2011).

Therefore, it is important to assess the current state of the plankton under prevailing

conditions. The MSFD explicitly requires the establishment or maintenance of GES

over marine sub-regions. Therefore, the reference condition should ideally be the

status of the plankton that is deemed to represent a state which has been subjected to

minimal human influence, i.e. GES.

Establishing reference conditions for the Water Framework Directive (WFD) has

proved troublesome, because it has involved finding water-bodies subject to no, or

very little, anthropogenic pressure (“pristine conditions”) with an associated historic

time series of data. The MSFD is a departure from this approach and defines GES in

terms of ecosystem functioning and the need for ecosystems to be “fully functioning”.

Discussion of how to assess the state of the plankton and determine whether (or not)

it represented GES took place during the pre-project workshop (see Gowen et al.,

2013). It was agreed that simply considering the composition of the plankton against

a notional expectation of what species should be present was inadequate and that a

more robust approach was that of Scherer and Gowen (2012) which assesses data on

the abundance and composition of the plankton in the context of the

ecohydrodynamic conditions of the water bodies within which the plankton live

(and to which species are adapted). Participants reviewed the approach of Scherer

and Gowen (2012) and agreed a modification based around a suite of questions to

aid the assessment process (Box 3). However, it was concluded that until there is a

31

better understanding of what represents GES and how it can be determined

objectively, expert judgement would be used to determine GES.

Participants at the pre-project workshop also agreed that during the project, the

modified Scherer and Gowen method would be used to undertake initial

assessments of the state of the plankton at selected sites (Table 3.1) for which there

were readily available supporting environmental data (e.g. physical and chemical

oceanographic data). These assessments were reviewed by project partners at the

March 2014 and March 2015 workshops and while they do not provide complete

Box 3: Questions used to aid the initial assessment of GES of the plankton

1. Does the assessed area represent a distinct ecohydrodynamic region?

2. Is the seasonal pattern of dissolved inorganic nutrients consistent with current

understanding of the biogeochemical cycling in shelf seas?

3. Is the seasonal cycle of plankton production and biomass consistent with current

understanding of the processes controlling plankton biomass and production in shelf

seas?

4. Is the seasonal succession of species of the assessed site consistent with what is

expected for the ecohydrodynamic conditions in temperate shelf sea?

5. Does the plankton support higher trophic levels?

6. Does the concentration of anthropogenic nutrient enrichment at the assessed site stay

below the relevant OSPAR thresholds for dissolved inorganic nutrients and

nutrient?

7. Has there been a long term change in plankton phenology and biomass?

8. Does the state of the plankton at the assessed site represent good environmental

status (GES?)

32

coverage of UK waters, the method which was developed and tested during the

project provides the basis for a future assessment of UK waters. Details of the

assessments which were made are presented in Annex A of this report.

Table 3.1 Sampling sites at which the state of the plankton could be assessed and the

institute responsible for the assessment.

Short code sentinel site latitude longitude Project Partner

WIS Western Irish Sea 53.78 -5.63 AFBI

LE Loch Ewe 57.84 -5.61 MSS

S Stonehaven 56.96 -2.13 MSS

FoF Firth of Forth 56.02 -3.17 SEPA

FoC Firth of Clyde 55.94 -4.89 SEPA

WD Wash/ Dowsing 53.32 1.32 Cefas

WG West Gabbard 51.59 2.05 Cefas

L4 L4 (western English Channel) 50.25 -4.22 PML

LY1 Firth of Lorne/ Loch Linnhe 56.48 -5.50 SAMS

3.1.3 Indicators

As discussed in Section 2, the use of plankton lifeforms provides a means of

summarising a large amount of information on the seasonal abundance of individual

species, especially seasonal succession.

Careful selection of lifeform pairs (see Table 3.2) provides information on the MSFD

descriptors Biological diversity (D1), Food webs (D4), the relevant component of

Eutrophication (D5.2.4) and Seafloor integrity (D6). Combining the values of the

index for the lifeform pairs that are used for each descriptor provides an average

value of the index for each descriptor. Combining all of the plankton indices for the

four descriptors gives a value for the holistic Plankton Index that can be used to

33

monitor changes in the structure and functioning of the planktonic component of

pelagic ecosystems.

The pairing of lifeforms was discussed by Gowen et al. (2011) and revised at the

March 2014 project workshop. The selection of lifeform pairs was based on expert

opinion of the role of individual species in the functioning of the pelagic ecosystem.

The rationale for the selection of the lifeform pairs is set out below and the final set

of lifeform pairs to be used in the UK integrated monitoring programme is shown in

Table 3.2.

Biodiversity

For the pelagic habitat element of the Biodiversity descriptor (D1) we have selected

lifeform pairs that are likely to reflect changes in the dominant groups of

phytoplankton and zooplankton. The lifeform pairs selected were: (i) diatoms and

dinoflagellates, evolutionary distinct groups with different attributes and general

biology; (ii) gelatinous zooplankton and fish larvae (including fish eggs), which

provide indicators of alternative ecosystem states and ecosystem services (fisheries);

(iii) crustacean and non-gelatinous and non-crustacean holoplankton, which

incorporates all evolutionary distinct holoplankton groupings not included in (ii)

above.

Food webs

The Food Web descriptor (D4) concerned ecosystem structure and function (or

energy flow). The lifeform pairs selected were: (i) phytoplankton and zooplankton

abundance, to provide an indication of changes in the transfer of energy from

primary to secondary producers; (ii) large (>20 μm) and small (<19.9 μm)

phytoplankters, to track change in the potential efficiency of energy flow to higher

trophic levels; (iii) large (>2mm) and small copepods (<1.9mm), for an indication of

food web structure and the efficiency of energy flow through the food web.

34

Eutrophication (relevant component)

The lifeform pairing for the relevant component of the Eutrophication descriptor

(D5.2.4) were: (i) diatoms and dinoflagellates, to track changes in the occurrence of

high biomass harmful blooms; (ii) ciliate and microflagellates, chosen to determine

whether nutrient enrichment causes a shift in floristic composition, especially

towards heterotrophic phytoplankton; (iii) potentially toxin producing diatoms and

dinoflagellates, to track changes in toxin producing algae and their impacts on

ecosystem services.

Seafloor integrity

The lifeform pairs for the Seafloor Integrity descriptor (D6) focuses on benthic/

pelagic coupling: (i) holoplankton (fully planktonic) and meroplankton (only part of

the lifecycle is planktonic, the remainder is benthic), to track changes in benthic/

pelagic coupling; (ii) pelagic and tychopelagic diatoms, to monitor seabed

disturbance and the frequency of re-suspension events (e.g. storms).

35

Table 3.2 The final set of lifeform pairs for the MSFD Biodiversity, Food web, Eutrophication and Seabed integrity descriptors.

Descriptor Lifeform pair 1 Lifeform pair 2 Lifeform pair 3

D1: Biodiversity Diatoms Dinoflagellates Gelatinous

zooplankton

Fish larvae Holoplanktonic

crustacean

Non gelatinous and

non -crustacean

holoplankton

Lifeform

feature(s)

All

diatoms

All dinoflagellates Ctenophores

& Cnidarians

Including fish eggs Excluding eggs

Reasoning: Evolutionary distinct groups with

different attributes and general biology

Indicators of alternative ecosystem

states and potential services in food

provision

Evolutionary distinct groupings that

capture all holoplankton not included in

Lifeform pair 2

Pressure(s): Nutrient enrichment; change in

hydrographic conditions

Fishing Fishing; Nutrient enrichment

D4: Food-Webs Phytoplankton Zooplankton Large

phytoplankton

Small

phytoplankton

Large copepods Small copepods

Lifeform

feature(s)

Chlorophyll

(mg m-3)

Abundance (m-3) > 20 μm < 19.9 μm > 2 mm <1.9mm

Reasoning: A gauge of the magnitude of energy

flow and the balance between

successive trophic levels

Energy transfer from primary to

secondary producers a size-based

gauge of the potential efficiency of

energy flow to higher trophic levels

Food web structure and efficiency of

energy flow through food web

Pressure(s): Fishing Fishing Fishing, nutrient enrichment, change in

stratification

D5:

Eutrophication

Diatoms Dinoflagellates Ciliates Microflagellates Potentially

toxin producing

Toxin producing

dinoflagellates

36

diatoms

Lifeform

feature(s)

All diatoms Autotrophs and

mixotrophs

Including

tintinids

All species < 20 μm Excluding P.

delicatissima

All species on the

Food Standards

Agency list

Reasoning: Shift in community composition

towards potentially harmful groups

Shift from primarily autotrophic to

a more heterotrophic system

Shift in algal community towards

dinoflagellate HABs

Pressure(s): Nutrient enrichment Nutrient enrichment Nutrient enrichment

D6: Sea floor

integrity

Holoplankton Meroplankton Pelagic

diatoms

Tychopelagic

diatoms

Lifeform

Feature(s)

Excluding fish larvae All species

Reasoning: Indicator of strength of benthic-

pelagic coupling and reproductive

output of benthic versus pelagic

faunas

Seabed disturbance and frequency

of re-suspension events (e.g. storms)

Pressure(s): Bottom trawl fishing, dredging Climate change

D1.7: Biodiversity Ecosystem Structure All lifeform pair combinations.

Reasoning: Changes in these lifeforms provide a comprehensive overview of the structure and functioning of the planktonic component

of marine ecosystems.

Pressure(s): Fishing; nutrient enrichment; aquaculture, industrial spills (e.g. oil, contaminants); river damming; seabed disturbance (inc.

contaminant re-suspension); renewable energy; warm water outflows; ocean acidification

37

3.2 Sampling strategy

To support cost effective monitoring, the UK integrated plankton monitoring

programme has been built on: (i) a network of existing sampling sites and CPR

routes that are used for different monitoring and research purposes; (ii) the

ecohydrodynamic approach (see Section 2) to reduce the need for large scale spatial

sampling. Currently there are 13 fixed point sampling sites (Figure 3.2) and ten CPR

routes (Figure 3.3) that will deliver data for the monitoring programme and which

were chosen to ensure best coverage of ecohydrodynamic water bodies within each

CP2 region. Table 3.3 gives an overview of all fixed point stations that are currently

active and will contribute to the monitoring programme. At some fixed point

sampling sites data are collected regularly using small boats or research vessels and

sampled using CTD, water bottles (for chemical properties and phytoplankton);

some stations also sample zooplankton with net tows and. At other sites,

instrumented buoys which measure water properties electronically at high

frequency and take and preserve water samples at regular intervals for nutrients and

phytoplankton have been deployed. Continuous Plankton Recorders (CPRs) are

towed behind ships of opportunity along established routes at regular intervals, and

sample mesozooplankton, microplankton, phytoplankton and 'plankton colour', an

indicator of phytoplankton biomass. Environmental data are available for some

routes.

Some of the main difficulties which arise when trying to compare or combine data

from different sampling programmes, especially data from fixed point locations and

the CPR, were discussed in detail at the pre-project (Gowen et al., 2013) and project

workshops (Scherer et al., 2014).

In summary, samples collected from fixed point sampling sites and by the CPR can

be considered as each providing a window through which insights into the structure

and functioning of the planktonic component of the pelagic ecosystem can be

38

gained. However, the view ‘seen’ by the two sampling methods differ. Fixed point

sampling sites provide a detailed but restricted spatial view while the CPR provides

a less detailed but broader spatial view.

At the level of the plankton as a whole, there is no reason why data from both fixed

point sampling sites and the CPR cannot be used to track changes in the state of the

plankton. Indeed, both CPR data (Edwards, 2006) and fixed point sampling site data

(Tett et al., 2007; 2008; Scherer, 2012) have been use in the development of the

lifeform and state space method. However, the pictures of the plankton gained from

fixed point sampling site and CPR data are likely to be considerable different. The

mesh size of the CPR is 270μm and as a consequence the CPR will underestimate the

abundance of small species of phytoplankton. There is therefore an expectation that

the level of underestimation will increase as organism size decreases. Furthermore,

‘soft bodied’ species of mesozooplankton (gelatinous species), microzooplankton

(ciliates), and ‘athecate’ (lacking a cell wall) dinoflagellates are not sampled

quantitatively. Although some taxa are under-sampled, the relative abundance of

plankton recorded by CPR sampling remains consistent over time, allowing the

detection of inter-annual changes in phytoplankton abundance. At the present time

it is unclear which factor(s) have the greatest influence on the level of under

sampling.

As a consequence of the way in which the CPR collects plankton, the state space

plots of lifeform pairs to be used in the monitoring programme (Table 3. 2), will be

biased towards those organisms which the CPR is most efficient at capturing (large

hard bodied zooplankton and large species of phytoplankton with a robust cell

wall). This is one reason why participants at the pre-project workshop (Gowen et al.,

2013) agreed that fixed point sampling site and CPR data cannot be directly

compared or combined until further research is undertaken. Furthermore, the PI

method requires that the reference condition and the new (time-series) data are

39

sampled by the same method. Following further discussion at the project workshop

in March 2014 (Scherer et al., 2014) the majority view of the expert group was that:

i) Samples from fixed point stations offer a scientific benefit in terms of

providing quantitative plankton data and with supporting environmental

data. In general therefore, data from fixed point sampling are preferable in

situations where data from both sampling methods (fixed point and CPR) are

available for the same ecohydrodynamic region;

ii) Comparisons should not be made between data sets collected from fixed

point sampling sites and the CPR (until further research has been undertaken)

because they do not employ the same sampling method;

iii) Monitoring of the plankton should be based on the collection of samples from

the main three ecohydrodynamic areas within each CP2 region, rather than

sampling over large spatial scales;

iv) Data from both fixed point sampling sites and the CPR should be used to

monitor changes in the state of the plankton but owing to (iii) above

duplication of effort (i.e. using fixed point and CPR data from the same

ecohydrodynamic area) should be avoided until a comparison of data has

been completed.

Since the integrated UK plankton monitoring programme is based on a network of

existing sampling sites and CPR routes, there is incomplete coverage of the main

ecohydrodynamic water bodies and full monitoring of the phytoplankton and

zooplankton is not undertaken at all of the sites. The possible options for

implementing these are presented in detail in Section 5.

40

Table 3.3 Overview of all the fixed point monitoring stations currently able to

contribute to the UK integrated monitoring programme: 1-AFBI mooring 38A

western Irish Sea; 2-Cefas Liverpool Bay SmartBuoy; 3-Cefas Dowsing SmartBuoy; 4

-Cefas West Gabbard SmartBuoy; 5-MSS Loch Ewe; 6-MSS Stonehaven; 7-PML L4; 8

-SEPA Firth of Forth; 9-SEPA Firth of Clyde; 10-SAMS Firth of Lorne/Loch Linnhe;

11-MSS Scapa (Orkney Islands); 12-MSS Scalloway (Shetlands); 13-EA Bristol

Channel

Station number

Station abbreviation monitoring site lat long

1 WIS Western Irish Sea 53.78 -5.63 2 LB Liverpool Bay 53.53 -3.43 3 WD Wash and Dowsing 53.32 1.32

4 WG West Gabbard 51.59 2.05

5 LE Loch Ewe 57.84 -5.61 6 S Stonehaven 56.96 -2.13 7 L4 L4 (western English Channel) 50.25 -4.22

8 FoF Firth of Forth 56.02 -3.17

9 FoC Firth of Clyde 55.94 -4.89

10 LY1 Firth of Lorne/ Loch Linnhe 56.48 -5.502 11 Scap Marine Scotland Scapa 58.74 -3.04 12 Scal Marine Scotland Scalloway 60.18 -1.23 13 IBS001P Inner Bristol Channel off Minehead A IBS001P 51.27 -3.38

41

Figure 3.2 A map showing the 13 locations of the currently active fixed point stations

that will contribute to the UK integrated monitoring programme for plankton.

42

Figure 3.3 A map showing the location of current CPR routes where phytoplankton

biomass (by means of the Phytoplankton Colour Index), and phytoplankton and

zooplankton composition are currently monitored and which will contribute to the

UK integrated monitoring programme. Phytoplankton biomass for each of the sea

areas will also be obtained from remote sensing (as in Charting Progress 2).

43

3.3 Quality Assurance

Quality assurance and quality control will be a key part of the identification and

enumeration of plankton samples. At the 2014 workshop (Scherer et al., 2014) it was

agreed that it was the responsibility of each institute contributing to the monitoring

programme to ensure high quality for the data they provide. Some institutes have

ISO 17025 accreditation for the identification and enumeration of toxin producing

phytoplankton and analysis of chlorophyll. For phytoplankton data, one example of

a good external QA scheme is the annual international Biological Effects Quality

Assurance in Monitoring Programmes (BEQUALM) ‘ring test’ for analysts. For

zooplankton a National Marine Biological Analytical Quality Control Scheme

(NMBAQC) recommendation was circulated. Furthermore, regular internal QAs will

help to maintain the quality of the data provided. Metadata will be submitted and

upload to EMECO with plankton data. Each institute participating in the monitoring

programme will be responsible for their own metadata but a common set of

metadata is required (see Section 4)

3.4 Reporting

3.4.1 EMECO – General background

The European Marine Ecosystem Observatory (EMECO) is a consortium of agencies

and institutes with responsibility for both the monitoring and assessment of threats

to the marine ecosystem and status (health) and also for improving understanding

through research in European shelf-seas. The consortium brings together existing

monitoring, modelling and research capabilities to create a European infrastructure.

EMECO was formed to improve the evidence base for formal environmental

assessments, provide integrated assessments (from physics to fish) and to meet legal

requirements imposed by the Marine Strategy Framework Directive. In this way,

44

EMECO is an "End-to-End" system from data to integrated policy relevant

information products.

Operationalisation of the lifeform and state space method included the use of the

EMECO data tool (www.emecodata.net ) to:

aggregate plankton taxa into lifeforms;

calculate values of the Plankton Index;

output reference envelopes for particular ecohydrodynamic water bodies;

output time-series of the indices.

The EMECO tool will also be used to prepare regional and national reports.

3.4.2 Data processing

Reference data

Before changes in the condition of the plankton in UK waters can be monitored,

reference conditions have to be established. Data for the establishment of these

reference envelopes had to be processed. The first step in this process was to create a

template that all partners had to adopt to start uploading the data to EMECO (Table.

3.4) (unprocessed data). Some datasets for the reference envelopes submitted by

partners were already processed, i.e. sorted into lifeforms and could be uploaded

directly to the PI computer script within EMECO (Figure 3.4) (processed data). The

tool was set up so that it would accept unprocessed and processed data. For

consistency both processes use the same master list for sorting species into lifeforms.

Table 3.4 An example of the adopted template for uploading data into EMECO

Abundance (cellsL-1) sampling date Taxa Site Organisation

200 30/01/2007 Thalassiosira spp. IBS001P EA

3800 30/01/2007 Fragilaria spp. IBS002P EA


45

1

Figure 3.4 A flow diagram of the uploading process of data into EMECO. 2

Format data for upload to

EMECO

EMECO

1 2 3 4 5 6 7

1

2

3

4

5

6


p: 0.90points: 153



log10


log 10

(din

ofla

gella

te a

bund

ance

/L)

1 2 3 4 5 6 7

1

2

3

4

5

6


log10


drawn by PCI1G on 25-Jul-2013MCI: 0.60new points: 15

binom p: 0.0022chi-sq: 13.5 (df=1)

Output 1

Straight into PI script

Processed data

PI script (MatLab)

Master-list

Sorting process

Reference condition

Assigning lifeforms

Comparison condition

Output 2

1 2 3 4 5 6 71

2

3

4

5

6

7

min set at: 10 mf: 0 (ref & comp)

p: 0.90

points: 135

months 12-3

months 4-5

months 6-8

months 9-11


log10

(silicate/L)

log 10

(non

-sili

cate

/L)

1 2 3 4 5 6 71

2

3

4

5

6

7Comparison: West Gabbard (2008 to 2009)

log10

(silicate/L)

drawn by PCI1G on 17-May-2011

MCI: 0.63

new points: 30

binom p: 0.0001

chi-sq: 21.3 (df=1)

Output 3

Time series of PI value

Annual datasets deliver Each comparison creates 1 PI value

Input

46

The second step required the establishment of a master list (for an example see Table

3.5). The final master list has been sent to project partners. This list contains every

phytoplankton and zooplankton species currently identified and recorded by the

institutes participating in the monitoring programme. However, when counting

samples of plankton different analysts used a variety of short-hand names for

individual species. The master list lists all of these abbreviated names and relates

each to the correct species name. It will be important therefore that any future

participants in the monitoring programme use one of the existing abbreviations and

that for any additional species to be included in the monitoring programme a

common abbreviated name is used by analysts. The master list also takes into

account incorrect spelling and old species names which have been corrected

according to the World Register of Marine Species (WoRMS). Development of the

master list was an essential part in the process of aggregating species into lifeforms

and is now an integrated step within EMECO.

Table 3.5 An example from the master list, showing the different annotations used

for a particular species.

Species identified Uniform name

Thalassiosira rotula Thalassiosira rotula

T. rotula Thalassiosira rotula

Thalassiosira_rotula Thalassiosira rotula

Thalasiosira rot. Thalassiosira rotula

In addition to removing inconsistencies from misspelling, abbreviations and

outdated names, the master list ensures consistent allocation of species to lifeforms

across the different participants in the monitoring programme. Furthermore, the

allocation of species to lifeforms cannot be altered by individual analysts.

The third step was to assign lifeform codes to each of the correct species names. In

many cases an individual species has been assigned to more than one lifeform (see

Table 3.6).

47

Table 3.6 An example from the sorting system within EMECO showing the

allocation of a single species to more than one lifeform.

Species name Assigned lifeform code

Thalassiosira rotula LF code 1 (diatom)

Thalassiosira rotula LF code 5 (large phytoplankton >20µm)

Thalassiosira rotula LF code 12 (pelagic diatom)

The final step (Table 3.7) involved pairing the abundance of the agreed lifeform pairs

that relate to the MSFD quality descriptors (Table 3.2).

Table 3.7 An example output of the final step and format that is uploaded into the

MatLab script for the creation of an envelope.

Biodiversity descriptor (D1) lifeform pair 1

Date Abundance LF 1 (diatoms) Abundance LF 2 (dinoflagellates)

20/05/2009 481,570 65,200

The station positions and all necessary Metadata have to be sent to EMECO in a

separate file but as part of the ‘data package’ that EMECO requires for the data

processing. The station positions and all other information will be linked to the

individual monitoring site.

Monitoring data

The uploading of the data required for monitoring changes in plankton community

structure follow the same procedure as the data for the reference envelopes. The

only difference being that they are used to make comparisons to derive PI values

which in turn adds to the holistic PI data point and time series of the PI.

3.4.3 Outputs

The key graphical outputs that the EMECO tool delivers for the integrated

monitoring programme are shown in a flow diagram (Figure 3.4) which illustrates

how the system currently works with the required inputs and the graphical outputs.

48

Reference conditions

The first output is the reference envelope or the envelopes representing the starting

condition for a lifeform pair at a monitoring site or from a CPR route. It was agreed

that envelopes for the starting conditions should reflect the state of the lifeform pairs

over 3 years (2008 to 2010) and would ideally be representative of GES. The time

period is the same for all monitoring sites. All reference envelopes are retained

within EMECO.

Calculating the Index

Once the envelopes for the reference conditions have been established, comparisons

can be made. For this the PI computer script integrated in the EMECO data tool,

processes monthly data (or monthly mean values if sampling frequency is higher

than monthly) for each year and plots the comparison into the reference envelope

and calculates the PI value. It also performs statistical analysis to determine whether

a value of the index is statistically different from the reference (for technical details

see 2.4.3).

Time-series of PI indices

With each lifeform pair comparison a PI value is calculated. The holistic PI value is

the average of all of the PI values and delivers one data point to the time-series of the

index. The time series is the third output of EMECO.

This time-series can then be analysed for long-term trends and cross correlated with

time-series of human and climate pressures (see section 3.1 Figure 3.1). As agreed in

the March 2014 workshop this statistical analysis will be undertaken by each

institute individually.

49

3.4.4 Reporting with the EMECO reporting tool

The main part of the lifeform and state space method to be used for the UK

monitoring programme is embedded in EMECO and has been operational since 20th

August 2014. The following is an illustration of how plankton data can be used with

the EMECO data tool for plankton.

An individual can get access to the lifeform and state space method to be used for

the monitoring programme by visiting the EMECO website (www.emecodata.net)

requesting a login. It is recommended to load the website in Google Chrome or

Firefox (Internet Explorer has a tendency to crash) and operate the EMECO tool with

these providers. Once a login has been granted, data can be uploaded or imported

under the “My Data” section. Currently the user can choose from two scripts: the PI

script that processes the fixed point sampling data and the CPR script that handles

the CPR data.

Figure 3.5 Screenshot of the interface for importing or uploading data in EMECO.


50

Under the section “Assessment Tool” projects and reports can be set up and shared.

Generally, a project has four components: data a subset of data queried from the

EMECO data tools (these could be the abundance of one lifeform category or a

lifeform pair in a specific ecohydrodynamic area, but also temperature data for a

CP2 region); processing any calculations or time filters that have been applied to

the data (in the case of our lifeform data we agreed a monthly average); outputs a

set of data and information products generated from the data query and processing

applied; confidence estimate of confidence applied to the data and outputs

(percentages of confidence in the data can be set manually with a scale bar - this

might not necessary apply for our data).

A project is created and saved within a folder or a sub folder on the EMECO

platform. Sub projects can also be created within projects. Project folders, sub

folders, projects and subprojects can be re-ordered, re-named, edited, deleted or

shared. To keep a better overview it is suggested to create a new project within one

folder each time new outputs are wanted.

51

Figure 3.6 Screen shot of the interface of setting up a new project under the

Assessment Tool section.

Currently all lifeform categories (in abundance) and lifeform pairs for the relevant

descriptors are included in the data platform and the reference envelopes (year 2008

to 2010) for all datasets submitted by partners are available.

After having set up a project the user can use the reporting tool which is a text editor

to write, share and save reports using the outputs, tables, graphs, etc. generated in

EMECO and those generated by other means (e.g. photographs, text written in

word). It can also be exported in PDF format. The project outputs are synchronised

with the report i.e. if a project is updated and saved, the outputs in the reports will

52

be automatically updated. At this point in preparing a report the results of time-

series analysis and correlation statistics can be incorporated into the report.

Figure 3.7 Screenshot of the interface of the reporting tool.

3.4.5 EMECO Reporting Tool

An example EMECO report in pdf format is provided in Annex B.

4. Operational readiness

This section of the report summarises the operational readiness of the UK integrated

plankton monitoring programme and identifies elements of the programme that

require further work. The latter is divided into two: (i) minor elements which will be

resolved in the light of experience gained since monitoring began in July 2014 and

through further discussion by project partners at the March 2015 post-project

workshop; (ii) elements which require additional funding.

4.1 State of readiness

The UK integrated plankton monitoring programme will incorporate a target and

indicators based on the lifeform and state space method for tracking change in the

condition of the plankton in UK coastal and shelf seas. Data will be collected from

existing monitoring and research programmes. There are nine fixed point sentinel

53

sites (Table 3.1) plus four other sites (giving 13 fixed point sampling stations) and 10

CPR routes (Table 5.1) located in the main ecohydrodynamic water bodies of each

CP2 region. The EMECO data tool which includes the computer script to calculate

the plankton indices will be used for: data manipulation; QA to ensure conformity in

species names and the allocation of species to lifeforms across participating

institutes; preparation of reports on the status of the plankton in UK waters.

The lifeform and state space method has been published in the peer review scientific

literature (Tett et al., 2007; 2008) and the theory was updated by Tett et al. (2013). The

method was also the subject of a Ph.D. studentship (Scherer, 2012) and has been

applied to phytoplankton time-series data from the Thau lagoon in France (Gowen et

al., 2015). A target (“The plankton community is not significantly influenced by

anthropogenic pressures.”) and plankton indicators have been established for the MSFD

quality descriptors for the plankton component of Biodiversity, Food web,

Eutrophication and Sea floor integrity. A holistic plankton indicator has also been

developed. For the purposes of the MSFD, monitoring began in July 2014, although it

should be noted that there are existing time-series of data for many of the sampling

sites and CPR routes.

The status of the plankton has been assessed at sentinel sites and will be used as

reference conditions against which future change in the condition of the plankton

will be determined.

The EMECO data tool is operational. Unprocessed (species data not sorted into

lifeforms) and processed data (lifeform data) can be uploaded by each institute

participating in the monitoring programme. Outputs from the data tool include

reference envelopes, calculated values of plankton indices and time-series plots of

indices. All of these plots can be incorporated into a report using the EMECO

reporting tool.

54

4.2 Outstanding matters

4.2.1 Minor elements to be completed at the March 2015 workshop

There are three minor elements of the monitoring programme which need to be

finalised at the March 2015 workshop.

1. To determine whether the target has been met, analysis will be performed on

time-series of the indices and any significant trends will be correlated with

trends in anthropogenic and climate pressures (see Section 3). This analysis

cannot currently be automated with EMECO and it was agreed (Scherer et al.,

2014) that each institute participating in the monitoring programme will take

responsibility for the statistical analysis of their own data. However,

agreement is required on a standardised approach based on the procedure

outlined in Section 3.1 and will be finalised in March 2015.

2. Metadata provides an important way of quality assuring data and the

provision of metadata to accompany plankton data was discussed at the

project workshop in March 2014. It was agreed that each institute

participating in the monitoring programme will be responsible for organising

their own metadata but that MEDIN will be asked to advice on the details

that should be supplied as metadata and where metadata should be stored.

3. As noted above, the EMECO data tool will be used for data manipulation, QA

and reporting. As in the case of data analysis, participating institutes will be

responsible for preparing reports based on the data they collect. However,

agreement is required on the content and format of these reports and how

these regional reports are integrated into a UK national report.

55

4.2.2 Elements that require additional funding

The assessment of the status of the plankton in those ecohydrodynamic water bodies

that will be sampled by the CPR and the Lorne Observatory in the Firth of Lorne

were not undertaken as part of this project. Funding from NERC (NE/M007855/1) is

now in place to analyse data from the Lorne Observatory and an assessment will be

undertaken and reported to the March 2015 workshop. At present no source of

funds has been identified to assess the status of the plankton in those

ecohydrodynamic water bodies sampled by the CPR.

As noted above, the plankton monitoring programme is built on a network of

existing sampling sites and CPR routes that are used for different monitoring and

research programmes. While this is cost effective, it does mean the individual

sampling programmes were not specifically designed for the purposes of the MSFD.

As a consequence, not all of the sampling sites sample phytoplankton and

zooplankton and for some CP2 regions the network does not cover all of the main

ecohydrodynamic water bodies.

Therefore, as it is currently constructed the UK integrated plankton monitoring

programme does not meet the minimum requirement that the 'Lifeform and State

Space' project partners agreed was necessary to deliver robust assessments of

changes in the status of the plankton in UK coastal waters and shelf seas (Scherer et

al., 2014). The following section presents options for upgrading the current

monitoring programme to meet the minimum requirement.

5. Recommendations to Defra

The recommended minimum requirement to deliver an integrated UK plankton

monitoring programme consists of:

56

Fixed point sampling sites and CPR routes in each of the three main

ecohydrodynamic water bodies of each CP2 region;

Full sampling of the phytoplankton and zooplankton at each fixed point

sampling station and CPR route.

As noted above, at present the monitoring programme does not fulfil the minimum

requirement and it will not be possible to deliver an integrated assessment of

changes in the condition of the plankton. This section presents an analysis to identify

gaps in the current sampling programme. To preserve the commercial

confidentiallity of project partners the options presented here were costed in a

separate confidential report to Defra.

5.1 Gap analysis

To identify gaps in the current monitoring programme a series of gap maps were

produced. The findings are summarised in Table 5.1. All of the existing sampling

sites and CPR routes require continued funding but some are more vulnerable than

others. The vulnerability of each site has also been included in Table 5.2.

The maps shown in Figure 5.1a - 5.1c graphically illustrate the information given in

Table 5.1. Since the CPR routes (see Figure 3.3) provide the necessary data for

phytoplankton, zooplankton and phytoplankton biomass (by means of the

phytoplankton colour index) for the UK integrated monitoring programme they are

not shown here again.

57

Table 5.1 Overview of all current (existing) fixed point monitoring stations that will contribute to the UK integrated monitoring

programme. The table indicates the findings of where delivery of data is currently not provided. The labelling is as follows:

1-AFBI mooring 38A western Irish Sea; 2-Cefas Liverpool Bay SmartBuoy; 3-Cefas Dowsing SmartBuoy; 4-Cefas West Gabbard

SmartBuoy; 5-MSS Loch Ewe; 6-MSS Stonehaven; 7-PML L4; 8-SEPA Firth of Forth; 9-SEPA Firth of Clyde; 10-SAMS Firth of

Lorne/Loch Linnhe; 11-MSS Scapa (Orkney Islands); 12-MSS Scalloway (Shetlands), 13-EA Bristol Channel

Station number

monitoring site lat long

Biomass as Chlorophyll

Phytoplankton Zooplankton

1 WIS Western Irish Sea 53.78 -5.63 2 LB Liverpool Bay 53.53 -3.43 3 WD Wash/ Dowsing 53.32 1.32

4 WG West Gabbard 51.59 2.05

5 LE Loch Ewe 57.84 -5.61

6 S Stonehaven 56.96 -2.13

7 L4 L4 (western English Channel) 50.25 -4.22

8 FoF Firth of Forth 56.02 -3.17

9 FoC Firth of Clyde 55.94 -4.89

10 LY1 Firth of Lorne/ Loch Linnhe 56.48 -5.502 11 Scap Marine Scotland Scapa 58.74 -3.04

12 Scal Marine Scotland Scalloway 60.18 -1.23

13 IBS001P Inner Bristol Channel off Minehead A IBS001P 51.27 -3.38

58

Figure 5.1a A map showing the locations of fixed point stations where

phytoplankton biomass (based on chlorophyll measurements) is currently

monitored. (Not undertaken at Scapa and Scalloway).

59

Figure 5.1b A map showing the locations of fixed point stations where

phytoplankton composition is currently monitored.

60

Figure 5.1c A map showing the locations of fixed point stations where zooplankton

biomass is currently monitored.

61

Table 5.2 Overview of all current (existing) plankton monitoring and their vulnerability in the CP2 regions of UK waters. Note that it is assumed

that where a monitoring is stated as “secure”, this is based on the understanding that funding is in place and will continue for the foreseeable

future. For explanation of the ecodydrodynamic type see section 2.3. The station numbering in parenthesis after the station name refers to the

map in Figure 2.2 indicating the station location.

CP2 Region Ecohydrodynamic Type Fixed point or CPR route Vulnerability

1 Central and northern North Sea

A, Seasonally stratified SAHFOS-CPR Secure

B, Indeterminate MSS - Stonehaven (6) Secure

C, Region of freshwater influence SEPA – Firth of Forth (8) Secure

2 Southern North Sea A, Permanently Mixed Cefas – Dowsing, The Wash and West Gabbard (3)

Medium

B, Indeterminate SAHFOS- CPR Secure

3 Eastern English Channel A, Permanently mixed SAHFOS – CPR Secure

B, Indeterminate SAHFOS – CPR Secure

4 Western English Channel and ‘Celtic Sea’ including the Bristol Channel

A, Seasonally stratified PML – L4 (7) Secure

SAHFOS – CPR Secure

B, Permanently stratified SAHFOS - CPR Secure

C, Region of Freshwater Influence EA – Bristol Channel (13) Secure

5 Irish Sea and Firth of Clyde A, Seasonally stratified AFBI – western Irish Sea (1) Secure

B, Region of freshwater influence AFBI/Cefas – Liverpool Bay (2) Secure

C, permanently mixed AFBI LBy06 (14) – proposed as new site

D, predominantly haline stratification SEPA- Inner Firth of Clyde (9) Secure

6 West coast of Scotland (including the Minch and (CW))

A, Complex seasonality MSS – Loch Ewe (5) Secure

B, predominantly haline stratification SAMS – LY1 (10) Secure

C, Indeterminate SAHFOS – CPR MSS – Loch Ewe (5) Secure

7 Outer Scottish Shelf: Malin to Shetland, west of Hebrides and Scottish north coast

A, Seasonally stratified MSS – Scalloway (12) Secure

B, Mixed inshore; tidal mixing MSS – Scapa (11) Secure

C, Seasonally stratified SAHFOS – CPR Secure

West of Hebrides SAHFOS – CPR Secure

8 Rockall Trough and plateau A, Oceanic seasonal stratification overlying permanent thermocline (except around Rockall)

SAHFOS – CPR (along the northern edge of the region) – Rockall trench??

Secure

62

The analysis shows that the biggest gap is the lack of zooplankton monitoring. Given

the key role played by zooplankton in the transfer of organic matter (and energy) to

higher trophic levels and therefore to ecosystem functioning, implementation of

zooplankton monitoring was considered the highest priority. It was decided by the

expert group that the implementation of zooplankton and, where missing,

phytoplankton has to be given priority over the establishment of new sites/routes

and investment in new technology (e.g. to improved identification of small sized

phytoplankton).

5.2 Additional monitoring sites

During the workshop in March 2014, there was some discussion on the uncertainty

of sampling coverage in the Bristol Channel. The information provided by Mike Best

(Environment Agency) after the workshop was that the station Bristol Channel Inner

South (IBS001P) would cover the ROFI (region of freshwater influence) in CP2

region 5 and would provide the best sampling frequency for zooplankton.

For the complete coverage of main ecohydrodynamic regions in the Irish Sea and

Firth of Clyde (CP2 region 5) it was proposed that AFBI could monitor the

permanently mixed region in the eastern Irish Sea as part of AFBI’s routine

oceanographic east-west transect across the Irish Sea.

5.3 Options

The following is a summary of the option selection to implement the minimum

recommended monitoring programme. The option selection and recommendation

are based on the majority view expressed by the project partners. It is assumed that

where monitoring is identified as being secure this is based on the understanding

that funding will continue from existing sources.

63

Option 1 – do nothing

This option presents the base case or doing nothing option. That is, the UK national

plankton monitoring programme would be based on existing (where funding is

currently secure) fixed point monitoring sites (13) and CPR routes (10) without any

additional sampling.

Current status

Phytoplankton is currently sampled at 13 out of 13 fixed point sites and zooplankton

is currently sample at only 5 of these sites. Phytoplankton and zooplankton are

sampled at all relevant CPR routes (10 out of 10).

Implications

This option would only provide:

i. 1 out of the 3 lifeform pairs for biodiversity (QD1);

ii. 1 out of 3 lifeform pairs for food webs (QD4);

iii. 1 out of 2 lifeform pairs for sea floor integrity (QD6);

iv. The complete (relevant) component (D5.2.4) of eutrophication (QD5);

This incomplete sampling would also mean that it would not be possible to produce

values for the holistic indicator. While this option may provide a near complete suite

of lifeform pairs for the CPR data (apart from the small sized phytoplankton and

micro-flagellate lifeforms) there would still be no sampling of zooplankton in the

Region of Freshwater Influence (ROFI) in the Bristol Channel. There would also be

no sampling of zooplankton in the seasonally stratifying region of the Irish Sea or in

the Firth of Lorne haline stratified waters.

The risk presented by this option would be a failure to have complete suites of

lifeforms for some ecohydrodynamic water bodies and a lack of sampling from

others. In both cases this would make scaling up to the CP2 regions difficult. The net

64

result would be an incomplete monitoring programme and failure to track changes

in the condition of the plankton throughout UK waters.

Option 2 – upgrade existing fixed points but no new ones

This option presents the case for upgrading existing fixed point stations but not to

establish new ones. This would give a monitoring programme based on existing

(where funding is secure) fixed point sites and CPR routes and would fill the current

gaps in phytoplankton and zooplankton monitoring.

Current status

Phytoplankton is currently sampled at 13 out of 13 fixed point stations and

zooplankton is currently sampled at 5 of these 13 sites. Phytoplankton and

zooplankton is sampled at all relevant CPR routes (10 out of 10).

Implications

This option would provide data on the full suite of lifeforms for the relevant MSFD

descriptors from existing fixed sampling sites and CPR routes. However, there

would be no sampling in the Firth of Lorne haline stratified waters or in the ROFI of

the Bristol Channel. For the Irish Sea there would be full sampling at the seasonally

stratifying region and the ROFI, but not at the permanently mixed waters (although

there is a CPR route through this area). This would lead to a failure to deliver a

complete UK integrated plankton monitoring programme.

Option 3 - CPR based UK monitoring

This option presents the case for basing the whole UK monitoring programme on

CPR routes.

65

Current status

There are currently 10 CPR routes that provide (semi-quantitative) data on

phytoplankton and (quantitative) data on zooplankton.

Implications

An additional three new CPR routes would be required to cover all three main

ecohydrodynamic water bodies in each CP2 region. Given the under sampling by

the CPR discussed above, the majority view of the project partners was that the UK

plankton monitoring programme should not be based solely on the CPR.

Option 4 – the minimum recommended monitoring programme

This option presents the case of bringing the current UK monitoring programme up

to the minimum recommended: the addition of phytoplankton and zooplankton

monitoring at existing fixed point stations with the establishment of three new fixed

point sites (Bristol Channel, outer Liverpool Bay and Firth of Lorne) to cover all

main ecohydrodynamic types in every CP2 region.

Implications

Implementing this option would provide the minimum UK integrated monitoring

programme recommended by the lifeform project expert group. This would: (i)

provide the full suite of lifeforms for all of the main ecohydrodynamic areas in each

CP2 region; (ii) deliver a monitoring programme which will detect changes in the

planktonic component for the MSFD quality descriptors (D) 1 (Biodiversity), 4 (Food

webs), 6 (Sea floor integrity), and the relevant component (D5.2.4) of the

eutrophication descriptor.

66

5.4 Options selection

Based on the majority view, the opinion of this expert group is that option 4 –

minimal additional monitoring is the best option and the one recommended to

Defra. Table 5.3 summarises the option sift.

Table 5.3 A summary of the options.

Option Implication

1 - Do nothing i) Lack of sampling in some ecohydrodynamic water

bodies in some CP2 regions

ii) Failure to provide adequate data to assess changes in

the plankton

iii) Failure to deliver assessment of GES for the plankton

habitat in UK waters

2 - upgrade

existing

sampling sites

i) Lack of sampling in some ecohydrodynamic water

bodies within CP2 regions

ii) Failure to deliver assessment of GES in UK waters

3 - CPR based

monitoring

programme

i) Semi quantitative sampling for small species and soft-

bodied species

ii) Lack of environmental data

iii) Duplication of effort where fixed point stations exist

iv) No sampling in the seasonally stratifying water of the

Irish Sea

4 - Implement

minimum

recommended

programme

Will deliver:

i) UK wide coverage of the three main ecohydrodynamic

water bodies areas within each CP2 region

ii) Delivery assessment of status (GES or not)

in each CP2 region

6. Future Research

1. A comparison between data collected by fixed point sampling stations and CPR

tows. As discussed above (and see also Gowen et al., 2013) methods of collecting

plankton at fixed point sampling sites and by the CPR differ and this is a

constraint on integrating data sets. To try and overcome this constraint and

determine whether (and the extent to which) or not fixed point and CPR data

67

can be integrated, a detailed investigation is required. Such an investigation

should quantify differences in sampling and address questions such as: (i) are

levels of under-sampling consistent over time and space? (ii) is under-sampling

inversely related to organism size; (iii) would it be cheaper to determine how to

use the two types of datasets (CPR and fixed point) together than to increase

monitoring?

2. Assessing the status (GES) of the plankton in those ecohydrodynamic regions

where CPR routes are used to collect data for the integrated UK plankton

monitoring programme.

3. Research on the concept of ecosystem health and defining GES for the planktonic

component for the pelagic ecosystem.

68

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77

Annex A

Assessments of the state of the plankton at sentinel sites

Determining the status of the microplankton

community in the western Irish Sea

Cordula Scherer and Richard Gowen

Agri-Food and Biosciences Institute, HQ, 18a Newforge Lane, Belfast, BT9 5PX,

Northern Ireland

78

Contents

Summary ................................................................................................................................ 81

Rational ................................................................................................................................... 82

1. Introduction ....................................................................................................................... 82

1.1 General Background ................................................................................................... 82

1.2 Assessing the status of the microplankton in the western Irish Sea .................... 83

2. Methods .............................................................................................................................. 85

3. Physical oceanography of the Irish Sea ......................................................................... 86

3.1 Introduction ................................................................................................................. 86

3.2 Adjacent Sea areas ...................................................................................................... 88

3.3 Flow and residence time ............................................................................................ 90

3.4 The seasonal cycle of temperature and salinity ...................................................... 91

3.5 The seasonal development of stratification ............................................................. 95

3.6 The sub-surface light climate .................................................................................. 103

3.7 Summary .................................................................................................................... 105

4 Dissolved Inorganic Nutrients ....................................................................................... 105

4.1 Introduction ............................................................................................................... 105

4.2 Adjacent Sea Areas ................................................................................................... 106

4.3 The Irish Sea ............................................................................................................... 110

4.4 External Nutrient Sources ........................................................................................ 115

4.5 Long-term change ..................................................................................................... 119

4.6 Summary .................................................................................................................... 120

79

5. Microplankton ................................................................................................................. 121

5.1 Introduction ............................................................................................................... 121

5.2 The seasonal cycle of biomass and production in the western Irish Sea. ......... 121

5.3 Microplankton species abundance and composition .......................................... 126

5.5 Summary .................................................................................................................... 132

6. Zooplankton ..................................................................................................................... 132

6.2 Long-term change in zooplankton ......................................................................... 133

7. Energy flow through the food web .............................................................................. 134

8. Assessing the state of the microplankton .................................................................... 135

8.1 Introduction ............................................................................................................... 135

8.2 Does the western Irish Sea represent a distinct ecohydrodynamic region? ..... 136

8.3 Is the seasonal pattern of dissolved inorganic nutrients consistent with current

understanding of biogeochemical cycling in shelf seas? ........................................... 136

8.4 Is the seasonal cycle of microplankton production and biomass consistent with

current understanding of the processes controlling microplankton biomass and

production in shelf seas? ................................................................................................ 137

8.5 Is the succession of species in the western Irish Sea consistent with what is

expected for a seasonally stratifying temperate shelf sea? ....................................... 139

8.6 Does the microplankton in the western Irish Sea support higher trophic levels?

............................................................................................................................................ 141

8.7 Is the western Irish Sea enriched with anthropogenic nutrients? ...................... 142

8.8 Has there been a long-term change in phytoplankton phenology and biomass?

............................................................................................................................................ 142

8.9 Does the state of the microplankton in the western Irish Sea represent good

environmental status (GES)? ......................................................................................... 143

80

9. Establishing reference conditions ................................................................................. 144

9.1 Introduction ............................................................................................................... 144

9.2 The lifeform state space approach .......................................................................... 145

9.3 Assigning species to lifeforms ................................................................................. 148

9.3.1 Biodiversity descriptor (D1) ................................................................................. 152

9.3.2 Food web descriptor (D4) ...................................................................................... 153

9.3.3 Eutrophication descriptor (D5) ............................................................................ 154

9.3.4 Sea floor integrity descriptor (D6) ........................................................................ 155

10. Conclusions .................................................................................................................... 156

References ............................................................................................................................ 158

81

Summary

The aim of the work presented in this report was to assess the state of the

microplankton in the seasonally stratifying region of the western Irish Sea. To

determine whether there was evidence of top down (fisheries) and bottom up

(nutrient enrichment) induced change and whether the state of the microplankton

was representative of GES for the purposes of the MSFD.

To assess the state of the microplankton the approach recommended by

Gowen et al. (2013) was followed and the microplankton data were interpreted in

the context of the ecohydrodynamic conditions in the western Irish Sea. The final

step in the assessment that is whether the microplankton was representative of GES

was based on expert judgement. Data on the physical and chemical oceanography of

the western Irish Sea together with data on the microplankton have been assembled

from peer review publications and unpublished data held by DARD/AFBI.

There is a recurrent annual cycle of seasonal stratification in the western Irish

Sea which characterises the region as a distinct ecohydrodynamic water body. There

is a low level of anthropogenic nutrient enrichment although time-series analysis

shows that there is a decreasing trend in the winter concentration of dissolved

inorganic phosphorus and that there is no long-term trend in winter nitrogen. The

microplankton data show that there is a recurrent annual cycle of phytoplankton

production. The beginning and duration of the production season is controlled by

the sub-surface light climate as a function of solar radiation and surface mixed layer

depth. During the production season there is a succession of species: diatoms

typically dominate the spring bloom and dinoflagellates increase in abundance

during the summer and early autumn. In some years there is an autumn bloom

which is dominated by diatoms.

Based on the data we concluded that the microplankton community in the

seasonally stratifying region of the western Irish Sea does not experience bottom up

82

or top down pressure and in our expert opinion, we also concluded that under

prevailing conditions the microplankton is in good environmental status and could

be used as reference conditions for other seasonally stratifying regions in UK waters.

Rational

The key aim of this report was to determine the status of the microplankton

community in the western Irish Sea providing an authoritative assessment of its

status. The method to achieve this aim was to take existing (published) and new

(unpublished data held by AFBI and DARD) data gained by surveys and remote

sampling at and around the mooring station 38A. It was further to apply a modelling

approach to quantify any changes in Irish Sea micro-plankton over the last decades

using AFBI long term mooring data.

1. Introduction

1.1 General Background

The term phytoplankton is the name given to the microscopic floating plants that are

found in freshwater and marine waters. Collectively these species are responsible for

the bulk of the primary production (carbon fixed during the process of

photosynthesis) in the world’s ocean and this supports the pelagic food web and

benthic production. Other microorganisms in the plankton include mixotrophic6 and

heterotrophic7 species that play an important role in the cycling of organic matter in

the pelagic component of marine ecosystem. Collectively, the autotrophs,

mixotrophs and heterotrophs make up the microplankton.

6Mixotrophs are autotrophic (fix carbon by photosynthesis) but are also capable of using

organic matter. 7 Heterotrophs require organic matter as a source of energy and nutrient elements.

83

Changes in the phytoplankton brought about through climate change (Edwards

2005) and both bottom up and top down anthropogenic pressures on the

microplankton can alter energy flow and influence ecosystem structure and

functioning (see Scherer and Gowen, 2013a and references cited therein: Report to

DARD (CA/033766/11)). Negative feedback from such changes can in turn influence

the delivery and sustainability of ecosystem services to humans, especially fisheries

(Ware and Thomson 2005).

The aim of the work presented in his report was to assess the status of the

microplankton in the western Irish Sea and determine whether there was evidence of

top down, fisheries (and bottom up – nutrient enrichment) induced change. An

additional outcome of the assessment of the status of the microplankton was to

determine whether status was representative of GES for the purposes of the MSFD

and whether the microplankton of the western Irish Sea could be used as reference

conditions for other water bodies in UK waters with similar physical, chemical and

biological (ecohydrodynamic) characteristics.

1.2 Assessing the status of the microplankton in the western Irish Sea

In October 2005, the European Marine Strategy Framework Directive (MSFD)

was presented by the Commission of the European Union (EU) and came into force

in 2006 (2008/56/EC). The overall aim of the MSFD is to protect and where necessary

re-store the European seas: ensuring sustainability for human use and providing

safe, clean, and productive marine waters. The directive covers all European waters

up to 200 nautical miles from the coastal baseline and there is therefore a small

geographical overlap with the Water Framework Directive (WFD). It includes the

water column, sea bed and its sub-surface geology and under the directive,

assessments of environmental (ecological) status will be based on eleven quality

descriptors (QDs) which are: biological diversity (QD 1), non-indigenous species

(QD 2), population of commercial fish/shell fish (QD 3), elements of marine food

webs (QD 4), eutrophication (QD 5), sea floor integrity (QD 6), alteration of

84

hydrographical conditions (QD 7), contaminants (QD 8), contaminants in fish and

seafood for human consumption (QD 9), marine litter (QD 10), introduction of

energy (including underwater noise) (QD 11). All member states are expected to

achieve “good environmental status” (GES) in the marine environment by 2020

(2008/56/EC). GES is defined as “the environmental status of marine waters where these

provide ecologically diverse and dynamic oceans and seas which are clean, healthy and

productive within their intrinsic conditions”.

To develop the necessary framework of targets and indicators for the MSFD, the

UK Department of Environment and Rural Affairs (Defra) established a programme

of work that included two scientific workshops. At the second workshop

(Birmingham, 29-30th March 2011), a ‘pelagic subgroup’ discussed methods of

detecting change in the plankton found in the coastal waters and seas around the

UK. The subgroup recommended a ‘lifeform functional group’ approach (Tett et al.

2008) that developed from a Defra-funded study (led by Cefas, CSA 6754/ME2204)

but identified several matters that required further consideration. To address these,

Defra funded two workshops at the Agri-Food and Biosciences Institute (AFBI) in

Belfast in June 2011 (Gowen et al. 2011) and in March 2013 (Gowen et al. 2013). The

plankton is considered under QD1, 4, 5 and QD 6 and at the second workshop

participants agreed and recommended an approach to assess the status of plankton

communities in UK waters. This approach formed the basis of the assessment

presented in this report.

This report is a final report to DARD for work package 4 of project

CA/033766/11. As agreed with the project steering group committee members, this

report will be used as the basis for assessing the state of the plankton in other coastal

regions of the UK as part of a Defra funded (AFBI project code 45073) project to

establish plankton targets and indicators for the MSFD. However, the assessment

presented here (together with the other assessments) will be the subject of peer

review in March 2014 and may therefore be updated.

85

2. Methods

Participants at the March 2013 workshop agreed that to simply consider the

composition of the plankton at selecting sites against a notional expectation of what

species should be present was an inadequate means for determining the state of the

plankton. Instead, the group concluded that a much more robust approach to

assessing state would be to interpret plankton data in the context of the

ecohydrodynamic conditions of the water bodies within which the plankton live and

to which species are adapted. However, the expert group pointed out that until there

is a better understanding of what represents GES and how it can be determined

objectively for the plankton, it would be necessary to use expert judgement to

determine whether the state of the plankton was representative of good

environmental status.

Detailed information on the methods used to collect data and samples and the

analyses used for particular variables can be found in the publications cited in this

report. The data that have been compiled to support the assessment are presented in

the following three sections.

To assess the state of the microplankton in the western Irish Sea, in (Section 8)

we asked a series of questions in an attempt to determine whether: (i) the data on the

microplankton in the western Irish Sea are consistent with current understanding of

the dynamics (and factors influencing those dynamics) of the microplankton in

temperate shelf seas; (ii) there has been any long-term climate or anthropogenic

driven change in the western Irish Sea microplankton. In accordance with the view

of the expert group, we have used our expert judgement to conclude whether or not

the state of the plankton in the western Irish Sea is representative of GES.

86

3. Physical oceanography of the Irish Sea

3.1 Introduction

The Irish Sea is a small (2534 km3) inner shelf sea that connects to the more

open shelf waters of the Celtic Sea to the south by St George’s Channel and to the

Malin Shelf in the north via the North Channel (Fig. 3.1). A deep trough 80-100 m

extends north south through the western Irish Sea. The deepest part of the sea is in

Beaufort’s Dyke (~300 m) in the North Channel. To the east the water is generally

less deep (< 50m) and there are extensive shallow (~20 m) coastal areas. The specific

features and locations mentioned in the text are shown in Fig. 3.1.

87

Figure 3.1 Maps of the western shelf region of the UK and Ireland (left) and of the northern Irish Sea (right) showing features

mentioned in the text.

10 9 8 7 6 5 4 3 2

Longitude (decimal)

48

49

50

51

52

53

54

55

56

57

Lo

ng

itu

de

(dec

imal

)

Malin Shelf

English Channel

Celtic Sea

Belfast

Dublin

North

Channel

Wales

France

SaintGeorge'sChannel

Eastern

Irish

Sea

Bristol

Channel

Western

Irish

Sea

Mal

in H

ead

ShelfBreak

ShelfBreak

Celtic Seafront

Islay front

Malin Shelf and Celtic Seatransect line

Beaufort's Dyke

Transect A

Liverpool Bay

6.4 6.1 5.8 5.5 5.3 5.0 4.7 4.4 4.1 3.8 3.5 3.3 3.0

Longitude (decimal)

53.0

53.2

53.4

53.6

53.8

54.0

54.2

54.4

54.6

54.8

Lat

itu

de

(dec

imal

)

4

6

13

16

21

26

38

45

50

57

47

LB

38A

Belfast

Eastern

Irish Sea

AFBImooring station:

62

88

3.2 Adjacent Sea areas

The seawater of the shelf and coastal seas of North West Europe has its origin in

the Atlantic Ocean. Small-scale processes modify the temperature and salinity

characteristics of this oceanic water as it is transported onto the continental shelf and

into shelf seas. This is particularly true of the Irish Sea and many of the physical (and

chemical) characteristics of the Irish Sea reflect its relative isolation from the ocean.

Early measurements of surface temperature and salinity show that the Celtic Sea

is more saline and warmer than the Irish Sea (Bassett, 1910; Matthews, 1914). For the

period 1903 to 1931, Bowden (1955) gave the annual mean surface temperature in the

Celtic Sea as 12° C compared to 10.5-10.75° C in the Irish Sea. Data collected recently

as part of Work package 2 of this project (Scherer & Gowen, 2013) also shows this

feature (Fig. 3.2) and also show that the outer Malin Shelf region is warmer than the

Irish Sea in winter (data shown in Fig. 3.2) and in summer. The figure also illustrates

the geographical isolation of the Irish Sea from the Atlantic Ocean.

89

Figure 3.2: A contour plot showing the distribution of temperature from the shelf break off

the Malin Shelf through the Irish Sea and Celtic Sea to near ocean waters off the Celtic Sea

shelf break. (Data were collected in January 2013/AFBI unpubl. data).

The salinity of water at the shelf break region of the Celtic Sea and Malin Shelf

is ~35.50 and 35.42 respectively. On occasion high salinity water can penetrate into

the Irish Sea from the south (Gowen et al., 2002). However, a gradual reduction in

salinity as Atlantic water is transported from the shelf break region into the Irish Sea

and mixes with riverine inflow and runoff from the land is more typical. Recent

measurements of salinity across the Malin Shelf (Scherer and Gowen, 2013) are

consistent with earlier observations (Ellett & Edwards, 1983) in showing that oceanic

water (salinity ≥ 35.00) water generally lies to the west of Malin Head Fig. 3.3).

0 74 148 222 296 370 444 518 592 666 740 814 888

Distance (km)

999

899

799

699

599

500

400

300

200

100

0

De

pth

(m

)

Outer malin Shelf Outer Celtic ShelfAFBIMooring

January 2013

90

Figure 3.3: The near surface distribution of salinity on the Malin Shelf in January 2013. The

filled dots show the positions of sampling station. The contour interval is 0.1 (AFBI unpubl.

data).

3.3 Flow and residence time

Flow through the region is generally considered to be northwards (Bassett, 1909)

with water from the Atlantic and Celtic Sea providing the source water for the Irish

Sea. However, once ocean water has moved onto the shelf, flow is not regular. Early

estimates of flow through the Irish Sea between Dublin and Holyhead range from

1.3 km d-1 (Knudsen cited in Bowden, 1950; Brown, 1991) to 0.3 km d-1 (Bowden,

1950).

Assuming a Dublin to Holyhead cross sectional areas of 7.2 km2, these flows

equate to a volume transport of between 2.1 and 9.2 km3 d-1. Volume transport

through the North Channel has been estimated to be between 2 and 8 km3 d-1

(Dickson and Boelens, 1988); 3.5 and 5.2 km3 d-1 (Simpson and Rippeth, 1998); 8.6

km3 d-1 (Brown and Gmitrowicz, 1995) and 6.7 km3 d-1 (Knight and Howarth, 1998).

Strong winds in winter might be expected to increase exchange across the

shelf break regions of the Celtic Sea and Malin Shelf. However, such winds tend to

10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0

Longiture

54.0

54.5

55.0

55.5

56.0

56.5

57.0L

atit

ud

e

91

increase the flow of the slope current (which travels along the slope of the European

continental shelf edge west of Ireland and Scotland) rather than increasing the

movement of water onto the shelf (Pingree and Le Cann, 1989). As ocean water is

transported onto the shelf it can be considered to age as it moves across the shelf into

the Irish Sea. Based on changes in nutrient concentrations and ratios in ocean water

as it extends onto the shelf, Hydes et al. (2004) estimated that water in the Celtic sea

was 2 years old but had aged to 6 years by the time it has reached the middle of the

Irish Sea. Using the same approach water in the outer region of the Malin shelf was

estimated to be 400 days years old and 600 days old in the inner shelf region near

Malin Head. The residence time of time of water in the Irish Sea is in the order of 12

months (Dickson and Boelens, 1988). The estimated age of the water in the central

Irish Sea therefore seems overly long. However, the 6 years includes the transit time

from the shelf break. Furthermore, it is apparent that the exchange of water between

the Irish Sea and adjacent sea areas is influenced by wind events (Knight and

Howard, 1998).

The situation in summer is rather more complex. The seasonal development of

tidal mixing fronts on the Malin Shelf (Simpson et al., 1979; Gowen et al., 1998) in the

Irish Sea (Simpson and Hunter, 1974) and Celtic Sea (Fasham et al., 1983) together

with changes in the patterns of water circulation (Hill et al., 1994; Horsburgh et al.,

1998) make it difficult to quantify the summer flow through the Irish Sea.

3.4 The seasonal cycle of temperature and salinity

Seasonal changes in water temperature in the Irish Sea are governed by the

annual cycle of solar heating and cooling. An example of the seasonal cycle of near

surface and bottom water temperature is shown in Fig. 3.4 (data from 2004).

Minimum near surface and near bottom temperatures of 7.7° and 7.6° C were

recorded on 24th March respectively. The maximum near surface temperature (16.6°

C) was recorded on the 17th August but the maximum near bottom water

temperature (13.7° C) was not reached until the 5th November. Data collected from

92

the AFBI instrumented mooring in the western Irish Sea between 1996 and 2013

shows that there is some inter-annual variability in the timing and values of the

maximum and minimum near surface and near bottom water temperatures but that

the seasonal changes are a recurrent annual feature (Fig. 3.5).

Figure 3.4: The seasonal changes in near surface and near seabed temperature (°C) at the

AFBI mooring site in the western Irish Sea. (Data from 2004)

Figure 3.5: Seasonal changes in near surface and near seabed temperature (°C) at the AFBI

mooring site in the western Irish Sea between 1996 and 2013.

The lowest salinity is found in the eastern Irish Sea and this reflects the inflow of

freshwater. Of the total riverine discharge into the Irish Sea (31 km3 y-1) some 24.9

km3 y-1 (80 %) flows into coastal waters of the eastern Irish Sea (Bowden 1955). The

Liverpool Bay region is much influenced by freshwater inflow and can be defined as

4.0

6.0

8.0

10.0

12.0

14.0

16.0

Jan Feb Mar May Jun Aug Sep Nov Dec

Tem

per

atu

re (

°C)

Near Surface

Near Seabed

5

10

15

20

96 97 98 99 00 01 02 03 04 05 06 07 08 09 10 11 12 13

Tem

per

atu

re (

°C)

Year

Near surface

Near bottom

93

a ‘Region Of Freshwater Influence’ (ROFI) meaning that there is tidal straining of the

horizontal salinity gradient and sporadic lenses of fresher water that are moved by

wind and mixed away when stirring increases. In the eastern Irish Sea, isohalines are

orientated north south (Fig. 3.6) and reflect the origin of freshwater inflow and are

suggestive of limited exchange between the eastern and western Irish Sea, although

the distribution of radionuclides indicate some east west transport (Leonard et al.,

1997). Most of this low salinity water leaves the Irish Sea via the North Channel

(McKay et al., 1986; Balls, 1987; Brown and Gmitrowics, 1995) although under certain

meteorological conditions, the northerly flow can be reversed and at such times a

tongue of low salinity water eastern Irish Sea water may be advected across the top

of the Isle of Man (Lee, 1960).

94

Figure 3.6: The spatial distribution of near surface salinity in the Irish Sea during January

1990. The contour interval is 0.1).

For offshore waters of the western Irish Sea, the annual mean surface salinity

given by Bowden (1955) for the period 1903 to 1931 was 34.1 to 34.4. The mean near

surface salinity at the AFBI instrumented mooring at station 38A is 34.20. The south

north salinity gradient in the western Irish Sea (Fig. 3.6) is indicative of a tongue of

relatively high salinity water extending northwards from the Celtic Sea. This feature

has been documented from earlier investigations of the Irish Sea (Bassett, 1909;

Matthews, 1913) and can therefore be regarded as a consistent winter feature. The

salinity at station 38A (34.20) indicates that ~3% of the water is freshwater. Over the

6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0

Longitude

53.2

53.6

54.0

54.4

54.8

55.2

55.6

56.0

Lat

itu

de

95

year, the seasonal range in salinity is small, 0.91 in 2006 (Fig. 3.7) with seasonal

maxima in winter and minima in late spring and summer.

Figure 3.7: The seasonal variation in near surface salinity at station 38A in the western Irish

Sea in 2006.

3.5 The seasonal development of stratification

The bathymetry of the Irish Sea, regional differences in tidal amplitude and

freshwater inflow give rise to distinct hydrographic regions (Gowen et al., 1995). In

St George’s Channel, much of the eastern Irish Sea and in the North Channel

turbulence generated by strong tidal flows is sufficient to maintain a vertically

mixed water column throughout most of the year. In contrast, early investigations

of the physical oceanography of the Irish Sea documented the presence of summer

stratified water to the south east of the Isle of Man (Matthews, 1913). In this region,

deep water (80 m) and weak tidal flows (< 0.5 m s-1) limit the downwards transfer of

heat and the water column stratifies (Fig. 3.8).

Stratification begins to develop in April (Fig. 3.8) although there is some

inter-annual variability in the timing of the onset of stratification and this influences

the timing of the plankton production season. Maximum stratification (up to ~6.0° C)

is typically observed in August. It is evident that the bottom water is not completely

33.40

33.60

33.80

34.00

34.20

34.40

34.60

01-Jan 22-Feb 15-Apr 06-Jun 28-Jul 18-Sep 09-Nov 01-Jan

Sal

init

y

Date

2006

96

isolated during summer since there is a gradual increase in temperature over the

summer. In 2002 for example, the bottom water temperature increase from 9.0° C on

the 19th of May to 12.0° C on the 30th August. This warming may be due to vertical

heat flux or movement of bottom water. Stratification begins to erode during late

summer. In 2002, the near surface to near bottom temperature difference was ≤ 1.0°

C on the 11th October and had fallen to 0.2° C on the 17th of that month.

Figure 3.8: Contour plots showing the seasonal development of thermal stratification at

station 38A in the western Irish Sea in 1997 and 2002. The contour interval is 0.5 °C.

North- south transects of the western Irish Sea places the temperature structure

at the AFBI mooring site in a wider geographical context (Fig 3.9). Data for the

section were collected during a survey in June 1992. The most intensely stratified

region is to the south west of the Isle of Man (stations 26 to 50). Here, warm surface

water (13.7° C) was separated from deeper cooler water by a thermocline with a

temperature gradient (∆T) of 2.9° C, over 10 m between 15 and 25 m. Below the

thermocline there was a ‘cold water dome’ of bottom water which is separated from

the surrounding, warmer bottom water by bottom density fronts. North of station 16

and south of station 57, tidal flows are stronger and there is greater mixing. As a

108

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63

48

32

17

2

Dep

th (

m)

1997

99

85

71

58

44

30

16

2

Dep

th (

m)

2002

J 01 F 22 A 15 J 06 J 28 S 18 N 09 D 31

97

consequence, and despite the greater depth in area of the North Channel, there is a

greater transfer of heat down the water column. At station 6 for example, surface

temperature in June 1992 was 11.7° C (compared to 13.7° C at station 38) and there

was no evidence of isolated cold bottom water. South of the region of intense

stratification, shallower water and increased tidal flows result in greater transfer of

heat down the water column. Surface water at station 57 was 11.0° C and the surface

to bottom difference was only 0.2° C.

Figure 3.9: The vertical distribution of temperature through the western Irish Sea in June

1992. The contour interval is 0.5°C. (See Figure 3.1 for station positions).

Vertical gradients in salinity are evident throughout the deeper part of the

western Irish Sea (Fig. 3.10). These gradients are small but can play an important role

in stabilising the water column such that heat is trapped in the surface layers and

rapid stratification of the water column takes place in early spring. By comparing the

contribution that temperature and salinity make to the density of the water (and

values of the potential energy anomaly (φ = phi) 8, Gowen et al. (1995) estimated that

8The potential energy anomaly: a measure of the amount of mechanical work (J m-3) necessary to

vertically mix the water column and values of < 10 and > 20 J m-3 indicate mixed and stratified water

respectively (Simpson et al., 1979; Simpson, 1981)

0 14 28 42 56 69 83 97 111 125 139

Distance (km)

160

140

120

100

80

60

40

20

Dep

th (

m)

S4 S6 S13 S16 S21 S26 S38 S50 S57 S62

98

salinity can account for ~50 % of the stratification during early spring. However, as

the surface layer warms, salinity becomes less important in stabilising the water

column.

Figure 3.10: A contour plot showing the horizontal and vertical distribution of salinity at

station 38 in the western Irish Sea during 1992. The contour interval is 0.15 and the dashed

lines are the sampling dates.

The seasonal development and spatial extent of stratification was investigated

by Gowen et al. (1995) by recording the changes in vertical gradients in density at a

grid of stations in the western Irish Sea during 1992. The density data were used to

plotting contour diagrams of φ (Fig. 3.11). In 1992, stratification developed first to

the south west of the Isle of Man in early May. The area of stratified water expanded

rapidly and by late May occupied most of the offshore region of the western Irish

Sea. For the region as a whole, stratification was most intense during July (Fig. 3.11)

and it is evident that there were two centres of stratification. The larger of the two

areas is located to the south west of the Isle of Man and the second between the Isle

of Man and the Northern Ireland coast. The area of weaker stratification between

these two centres may be due to an area of shallower water and greater mixing.

Throughout the region, stratification was weaker in August and the more northerly

100

88

75

63

51

38

26

13

1

Dep

th (

m)

Mar 25 May 5 Jun 16 Jul 15 Aug 16 Sep 24 Nov 17 Jan 06Oct 20

99

region of stratification had been eroded. In the vicinity of stations 4 and 6, the period

of stratification may only last 3 months compared to 5 months at the AFBI mooring

site. The observations of the seasonal development and erosion of stratification made

in 1992 by Gowen et al. (1995) are supported by data collected during an intensive

series of surveys conducted in 1995 by Horsburgh et al. (2000).

1

2

0

4

0 20

1711

12

3

1

12

313

31

10

3

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18

April 28-30

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imal

)

0

16

1

6

40

2 4

01

1

1

0

3

1

2

1

April 6-8 May 18-22

53.3

53.5

53.8

54.0

54.3

54.5

54.8

Lat

itu

de

(dec

imal

)

June 15-18July 13-16 August 15-18

6.4 6.1 5.8 5.5 5.1 4.8 4.5

Longitude (decimal)

53.3

53.5

53.8

54.0

54.3

54.5

54.8

Lat

itu

de

(dec

imal

)

1 3

1734

1

12

13

11

6 14

80

0 0 1

4

0

0

40

0 7

2

9

9

8

Sept 21-25

13

0

1

0

0

0

1

0

0

0

0

00

0

1

0

0

0

0

6.4 6.1 5.8 5.5 5.1 4.8 4.5

Longitude (decimal)

1

0

3

1

0

0

6

0

2

0

0

0

0

4

21

6.4 6.1 5.8 5.5 5.1 4.8 4.5

Longitude (decimal)

Oct 20-22 Nov 17-18

100

Figure 3.11: Contour plots of the potential energy anomaly (Phi, ϕ) illustrating the seasonal

development of stratification in the western Irish Sea in 1992. The contour interval is 10 J m -3

(from Gowen et al., 1995).

Bottom fronts separate the ‘cold water dome’ from warmer bottom water (Fig.

3.12) and drive a cyclonic gyre of near surface water (Hill et al., 1994). Recent

investigations of the gyre including the results of drifter studies have been reviewed

by Horsburgh et al. (2000). As noted above, vertical gradients in salinity may play an

important role in the initial stages of stratification and the gyre may establish in

April when temperature gradients are small. The bottom fronts that drive the gyre

are more stable than the near surface fronts and thermocline and the gyre can persist

into October when the surface features have been eroded. Data from drifters show

that the gyre encompasses both centres of stratification (Fig. 3.11) although the

degree of coupling between then is less clear. According to Hill et al. (1994) northerly

flows can reach up to 20 cm s-1 on the eastern flank of the gyre but southerly flows

are weaker (9 cm s-1) on the western flank; the transport time of drifters around the

gyre is ~42 days with a mean speed of 10 cm s-1. Towards the centre of the gyre, near

surface waters appear to become isolated although loss of drifters from the region

(particularly in early summer) suggests than some exchange between the gyre and

adjacent waters does occur (Hill et al., 1994).

Figure 3.12: The thermal structure of the western Irish Sea front showing the bottom front in

1999 (from Trimmer et al., 2003).

F1S1 S2 M1 M2

92

80

69

58

47

36

24

13

2

Dep

th (

m)

14.3

101

The south eastern boundary of the gyre forms the western Irish Sea tidal mixing

front (Simpson and Hunter, 1974) between the seasonally stratified western Irish Sea

and mixed eastern Irish Sea water (Fig. 3.12). The front extends from the southern

point of the Isle of Man to Dublin and in can be identified by the 14.2° C isotherm

(Fig. 3.13). The transition from stratified to mixed waters occurs over a distance of

~20 km. The front becomes established in April-May, once the water column in the

western Irish Sea begins to stratify, and persists until at least August (Simpson and

Hunter, 1974). However as discussed above, stratification of the water column

begins to weaken in August. It is rapidly eroded and by September/ October the

surface front loses its integrity. Recent measurements of benthic mineralisation rates

(Trimmer et al., 2003) show a striking correspondence between the position of the

front and the transition in benthic activity over a distance of 13 km. Since the benthic

characteristics on either side of the front reflect the longer-term pattern, this sharp

demarcation in sedimentary characteristics supports the conclusion of Simpson and

Bowers (1979) that there is limited (5 km) movement in the position of the front.

102

Figure 3.13: The surface distribution of temperature (°C) showing the position of the western

Irish Sea front (denoted by dashed line which represents the 14.2°C isotherm). The contour

interval is 0.2°C. (DARD/AFBI data collected during July 2001).

The seasonally stratifying region of the western Irish Sea is a depositional area.

The reduction in turbulence associated with stratification allows seston (living

planktonic organisms and non-living detrital material) to settle out of the water

column (Fig 3.14). The depositional nature of the western Irish Sea is reflected in the

composition of bottom sediments (fine silt/clays) and there is important chemical

and biological coupling between the water column and sediment. For example,

Trimmer et al. (2003) found that the sediment on the stratified side of the front had

higher concentrations of chlorophyll and higher rates of oxygen uptake and nutrient

efflux compared to the sediment on the mixed side. The gyre may augment the

6.4 6.2 6.0 5.8 5.6 5.4 5.2 5.0 4.8 4.6 4.4 4.2 4.0

Longitude

53.2

53.3

53.4

53.5

53.6

53.7

53.8

53.9

54.0

54.1

54.2

Lat

itu

de

Dublin

Isle ofMan

103

depositional nature of the western Irish Sea by retaining seston within the region

and increasing the likelihood that material will settle out.

Figure 3.14: Changes in the stock of particulate aluminium (mg m-2) in the upper 25 m of the

water column in the stratified region of the western Irish Sea during spring 1997.

3.6 The sub-surface light climate

As discussed later in section 5, the sub-surface light climate plays an

important role in determining the onset and duration of the phytoplankton

production season in coastal waters and shelf seas. Examples of the vertical profile of

down-welling photosynthetically active radiation (PAR) from the seasonally

stratifying region of the western Irish Sea are shown in Fig. 3.15. From such

measurements the attenuation coefficient (kd) can be calculated and used to calculate

the depth at which irradiance is 1% of surface irradiance. This depth is the euphotic

zone depth: the surface layer within which there is sufficient light for

photosynthesis. Estimates of kd and euphotic zone depth from the irradiance profiles

in Fig. 3.16 are given in Table 3.1. From this small data set mean euphotic zone depth

is 25.7 m which encompasses the surface mixed layer and thermocline (Figs. 3.8, 3.8

and 3.12).

0

100

200

300

400

500

600

700

800

900

1000

03-Mar 25-Mar 17-Apr 10-May 01-Jun

Par

ticu

late

Alu

min

ium

(m

g m

-2)

Date

104

Figure 3.15: Examples of the attenuation of down-welling photosynthetically active radiation

in the seasonally stratifying region of the western Irish Sea during spring 1992 and in July

1996.

Table 3.1 Estimates of the attenuation coefficient of down-welling photosynthetically active

radiation and the corresponding euphotic zone depth.

Date Attenuation coefficient

(m-1)

Euphotic zone

depth (m)

Mar 25 0.1994 23.0

Apr 06 0.2415 19.1

Apr 14 0.1947 23.7

Apr 28 0.1691 27.2

May 05 0.1281 35.9

July 05 (1996) 0.1846 24.5

0

5

10

15

20

25

0 500 1000

Dep

th (

m)

Irradiance (μE m-2 s-1)

Mar-25

Apr-06

Apr-14

Apr-28

May-05

Jul 06 (1996)

105

3.7 Summary

The Irish Sea is a small geographically isolated inner shelf sea and this is reflected in

the age of water in the western Irish Sea which may be up to 6 years old relative to

near ocean water at the Celtic Sea shelf break. Deep water and weak tidal flows in

the western Irish Sea allow the water column to stratify (φ ≥ 20 for 120 days) for up

to 5 months between April and September. Stratification results in a surface mixed

later of ~25 m, which is separated from cold bottom water by a thermocline. During

spring and summer the euphotic zone is typically the same as mixed layer depth.

Bottom density fronts drive a near surface gyre with a rotation of approximately 42

days. The seasonally stratifying region is a depositional area in which seston settles

to the seabed and this is reflected by the fine silt/clay bottom sediments.

4 Dissolved Inorganic Nutrients

4.1 Introduction

Dissolved inorganic forms of nitrogen and phosphorus are essential nutrients for the

growth of phytoplankton (and higher plants). In addition, diatoms and

silicoflagellates require silicon (Si, as dissolved silica or silicic acid, Si(OH)4) for cell

wall formation. Nutrients may ‘limit’ both the growth and the yield of

phytoplankton populations. The former relates to the rate of increase in biomass,

and the latter to the absolute amount of biomass generated per unit of nutrient

available. The relationship between nutrients and populations of micro-organisms

can be described by a number of theories (Monod, 1942; Dugdale, 1967; Droop, 1968;

Droop, 1983; Davidson and Gurney, 1999; Flynn, 2005).

In temperate coastal waters and shelf seas, dissolved inorganic nutrients reach

their annual maximum during winter when the rate of re-supply exceeds the

106

demand by phytoplankton. For this reason, investigations of nutrient sources and

the nutrient status of coastal waters are typically based on winter data. In this report,

dissolved inorganic nitrate + nitrite is denoted as TOxN; dissolved inorganic

phosphate as DIP and silicate as Si.

4.2 Adjacent Sea Areas

The Atlantic Ocean is the source of water for the Celtic and Irish seas and this

oceanic water determines the background nutrient levels for the two seas. Present

day Atlantic concentrations of dissolved nutrients have been established as a result

of large scale biological, chemical and physical processes which have been active

over geological time-scales. However, as noted above the residence time of water in

the Celtic and Irish Seas is in the order of years, and this timescale implies that

considerable recycling of dissolved N, P and Si will take place within these shelf

seas.

Gowen et al. (2002) and Hydes et al. (2004) presented winter (January/ February)

nutrient data from near surface waters at the Malin Shelf and Celtic Sea shelf break

(Table 4.1). The more recent winter data in Table 4.1 are from surveys of the Malin

Shelf (Scherer and Gowen, 2013) undertaken as part of EFF project (CA/033766/11) of

which this report is part and AFBI surveys of the Celtic Sea. Quasi-synoptic data

collected in January 2013 have been plotted as a section from near ocean waters off

the continental shelf west of the Malin Shelf through the Irish and Celtic seas to near

ocean waters of the South West approaches (Fig. 4.1). A consistent feature of near

surface concentrations of dissolved inorganic nutrients is that winter concentrations

are higher in near ocean waters of the Malin Shelf. Hydes et al. (2004) attributed this

to deeper winter mixing off-shelf which introduced more nutrients into near surface

waters at the Malin Shelf compared to the depth of mixing in near ocean waters

beyond the Celtic Sea shelf break. More recent data supports this view and the

107

temperature data plotted in Fig. 4.2 show the greater depth of winter mixing in near

ocean waters off the Malin Shelf.

A second feature of the quasi-synoptic winter nutrient data shown in Fig. 4.1 is

the lower concentration (6.0µM) of TOxN in the western Irish Sea compared to the

outer shelf and near ocean. One reason for this might be because deep winter mixing

off the shelf restores winter nutrient concentrations to surface waters more quickly

that recycling restores inorganic nutrient concentrations in the inner shelf (Hydes et

al., 2004). The finding that concentrations of TOxN in the western Irish Sea were

significantly higher in March compared to January/ February (Gowen et al. 2002)

supports this view.

108

Table 4.1: Winter concentrations (µM) of dissolved inorganic phosphorus (DIP), nitrate +

nitrite (TOxN) and silicate (Si) in near surface (upper 20 m) waters of the shelf break region

of the Celtic Sea and Malin Shelf.

Concentration (µM) Ratios Source

Date DIP TOxN Si TOxN:DIP TOxN:Si

Celtic Sea

Feb 1994 - 7.80 - Gowen et al. 2002

Feb 1998 0.46 6.65 2.57 14.5 2.5 Gowen et al. 2002

Jan 1999 0.43 7.56 3.29 17.6 2.3 Gowen et al. 2002

Jan 2009 0.43 7.34 2.52 17.1 2.9 AFBI data

Jan 2011 0.55 8.17 3.24 14.7 2.5 AFBI data

Jan 2012 0.48 6.27 2.36 13.1 3.3 AFBI data

Jan 2013 0.48 6.91 1.90 14.4 3.6 AFBI data

Open Ocean (off the Malin Shelf)

0.68 11.00 4.75 16.2 2.3 Hydes et al. 2004

Malin Shelf

0.53 7.40 3.30 Hydes et al. 2004

Jan 2012 0.73 10.49 3.42 14.4 3.1 AFBI data

Jan 2013 0.64 10.29 3.66 16.1 2.8 AFBI data

109

Figure 4.1: A contour plot showing the distribution of TOxN (μM) along a section from near

ocean waters off the Malin Shelf (station M29) through the western Irish Sea (AFBI mooring

site, station 38A) to near ocean waters off the Celtic Sea shelf edge (Station CS08) during

January 2013. (The approximate location of the transect is show in Figure 3.1)

Figure 4.2: Vertical profiles of temperature (°C) in near ocean waters at station M29 (Malin

Shelf) and CS08 (Celtic Sea) in January 2013.

0 89 178 266 355 444 533 622 710 799 888

Distance (km)

999

874

749

624

500

375

250

125

0D

epth

(m

)

11.0

16.0

M29 M32 M34 M42 M44 M50 38A B11 B9 B7M12A M45 62 CS08CS06CS05CS03CS01 CS04

7.0 8.0 9.0 10.0 11.0 12.0 13.0

0

200

400

600

800

1000

7.0 8.0 9.0 10.0 11.0 12.0 13.0

Dep

th (

m)

Temperature (°C)

M29

CS08

110

4.3 The Irish Sea

Near surface winter concentrations of nutrients in the Irish Sea exhibit spatial

and temporal variability. Concentrations are typically higher in the eastern Irish Sea

(Fig. 4.3) and reflect the high concentrations of nutrients in freshwater flowing into

the region. At the Liverpool Bay station worked in 1997 for example, the maximum

winter concentrations of TOxN and P were 29.2 and 1.7 µM respectively (Gowen et

al., 2002). There are pronounced seasonal cycles in the concentration of all three

nutrients in the Irish Sea (Fig. 4.4) and the AFBI time-series of nutrient data from

station 38A shows that this seasonal pattern is a recurrent annual feature of the

western Irish Sea (Fig. 4.5).

Typically, there is a slow build up of nutrients over the winter and maximum

concentrations in offshore waters of the western Irish Sea reach their maximum in

March. Gowen and Stewart, 2005 gave mean (1998-2002) March concentrations of:

8.3 µM TOxN, 0.7 µM DIP and 6.6 µM Si). This is followed by a rapid removal of

nutrients from the surface mixed layer. The ratios (TOxN:DIP and TOxN:Si) of this

nutrient drawdown are typically 11.5 and 1.26 for TOxN:DIP and TOxN:Si

respectively (mean values for 1992, 1997-1999 and 2001). The timing of this

drawdown is variable. Taking the date on which the near surface concentration has

fallen to 50% of the maximum winter concentration as the mid-point of the

drawdown then between 1992 and 2013 (insufficient data for 1994, 1999 and 2000)

the mean midpoint was 21st April and ranged from 30th March to the 23rd May (Fig.

4.6).

111

Figure 4.3: The spatial distribution of TOxN, DIP and Si (μM) in near surface waters of the

Irish Sea during January 2000.

6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5

Longitude

53

53.2

53.4

53.6

53.8

54

54.2

54.4

54.6

54.8

Lat

itu

de

(dec

imal

)

6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5

Longitude

53.0

53.2

53.4

53.6

53.8

54.0

54.2

54.4

54.6

54.8L

atit

ud

e

TOxN DIP

Si

112

Figure 4.4: The seasonal cycle of inorganic nutrients at station 38 in the western Irish Sea

during 1992.

Figure 4.5: The TOxN time-series at the AFBI mooring site (station 38A) in the western Irish

Sea.

0

1

2

3

4

5

6

7

8

9

10

25-Mar 29-Apr 04-Jun 10-Jul 15-Aug 20-Sep 26-Oct 01-Dec 06-Jan

Co

nce

ntr

atio

n (

μM

)

Date

TOxN

DIP

Si

0

2

4

6

8

10

12

95 96 97 98 99 00 01 02 03 04 05 06 07 08 09 10 11 12 13

Co

nce

ntr

atio

n (

μM

)

Year

113

Figure 4.6: Inter-annual variability in the timing of the spring drawdown of winter nutrients

in the western Irish Sea (see text for details). There were insufficient data for 1994 and 1999.

Measurements made at station 38A in 1999 and 2000 show that over the spring

period 88 and 95 % of the TOxN were removed from the upper 30 m of the water

column. There is also a net removal of nitrogen from the water column. During the

spring of the same two years, between 22 and 35% of the total nitrogen stock was

removed from the euphotic zone (Table 4.3).

Table 4.2: Changes in the stock (concentration as µM summed over the upper 23 m of the

water column to give the stock as mmol m-2) of different nitrogen fractions at station 38 in

the western Irish Sea during spring 1999 and 2000.

Nitrogen fraction (mmol m-2)

Date Dissolved

inorganic

Dissolved

organic

Particulate

organic

Total

1999

March 03 287.4 141.6 43.4 472.5

April 07 23.4 222.3 195.3 441.0

May 11 35.1 224.0 107.1 366.5

June 22 67.7 263.0 102.7 433.4

2000

-30

-20

-10

0

10

20

30

40 1

99

2

19

93

19

95

19

96

19

97

19

98

20

01

20

02

20

03

20

04

2005

20

06

20

07

20

08

20

09

20

10

20

11

20

12

20

13

Tim

ing

of

spri

ng

blo

om

Year

114

April 06 186.9 263.1 32.2 482.2

May 01 194.3 191.1 42.6 428.0

May 09 8.2 183.1 87.1 278.4

May 11 9.3 255.9 65.1 330.4

As noted in section 3, thermal stratification of the water column in the western Irish

Sea isolates the bottom water. The drawdown of nutrients observed in the surface

mixed layer does not occur in the bottom water and a ‘pool’ of nutrient rich water

persists throughout the summer in the western Irish Sea (Fig. 4.7).

Figure 4.7: A contour plot of nitrate showing the seasonal depletion in the surface mixed

layer. (DARD/AFBI data collected from the seasonally stratifying region of the western Irish

Sea during 1992).

Concentrations of all three nutrients remain low in the surface mixed layer

throughout the summer (≤ 0.3 µM TOxN, 0.2 DIP µM and 0.7 Si µM) but by late

summer concentrations begin to increase. The breakdown in stratification brings

about the redistribution of nutrients with the mixing of nutrient rich bottom water to

the surface. In 1992 for example, DIN, DIP and Si were uniformly mixed throughout

the water column by October. However, autumnal mixing does not return nutrient

concentrations to their winter maxima. Recycling in the water column and sediment

efflux contribute to the restoration of the winter maxima.

110

97

83

70

57

44

30

17

4

Dep

th (

m)

Mar 25 May 19 Jun 16 Jul 15 Aug 16 Sep 24 Oct 20 Nov 17 Jan 06Apr 22

115

Changes in the concentrations of the different nitrogen fractions in the water

column during the autumn and early winter provide evidence for recycling within

the water column. Nitrogen is transferred from the dissolved organic fraction to the

dissolved inorganic fraction. Between November 2, 2000 and January 8, 2001, the

concentration of DON decreased by 100 mmol m-2 whereas TOxN increased by 343

mmol m-2. The fact that the latter exceeded the decrease in DON indicates that over

the winter period there was a net increase in the total nitrogen stock within the Irish

Sea. Some of this new nitrogen comes from the sediment through the

remineralisation of organic matter in the sediment (Trimmer et al. (1999) measured a

nitrate efflux rate of 10 μ mol m-2 h-1).

The cycling of silicate does not follow the same pathway as N and P. Silicate is

used in the cell walls of diatoms and silicoflagellates and is not digested by

zooplankton grazing on phytoplankton. As a result much of the particulate silicate

settles to the seabed as phytodetritus (living and dead algal cells) and zooplankton

faecal pellets. In the sediment, silicate is returned to the dissolved form by

dissolution. Estimates of silicate efflux from western Irish Sea sediments (Irish

coastal waters) range from 42 to 123 mol m-2 h-2 (Gowen et al., 2000).

4.4 External Nutrient Sources

Inorganic nutrients in the Irish Sea may come from three external sources: marine

(Atlantic), freshwater (anthropogenic and natural sources) and atmospheric. If as has

been suggested, winter concentrations in near ocean waters at the Celtic Sea shelf

break set background concentrations of nutrients in the Irish Sea, it follows that

differences between Atlantic water and Irish Sea concentrations will reflect internal

nutrient cycling and the influence of anthropogenic sources. Such differences have

been used to quantify the contribution that anthropogenic nutrient sources make to

116

nutrient levels in the Irish Sea (Gowen et al., 2002; Hydes et al., 2004) and assess the

eutrophication status of the Irish Sea (Gowen et al., 2008).

Estimating the Atlantic source term is not a trivial task and some of the

difficulties were discussed by Gowen and Stewart (2005). One of the key unknowns

is the on-shelf movement of oceanic water and volume transport through the Celtic

Sea. Much of the transport might be expected to occur during the winter however, as

noted in section 3, stronger winter winds tends to transport water parallel to the

shelf break rather than onto the shelf and the presence of a shelf break salinity front

in the winter (Hydes et al., 2004) may further restrict movement of ocean water onto

the shelf. Gowen and Stewart (2005) estimated the daily input of dissolved inorganic

nutrients into the Irish Sea to be 540 t TOxN, 78 t DIP and 840 t Si and gave what

they considered to be crude estimates of the annual input as: 82,000 TOxN, 12,000 t

DIP and 127,000 t Si. Most of the freshwater nutrient input to the Irish Sea is via river

inflow. The annual input of dissolved nutrients via the main UK and Irish rivers

flowing into the Celtic and Irish seas is estimates as 150000 t TOxN, 12000 t DIP and

34000 t Si (Gowen et al., 2002). The atmospheric input of DIN is 43000 t (Gillooly et

al., 1992) and the input of atmospheric DIP (in soil dust) is 2000 t. It would therefore

appear that the anthropogenic input of TOxN to the Irish Sea is approximately

equivalent to the natural input from the Atlantic.

A comparison between Atlantic and Irish Sea winter concentrations shows that

the latter is enriched with DIP and Si (Gowen et al., 2002). Evidence for TOxN

enrichment of the western Irish Sea is less clear. The mean January/February shelf

break concentration of 7.15 µM TOxN (Table 4.1) compares with moored water

sampler data (1996 to 2002 [not 1999]) of 7.44 µM (n = 126). March/ early April

concentrations of TOxN are higher (1997 to 2001, 8.57 µM (n = 88)) suggesting

enrichment relative to the January /February shelf break concentration. However,

comparing March/ early April western Irish Sea data with January/ February shelf

break data assumes that the latter represent the winter maximum and this may not

117

be the case if there is a late winter increase in the depth of winter mixing (Hydes et

al., 2004).

Concentrations of dissolved inorganic nutrients are much higher in freshwater

than in marine waters. As a consequence, small volumes of freshwater have a

disproportionately large influence of nutrient concentrations in coastal waters. The

following assessment assumes that the main source of freshwater to the western

Irish Sea is from Irish rivers. For this region, the mean near surface autumn/winter

(September 2001 – March 2002 moored CTD data) salinity was 34.20. This represents

a dilution of 3.7% compared to an oceanic salinity of 35.50. Irish rivers flowing into

the western Irish Sea have DIP and TOxN concentrations of ~3.0 and 220 µM

respectively (PARCOM data source). The silicate concentration is likely to be 83 µM

(Gowen and Stewart, 2005). The contribution of freshwater nutrients to near surface

western Irish Sea water is shown in Table 4.3.

Table 4.3: Estimates of the influence of anthropogenic nutrients on the winter concentration

in the western Irish Sea.

Nutrient Concentration Contribution Western Irish Sea

concentration

Oceanic Riverine Oceanic

(96.3%)

Riverine

(3.7%)

Predicted Measured

DIP 0.45 3.00 0.43 0.02 0.54 0.75

TOxN 7.15 220.00 6.89 8.14 15.03 8.75

Si 2.65 83 2.55 3.07 5.62 6.32

TOxN:DIP 26.7 11.7

TOxN:Si 2.67 1.38

Of the three nutrients, silicate had a predicted concentration that was closest to

the measured concentration (- 12%).The reason for this might be because the

biogeochemical cycling of silicate involves fewer chemical forms and loss terms

compared to TOxN and DIP. For DIP, the measured concentration is higher

implying a loss of DIP from the system or underestimate of the freshwater term.

118

Riverine waters are high in particulate phosphate and for Irish rivers, DIP is only 50-

60% of the total phosphorus load (PARCOM data source). It is likely therefore that

the freshwater supply of DIP is higher than that suggested by DIP alone, although

the final input of DIP will depend on the equilibrium between the particulate and

dissolved forms. The predicted concentration of TOxN for surface waters of the

western Irish Sea is 15.03 µM (85% more than the mean measured value). It would

therefore appear that the supply of TOxN to the western Irish Sea is greater than can

be accounted for by near surface concentrations.

The results of the above assessment can be shown graphically by plotting near

surface nutrient concentrations against salinity and comparing each plot with a

theoretical mixing line between oceanic water (salinity 35.50, 0.47 µM DIP, 7.15 µM

TOxN and 2.65 µM Si) and freshwater (zero salinity, 3.0 µM DIP, 220 µM TOxN and

83 µM Si). For DIP and Si the measured values are close to the theoretical mixing line

but for TOxN measured concentrations are much lower than predicted (Fig. 4.8). The

transport of ‘old’ Atlantic water into the Irish Sea and recycling of N in estuaries

would reduce both source concentrations of TOxN. In addition, sediment

denitrification would remove nitrogen from the system. Denitrification may be the

single most important process by which comparatively low concentrations of TOxN

are maintained in the western Irish Sea. A denitrification rate of 18 mol m-2 h-1

(Trimmer et al., 1999) equates to an annual loss of 2.2 tonnes of nitrogen per km2. If

the area of muddy sediment is 3504 km2, then 7735 t of nitrogen would be lost from

the system which is similar to the annual nitrogen load from the Boyne (PARCOM

data source).

119

Figure 4.8: A comparison between theoretical salinity nutrient mixing relationships (dashed

lines) and measured concentration in the western Irish Sea (Nutrient data were collected

between 1998 and 2002).

4.5 Long-term change

Long-term change in the nutrient status of the Irish Sea was investigated by Gowen

et al. (2002) and Gowen et al. (2008). In both studies the time-series of nutrient data

collected by the Port Erin Marine Station from a location approximately 5 km from

the shore was analysed for long-term trends. In the first study data between 1955

0.0

0.5

1.0

1.5

31.00 32.00 33.00 34.00 35.00 36.00

DIP

(µ

M)

Salinity

0

5

10

15

20

25

31.00 32.00 33.00 34.00 35.00 36.00

TO

xN

(µ

M)

Salinity

0

5

10

15

20

25

31.00 32.00 33.00 34.00 35.00 36.00

Si

(µM

)

Salinity

120

and 1999 (DIP), 1960 and 1999 (nitrate) and 1959 to 1999 (Si) were analysed and

Gowen et al. (2002) concluded that there was as an indication that DIP had declined

since the late 1980s, nitrate had remained stable since the mid 1970s and there was

no long-term trend in Si. In the second study data from the beginning of each time-

series up to 2005 were analysed using the same statistical technique (Mann-Kendal

test for monotonic trends). Based on the longer data set, Gowen et al. (2008)

concluded that the reanalysis confirmed the lack of any trend in nitrate since the mid

1970s and the decrease in DIP since 1989.

4.6 Summary

There is a pronounced and recurrent seasonal cycle of dissolved inorganic nitrogen

(nitrate + nitrite) phosphate and silicate in near surface waters of the western Irish

Sea. Maximum concentrations (1998-2002 mean concentrations: 8.3 µM TOxN, 0.7

µM DIP and 6.6 µM Si) are found in March. This is followed by a rapid removal of

these nutrients from near surface waters and concentrations remain low in the

surface mixed layer (≤ 0.5 μM TOxN, 0.2 DIP µM and 0.7 Si µM) throughout the

summer. By late summer concentrations begin to increase.

Concentrations of all three nutrients in the western Irish Sea are elevated relative

to near ocean concentrations but the level of nitrogen enrichment is constrained by

sediment denitrification. Analysis of the Isle of Man time-series shows that the

winter concentration of DIP has decreased since 1989 and the winter concentration of

TOxN has been stable since the late 1970s.

121

5. Microplankton

5.1 Introduction

Microplankton encompasses all the planktonic unicellular micro-organisms in

fresh and marine waters. Approximately 4,000 species make up the phytoplankton

worldwide (Sournia, 1991) and these microscopic floating plants are responsible for

the bulk of primary production in coastal waters and shelf seas beyond the limits of

macro algal and higher plant (sea grass) growth. The phytoplankton forms the base

of the pelagic food web but in addition to the phototrophic plants, mixotrophic and

heterotrophic organisms play an important role in the cycling of organic matter

through the pelagic component of marine ecosystems. Collectively these micro-

organisms are referred to as the microplankton.

5.2 The seasonal cycle of biomass and production in the western Irish Sea.

Estimates of phytoplankton biomass (as the green pigment chlorophyll

hereafter denoted as Chl) were made during an early study of the western Irish Sea

be Slinn (1974) who reported summer concentrations of approximately 4 mg m-3 in

early June and lower concentrations in July (>1 mg m-3). Richardson et al. (1985)

measured summer (1984) Chl concentrations of between < 2 and > 4 mg m-3 in the

western Irish Sea. However, the first detailed study of the seasonal cycle of

phytoplankton biomass and production was carried out in 1992 by Gowen et al.

(1995) who showed that there was a pronounced seasonal cycle of phytoplankton

biomass in the seasonally stratifying region of the western Irish Sea. Comparing

these early observations with more recent data (Fig. 5.1) shows that the pattern of

phytoplankton growth in this region of the Irish Sea is a recurrent annual event. The

way in which the production season evolves in the water column is illustrated in

Figure 5.2.

122

Figure 5.1: The seasonal cycle of phytoplankton biomass at station 38A in the seasonally

stratifying region of the western Irish Sea. The data were collected between 1992 and 2011.

(DARD/AFBI unpubl. data).

0

2

4

6

8

10

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18

01-Jan 22-Feb 14-Apr 05-Jun 27-Jul 17-Sep 08-Nov 31-Dec

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g m

-3)

Date

1992 1993 1994

1995 1996 1997

1998 1999 2000

2001 2002 2003

2004 2010 2011

2008

123

Figure 5.2: Contour plot of the seasonal chlorophyll cycle (mg m-3) at mooring station 38A in

2008 and 2009. The contour levels are 0.5 mg m-3. (from Scherer Ph.D. thesis 2012)

In the seasonally stratifying region of the western Irish Sea the microplankton

production season begins in April/ May and coincides with the onset of stratification

(Fig. 5.3). The relationship amongst sub-surface irradiance, mixed layer depth and

the timing of the spring bloom in the western Irish Sea was investigated by Gowen

et al. (1995) who found that a threshold mean surface mixed layer irradiance of

between 183 and 245 Wh m-2 (equivalent to between 12.2 and 16.3 W m-2 for a day

length of 15 hours, or 6 to 10% of daily irradiance) was required to trigger the start of

the spring bloom.

7.0 9.0 11.0 13.0

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124

Figure 5.3: The relationship between thermal stratification (temperature in °C) of the water

column and the development of the spring bloom (Chl = chlorophyll in mg m-3) in the

seasonally stratifying region of the western Irish Sea in 2001. (AFBI unpubl. data)

Based on data collected in 1992 and 1993, Gowen and Bloomfield studied the

regional differences in the seasonal cycle of biomass and production in the western

Irish Sea. These workers found what appeared to be a wave of production which

appeared to begin in shallow (20 m) Irish coastal waters and extend first to the

seasonally stratifying region and then to waters of the North Channel (Fig. 5.4).

There were also differences in the duration of the production season which was

longest (6 months) in Irish coastal waters, lasted 4-5 months in the summer stratified

region and only ~3 months in the North Channel. Following the spring bloom,

biomass in the surface mixed layer remains low (Fig. 5.1) although there is often a

sub-surface peak in chlorophyll situated close to the thermocline. In early autumn,

there is a small increase in biomass in some years (Fig. 5.1).

7.0 9.0 11.0 13.0

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35

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45

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125

Figure 5.4: Contour plots of euphotic zone, chlorophyll standing stock (mg m-2) in the

western Irish Sea during 1992. (From Gowen and Bloomfield, 1996)

The differences in the length of the production season gave rise to regional

differences in seasonal production. Gowen and Bloomfield (1996) gave estimates of

194 g C m-2 for seasonal production in Irish coastal waters; 140 and 194 g C m-2 for

the summer stratified region and North Channel respectively. These estimates

compare with only 96 g C m-2 given by Gowen et al (1995) for mixed waters to the

south of the seasonally stratifying region. Scherer and Gowen (2013) combined more

recent (2010 and 2011) data on carbon fixation to that from the earlier studies and

derived an estimate of annual production of 204 g C m-2 y-1 with a range from 157 to

291 g C m-2 y-1. The estimate of 204 g C m-2 for annual gross primary production

126

appears reasonable compared to estimates from the North Sea. Gieskes and Kraay

(1975) gave an estimate of 250 g C m-2 for the central North Sea and estimates of 119

and 199 g C m-2 for the southern and central North Sea respectively were given by

Joint and Pomroy (1993).

5.3 Microplankton species abundance and composition

Changes in phytoplankton cell abundance and biomass (carbon) reflect the seasonal

cycle of chlorophyll biomass (Fig. 5.5 and 5.6), with a spring increase, summer

minimum and in 2009, an autumn bloom.

Figure 5.5: The seasonal changes in microplankton abundance (cells L-1) in the western Irish

Sea during 2009. (From Scherer PhD thesis, 2012)

127

Figure 5.6: The seasonal changes in microplankton biomass (mg C m-3) in the western Irish

Sea during 2009. (From Scherer PhD thesis, 2012)

There are relatively few published studies of microplankton in the western Irish

Sea and most of these have focussed on the spring bloom. Beardall et al. (1978)

reported that the 1977 spring bloom was dominated by diatoms which declined in

abundance during the summer and that micro-flagellates were present in high

numbers throughout the year. McKinney et al. (1997) undertook a detailed study of

diatom abundance at the AFBI mooring site (station 38A) between April and August

1995 and identified a total of 39 diatom species. Skeletonema costatum was the most

abundant spring bloom species although species of Chaetoceros, Pseudonitzschia, and

Thalassiosira formed an important part of the phytoplankton. Gowen et al. (1998)

reported that in 1997 the spring bloom was dominated by microflagellates (≤ 10μm)

and the silicoflagellate (Dictyocha speculum). Analysis by Scherer (2012) gave a total

of 53 diatom species between April and August 2009. The dominant spring bloom

species was Guinardia delicatula. Gowen et al. (2012) compiled data on the 10 most

abundant spring bloom diatom species and found that there was considerable inter-

annual variability in the most abundant spring bloom species (Table 5.1).

In general, dinoflagellates in the western Irish Sea appear to be most abundant

and reach a higher biomass in summer and autumn (Fig. 5.7). The abundance of

128

large dinoflagellate species of Gymnodinium and Gyrodinium, together with

Protoperidinium crassipes and Protoperidinium depressum was highest during summer

and autumn in 2008 and 2009 (Scherer 2012). There is therefore a succession of

species in the western Irish Sea. Scherer (2012) divided the microplankton species

into four functional-taxonomic groups: diatoms, dinoflagellates, microflagellates and

ciliates and found that there was a marked seasonal succession of these four groups

in 2009 (Fig. 5.7). Diatoms dominated the spring bloom but were much less

abundant during the summer. In contrast, dinoflagellates and microflagellate

biomass increased during the summer, both in absolute amounts and as a proportion

of the total microplankton abundance. Ciliates were a minor component of the

microplankton community throughout the year, albeit with a slightly higher biomass

in spring.

Table 5.1: The ten most abundant diatoms (cells mL-1) during the spring bloom (April - May)

in near-surface offshore waters of the western Irish Sea. (From Gowen et al. 2012)

Species 1995 1998 2000 2001 2002 2003 Asterionellopsis glacialis 2.6 - - - - - Cerataulina pelagica 0.7 - 1.2 - 2.2 1.3 Chaetoceros spp. 17.4 2.2 282.9 0.2 250.2 705.0 Cylindrotheca closterium 0.8 0.7 1.1 - - 0.4 Detonula spp. - - 0.9 - - - Ditylum brightwelli - 0.4 - - 2.8 - Eucampia zodiacus - - 0.5 - - 0.4 Guinardia delicatula 0.5 0.2 9.0 0.1 38.5 2.3 Guinardia flaccida - - 2.1 - 0.9 - Guinardia striata - - - - - 0.6 Lauderia annulata 1.3 - - - 0.9 - Leptocylindrus danicus 2.0 0.2 - 0.1 2.4 4.2 Leptocylindrus minimus - - - 0.1 1.2 53.5 Paralia sulcata - 0.1 - - - - Pennate diatoms (small) - 0.3 - 0.1 - - Pseudo-nitzschia spp. 9.8 1.1 0.7 1.1 11.2 16.5 Rhizosolenia setigera - - - 0.2 - - Rhizosolenia styliformis - - 0.4 - - - Skeletonema (costatum) 173 - - 3.0 - - Thalassionema nitzschiodes - 0.2 - 0.1 - - Thalassiosira spp. 4.1 3.0 0.4 4.8 5.9 63.3

129

Figure 5.7: The seasonal succession of the four functional-taxonomic microplankton groups:

diatoms, dinoflagellates, microflagellates, and ciliates. (From Scherer PhD thesis, 2012)

5.4 Long term trends

If as is generally accepted, nutrient availability (especially nitrogen) determines the

level of production during the production season, then an increase in nutrients

might be expected to result in an increase in primary production. From the

preceding section it is evident that there has been a low level of enrichment (~3 μM

N [nitrate + nitrite]) in the western Irish Sea and a small increase in phytoplankton

production might therefore have been expected. This argument is consistent with an

increase in spring (May-June) chlorophyll (~ 2 mg m-3) reported by Allen et al. (1998)

at the Isle of Man time-series station approximately 5 km off the south west coast of

the Isle of Man. For the production season as a whole however, Allen et al. (1998)

found no change in phytoplankton biomass In contrast, Lynam et al. (2010) reported

that a step change increase in a phytoplankton colour index (a proxy for chlorophyll)

took place in 1989. However, re-analysis of the Isle of Man data using data up to

2005 showed no significant trend (Gowen et al., 2008). Furthermore, Gowen et al.

(2008) compared the Isle of Man chlorophyll data collected between 1966 and 1971

130

with more recent data from the western Irish Sea (Fig. 5.8) and concluded that there

had not been any major changes in the seasonal pattern of phytoplankton biomass or

elevation in biomass in the western Irish Sea.

Figure 5.8: A comparison between the seasonal envelope of variability (solid lines) of

chlorophyll (mg m-3) from the Isle of Man time-series data (1966-1971) and more recent (1992

– 2004) data from the western Irish Sea (blue filled circles). (From Gowen et al., 2008)

Scherer and Gowen (2013) used the 1992 and 1993 chlorophyll and primary

production data of Gowen and Bloomfield (1996) to define the seasonal cycle of

biomass and production in the western Irish Sea (Fig. 5.9A and B). These seasonal

envelopes of variability were used as reference envelopes against which more recent

data were compared. On this basis, Scherer and Gowen (2013) concluded that there

was evidence to show that the seasonal cycle and biomass of phytoplankton in the

western Irish Sea has not changed over the last 20 years. The absence of a long-term

trend of increasing nitrate concentration and the decreasing trend in DIP (Gowen et

al., 2008) support the conclusion that there has not been a long-term increase in

phytoplankton biomass in the western Irish Sea.

-1.5

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131

Figure 5.9: The annual cycle of chlorophyll standing stock (mg m-2) (upper graph) and

primary production (mg C m-2 d-1) (lower graph) in the seasonal stratified offshore region

(station 38A) of the western Irish Sea over the last 20 years. (DARD/AFBI unpubl. Data).

There are no time series data on microplankton species abundance or

community structure with which to determine whether there have been any long-

term changes.

0

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2010-11

1997

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5.5 Summary

There is a recurrent seasonal cycle of phytoplankton biomass and production in the

seasonally stratifying region of the western Irish Sea. The production season

typically begins with a spring bloom which coincides with the development of a

shallow (25 m) surface mixed layer and thermal stratification of the water column in

April/May. The production season lasts for up to 5 months; gross seasonal

production is in the range of 101 - 140 g C m-2 and annual gross production is 204 g C

m-2 with a range from 157 to 291 g C m-2. The spring bloom is usually dominated by

diatoms, although in some years the bloom has been dominated by microflagellates

and silicoflagellate. There is a succession of functional groups, from diatoms in the

spring to dinoflagellates and microflagellates in the summer and autumn. There is

evidence to show that there have not been any changes to the onset and duration of

the microplankton production season or the level of phytoplankton biomass in the

stratifying region of the western Irish Sea at least since the early 1990s.

6. Zooplankton

The zooplankton are the animal component of the plankton, some are herbivores,

feeding upon phytoplankton, while others are carnivorous feeding upon other

members of the zooplankton. The zooplankton is comprised of a very wide range of

organisms from planktonic copepods (~0.5 - 1 mm in size) to large zooplankton such

as jellyfish (~ 0.5 m). Some commercial fish species and the Norway lobster Nephrops

norvegicus have planktonic larval stages.

Zooplankton in the western Irish Sea is numerically dominated by copepods

(Scrope-Howe and Jones, 1985) and small neritic species Psuedocalanus elongatus,

Acartia clausi Oithona similis and Temora longicornis tend to be the most abundant

(Gowen et al., 1998). The conspecific species Calanus finmarchicus and Calanus

helgolandicus are generally less abundant than the small species (Gowen et al., 1998).

Studies by Scrope-Howe and Jones (1985), Nichols (1995), and Dickey-Collas et al.

133

(1996) showed that the summer stratified region in the western Irish Sea supports a

higher biomass of copepods than the coastal and mixed waters of the western Irish

Sea. Gowen et al. (1998) observed a seasonal cycle of copepod abundance (Fig. 6.1)

which was closely coupled to that of phytoplankton biomass and production.

Figure 6.1: The seasonal abundance of planktonic copepods (x 103 individuals m-2) in

the western Irish Sea between 1992 and 1996. (From Gowen et al. 1998)

At certain times of the year, fish larvae (Dickey-Collas et al., 1996) and the larvae

of N. Norvegicus (Hill, 2007) are an important component of the zooplankton in the

western Irish Sea. Dickey-Collas et al. (1996) also found that the abundance of

pelagic fish larvae was positively correlated with depth and the stratification

parameter ϕ.

6.2 Long-term change in zooplankton

Kennington and Rowlands (2005) reported CPR data that showed the

averaged copepod abundance (for nearly all species) in the Irish Sea has declined

from the 1970s to the present. Lynam et al. (2010) also reported the decline in

zooplankton. Lynam et al. (2010) further state that due to a rise in temperature since

0

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134

the 1980s the abundance of jellyfish and the importance in their role in the ecosystem

has increased.

7. Energy flow through the food web

In the seasonally stratifying region of the western Irish Sea, the water is too

deep for benthic plants, attached macro-algae (seaweeds) and benthic micro-algae to

grow. Energy flow through the food web is therefore largely derived from

phytoplankton production with transfer to both the pelagic and benthic components

of the ecosystem. However, there have been few studies of energy flow through the

food web in the Irish Sea. The main studies that have been undertaken in the

seasonally stratifying region are those by Gowen et al. (1999), Trimmer et al. (1999;

2003) and Hill (2007).

Gowen et al. (1999) attempted to quantify copepod grazing on the spring

bloom and estimated that over the course of the bloom copepods grazed 22% of

phytoplankton production. Towards the end of the spring bloom copepods grazed

up to 76% of daily production.

The input of phytoplankton derived organic matter to the benthos was

reported by Trimmer et al. (1999) who measured an increase in sediment chlorophyll

and benthic oxygen consumption soon after the peak of the spring bloom. Hill (2007)

also reported a close relationship between the seasonal pattern of chlorophyll in the

euphotic zone and the increase of chlorophyll concentration in the bottom water

(Fig. 7.1) as well as increased chlorophyll in the sediment. Based on measurements of

sediment oxygen demand Trimmer et al (1999) estimated that 41 % of carbon fixed

during the spring bloom settled to the seabed. More recently, Scherer and Gowen

(2013), estimated the annual input of phytoplankton carbon to the benthos in the

seasonally stratifying region of the western Irish Sea to be 17.2 g C m-2 and that the

2011 catch of Norway lobster (Nephrops norvegicus) represented 31% of the

135

phytoplankton carbon which was available for animals at the third trophic level in

the food web.

Further study is required to determine whether there has been any long term

change in energy flow and the pathways by which energy is transferred through the

food web.

Figure 7.1: Chlorophyll concentration in bottom water (~85 m) (closed circles) and

euphotic zone (0 – 23 m) (open circles) at Station 38A in the western Irish Sea from

February 2004 to January 2005. (From Hill PhD thesis, 2007)

8. Assessing the state of the microplankton

8.1 Introduction

In preceding sections data on the physical and chemical oceanographic

characteristics of the western Irish Sea and the microplankton have been presented. .

In this section we have assessed the state of the microplankton by asking a series of

questions using the data presented in the earlier sections to support our answers. For

the assessment the ecohydrodynamic approach (see methods) was used. That is, the

136

microplankton data from the western Irish Sea were analysed in the context of the

structure and functioning of the planktonic component of the pelagic system.

Finally, we used expert judgement to determine whether the status of the

microplankton in the western Irish Sea is representative of good environmental

status.

8.2 Does the western Irish Sea represent a distinct ecohydrodynamic region?

The answer to this question is yes. Within the western Irish Sea, water depth and

weak tidal flows result in the development of seasonal stratification (Fig. 3.9). This is

a recurrent annual event that typically begins in April (although the timing is

variable: ± 3 weeks) and lasts for up to 5 months (Fig. 3.5, 3.8). Stratification results in

a surface mixed layer of ~25 to 30 m (which corresponds to the depth of the euphotic

zone) and isolates bottom water from the surface (Fig. 3.4., 3.9) The onset of

stratification and the establishment of bottom density fronts (Fig. 3.12) sets up a

cyclonic gyre of near surface water which retains seston within the region. The

depositional nature of the seasonally stratifying region allows seston to settle to the

seabed and facilitates close benthic/pelagic coupling.

8.3 Is the seasonal pattern of dissolved inorganic nutrients consistent with

current understanding of biogeochemical cycling in shelf seas?

The answer to this question is yes. It is widely accepted that in most coastal waters, it

is the availability of dissolved inorganic nitrogen (N) as ammonium (NH4+), nitrate

(NO3−), and nitrite (NO

2−) that is most likely to constrain phytoplankton growth

(Ryther and Dunstan, 1971), although diatoms and silicoflagellates can be silicate

limited. Nitrogen limitation is the expected situation in northern European marine

waters, but in some locations, such as parts of the Baltic Sea, phosphorus (P) as

dissolved inorganic phosphate (PO4

3−) is considered to be the limiting nutrient

137

(Andersson et al., 1996). Phosphorus limitation has also been demonstrated for the

eastern Mediterranean (Krom et al., 2004).

There is a recurrent seasonal cycle of TOxN, DIP and Si with winter maxima

(Fig. 4.4, 4.5), a rapid drawdown during the spring and summer minima (Fig. 4.4).

The ratios of winter concentrations are similar to the Redfield ratios of 16:1,

TOxN:DIP and 1:1 for TOxN:Si. The seasonal pattern observed in the western Irish

Sea (Fig.4.4) is consistent with cycle of production and decay observed in temperate

shelf seas: (i) uptake of nutrients by phytoplankton (in ratios close to Redfield) at the

beginning of the production season; (ii) the isolation of bottom water which restricts

the supply of new nutrients to the surface mixed layer during summer (Fig. 4.7); (iii)

the re-establishment of winter concentrations as resupply exceeds uptake by

phytoplankton. The depletion of TOxN before DIP and Si provides prima facia

evidence of N limitation (Fig. 4.8) which is the expected norm in most Northern

European waters outside of the Baltic Sea.

8.4 Is the seasonal cycle of microplankton production and biomass consistent

with current understanding of the processes controlling microplankton

biomass and production in shelf seas?

The answer to this is yes. The seasonal cycle observed in the western Irish Sea (Fig.

5.1) is similar to that observed in many temperate coastal seas and is consistent with

the widely accepted theory that the sub-surface light climate (as a function of the

solar cycle of radiation, mixed layer depth and attenuation) and nutrient supply are

the two main factors that determine the seasonal cycle of microplankton biomass

and production (Gran & Braarud, 1935; Sverdrup, 1953; Pingree et al., 1978;

Smetacek et al., 1990; Tett, 1990).

In winter, wind and tidal stirring generally keep the water column vertically

mixed and this together with short day length and low angle of the sun means that

light limits microplankton growth.

138

As discussed in section 4, dissolved inorganic nutrients accumulate from

different sources during the winter when microplankton growth is minimal. In early

spring, increasing day length and angle of the sun lead to an increase in sub-surface

irradiance. The development of seasonal stratification results in the formation of a

shallow surface mixed layer and when irradiance in this level reaches a critical level,

light ceases to be limiting for microplankton growth (Fig. 5.3). At this point, with

nutrient concentrations in excess (i.e. above limiting concentrations), the production

season begins.

The production season often begins with a burst of growth, the ‘spring bloom

(Marshall and Orr, 1927) and microplankton is most abundant in the euphotic zone

(Fig. 5.2). Production exceeds losses from algal respiration, grazing and sinking and

biomass accumulates. However, this event is short lives and as nutrients become

depleted, growth slows, the loss terms exceed the rate or new biomass production

and biomass decreases (Tamigneaus et al., 1999). During the summer there is limited

resupply of nutrients and production is based on regenerated production (Dugdale

and Goering, 1967). In the post-spring bloom period, nutrients are recycled through

heterotrophic/autotrophic linkages involving the microbial loop (Azam et al., 1981;

Malone et al., 1988; Rivkin et al., 1996; Tamigneaux et al., 1999). In autumn, wind

mixing can cause a deepening of the thermocline and the entrainment of nutrients

into the surface mixed layer. This new nitrogen often triggers an ‘autumn bloom’

(Fig. 5.5, 5.6).

In the western Irish Sea the burst of growth and rapid increase in biomass (the

spring bloom) observed at the beginning of the production season in April/May is a

recurrent event (Fig. 5.1, 5.9). Spring bloom biomass can reach levels of up to 16 and

23 mg chlorophyll m-3 (Gowen and Bloomfield 1996) and this spring increase

coincides with the drawdown in inorganic nutrients. The seasonal production season

lasts for five months in the summer stratified regions of the western Irish Sea and in

some years a late summer/autumn bloom is apparent triggered by wind mixing that

139

provides new nitrogen from deeper layers. By October microplankton growth has

decreased and chlorophyll levels remain low during winter.

8.5 Is the succession of species in the western Irish Sea consistent with what is

expected for a seasonally stratifying temperate shelf sea?

The answer to this is yes. In the seasonally stratifying region of the western Irish Sea

there is a succession of species from diatom such as Skeletonema costatum and

Guinardia delicatula and species of Chaetoceros, Pseudonitzschia spp. and Thalassiosira

spp. which typically dominate the spring bloom (Table 5.1). This is followed by a

period of increased dinoflagellate abundance (Protoperidinium crassipes and

Protoperidinium depressum together with species of Gymnodinium and Gyrodinium)

during the summer. The summer assemblage is replaced in autumn by a second

diatom dominated assemblage although the dominant species (Rhizosolenia spp.,

Eucampia zoodiacus, Paralia sulcata) are different from the spring species.

Microflagellates appear to be abundant throughout the production season the

western Irish Sea (Fig. 5.7).

This seasonal pattern has been widely reported from different coastal and

shelf seas in temperate regions of the world (see review by Smayda, 1980). In the

warm temperate waters of the Mediterranean Sea, Margalef (1963; 1967) identified

four stages of succession, with each stage characterised as follows:

• small, colony forming flagellates and diatoms like Skeletonema and Chaetoceros;

• medium to large sized chains of diatoms (e.g. species of Thalassiosira and

Guinardia and small to medium sized dinoflagellates like Ceratium and

Prorocentrum;

• large, cylindrical celled diatoms like Rhizosolenia and an increasingly larger

dinoflagellate population;

140

• large motile dinoflagellates dominating the biomass and micro-flagellates

representing the highest abundance

Kilham and Kilham (1980) argued that Margalef’s fourth stage of succession

was rarely if ever reached in temperate coastal and estuarine and waters because the

duration of seasonal stratification is too short for the fourth stage to develop. In the

western Irish Sea, stratification lasts for ~5 months and Scherer (2012) concluded that

the late summer early autumn assemblage was consistent with Margalef’s stage 3.

Kilham and Kilham (1980) also pointed out that microflagellates are often

numerically the most abundant group of species. Jones and Gowen (1980) found that

microflagellates were numerically the dominant lifeform in coastal waters around

the British Isles but that abundance was not related to the irradiance and

stratification regime. Microflagellate abundance in the seasonally stratifying region

of the western Irish Sea appears to follow the same pattern. They were generally

abundant and numerically dominant throughout the production season (Scherer,

2012).

The pattern of seasonal succession observed in the seasonally stratifying region is

consistent with the generally accepted theory that variation in the supply of external

energy in the form of light, turbulence and nutrients are the main factors controlling

the seasonal composition and succession of microplankton (Margalef, 1978; Smayda,

1980; Reynolds, 1996; Peperzak et al., 1998; Escaravage et al., 1999; Smayda and

Reynolds, 2001; Tett et al., 2008). Tilman et al. (1982) and see also (Officer and Ryther

1980; Tett et al. 2003) proposed three broad factors influencing microplankton

succession:

physics: utilisation of differences in the capacity of species or lifeforms to

grow in physical environments that differ especially in their vertical mixing

intensity;

nutrient ratios: the relationship between the ratio of nutrient elements needed

for growth and the ambient ratio of these elements;

141

grazing: variable loss rate due to grazing by protozoans or zooplankton that

preferentially take some species or lifeforms rather than others.

8.6 Does the microplankton in the western Irish Sea support higher trophic

levels?

The answer to this question is yes. Studies by Gowen et al. (1998; 1999) show that

during the late 1990s the seasonal peak in copepod abundance occurred in spring

(Fig. 6.1) and was after the spring bloom of phytoplankton.. Grazing by copepods

accounted for up to 76% of daily gross primary production and overall, 22% of gross

spring bloom production.

There is close coupling between the water column and the benthos. Trimmer

et al. (1999) observed the seasonal deposition of pelagic production in the benthos in

the western Irish Sea and found an increase in sediment phytodetritus and a pulsed

increase in benthic oxygen consumption soon after the peak of the spring bloom. Hill

(2007) reported a close relationship between the seasonal pattern of chlorophyll in

the euphotic zone and the increase of chlorophyll concentration in the bottom water

as well as increased chlorophyll in the sediment (Fig. 7.1).

A number of studies show that there is close coupling amongst the seasonal

development of stratification, the seasonal cycle of plankton and the plankton life

history stages of some higher trophic level animals. White et al. (1988) suggested that

the cyclonic gyre of near surface water in the western Irish Sea acts as a retention

mechanism for plankton. Dickey-Collas et al. (1996) observed spatial and temporal

differences in the distribution and abundance of the larvae and 0-group stage of

pelagic fishes. Spawning takes place in shallow inshore waters where there is an

early spring bloom (Gowen et al. 1996) but later in the year fish larvae were more

abundant in the offshore stratifying region where the spring bloom and peak

copepod abundance occur later (Gowen et al. 1996; 1998). Finally, Hill (2007)

142

observed close coupling between the onset of stratification, the timing of the spring

bloom and the appearance of larval Nephrops in near surface waters of the stratifying

region.

8.7 Is the western Irish Sea enriched with anthropogenic nutrients?

The answer to this question is yes. The western Irish Sea is enriched with TOxN,

DIP and Si relative to near ocean waters at the Celtic Sea shelf break. The Si

enrichment is probably natural because rock formations in the UK and Ireland are

rich in silica. However, a comparison between the Isle of Man data from the early

1960s and data collected by DARD and AFBI (2000-2004) shows that there has not

been any change in the seasonal pattern of TOxN and Si cycling in the western Irish

Sea over the last 40 years (Fig. 5.8). The level of TOxN enrichment is low (about 2-3

µM) and the available data indicate that prior to anthropogenic nutrient enrichment,

the western Irish Sea may have been nutrient poor (5-6 µM). Salinity mixing

diagrams suggest that the level of TOxN enrichment should be much higher (~15

µM) given riverine nitrogen loadings (Fig. 5.3) but most of the additional nitrogen is

lost from the western Irish Sea by sediment denitrification.

Analysis of the Isle of Man long-term nutrient data set (IOM) shows that median

winter (January and February) concentrations of TOxN have been stable since the

mid-1970s and that the median winter concentration of DIP has decreased since

1989.

8.8 Has there been a long-term change in phytoplankton phenology and

biomass?

The answer to the first part of the question is no. There is evidence of inter-

annual variation in the timing of the spring bloom (Fig. 4.6) but a comparison

between data from the late 1960s (Isle of Man time-series) and data collected by

DARD/AFBI between 1992 and 2004 (Fig. 5.8) and between DARD/AFBI data

143

collected between 1992-1993 and between 1994 and 20 (Fig. 5.9), shows that there has

not been a long-term change in the seasonal pattern of phytoplankton biomass.

The answer to the second part of the question is more equivocal. Since the

western Irish Sea exhibits a low level of enrichment, if nutrient availability

(especially nitrogen) determines the level of production during the production

season then a small increase in production is likely to have occurred. This argument

is consistent with the significant increase in spring chlorophyll reported by Allen et

al. (1998). The more recent study by Lynam et al. (2010) identified an increase (step

change) in the CPR phytoplankton colour index which occurred in 1998. However, a

comparison between Isle of Man chlorophyll data collected between 1966 and 1971

and more recent data from the stratifying region of the western Irish Sea does not

show this increase (Fig. 5.8).

8.9 Does the state of the microplankton in the western Irish Sea represent good

environmental status (GES)?

At the present time it is not possible to provide a definitive answer to this

question but we conclude that the state of the microplankton in the western Irish Sea

is representative of GES. As noted above, until there is a better understanding of

what represents GES and how it can be determined objectively for the plankton, it is

necessary to use expert judgement to determine whether the state of the plankton

was representative of good environmental status. We have attempted to interpret the

microplankton data in the context of the ecohydrodynamic conditions in the western

Irish Sea: conditions to which the microplankters might be expected to be adapted.

In our opinion any anthropogenic perturbation of the microplankton in the western

Irish Sea has been minimal and the data presented in this report are consistent with

current scientific understanding of microplankton dynamics (seasonal patterns of

biomass, production and species) and the factors that control these dynamics in

temperate coastal waters and shelf seas.

144

9. Establishing reference conditions

9.1 Introduction

Having concluded that the microplankton in the seasonally stratifying region of the

western Irish Sea is representative of GES, the next step is to use the data to establish

reference conditions against which future change can be quantified in the western

Irish Sea and which can be used as reference conditions for other seasonally

stratifying regions in UK waters.

The plankton experiences an inherently variable environment largely as a result

of seasonal and day-to-day variability in weather and water movement. This is

particularly true in UK seas, which are for the most part tidally energetic and subject

to fluctuating weather conditions as well as seasonal weather patterns. As a

consequence, the plankton exhibits variability on a range of spatial and temporal

scales and the assemblage of species and populations of individual species are not

fixed in time and space but are dynamic. Overlaying this variability there are higher-

order constancies: the recurrent annual cycle of phytoplankton growth in coastal

waters (Tett and Wallis 1978; Smayda, 1998; Gowen et al., 2008); the succession of

lifeforms in seasonally stratifying coastal seas (Margalef, 1978). Despite these higher

order consistencies, detecting changes in planktonic communities due to human

pressures or climate is not easy. Any method must be capable of quantifying the

natural dynamic variability of plankton populations and take account of the seasonal

succession of some species.

One obvious approach to characterising the microplankton is simply to list the

abundances of all the species present. However, O’Neill (2001) argued that defining

ecosystems using species lists was inherently problematic and there is an obvious

practical difficulty in that any such list might comprise hundreds of species. It is

doubtful therefore that simple lists of species or thresholds of abundance of

individual species would adequately discriminate between natural variability and

145

human pressure driven change. In one sense the problem with species lists and data

on abundance is that there are often too many data.

There does not seem to be any single species of plankton that can be used as a

universal indicator of the condition of the plankton. There are several reasons for

this. First, no single species of pelagic animal or plant has a controlling effect on the

plankton as a whole. Second, the spatial heterogeneity of the plankton community

means that the species important in one region, or under one set of hydrodynamic

conditions, may be rare in another region. The third reason is that putative

relationships between particular organisms and specific pressures have often been

based on limited or over-interpreted evidence (e.g. the use of harmful species of

phytoplankton as indicators of eutrophication Gowen et al. 2012).

Gowen et al. (2011) reviewed methods of detecting change in the plankton.

Biodiversity indices (Shannon, 1948) or multivariate statistics (Edwards, 2005) have

proven to be powerful tools in analysing microplankton data. Devlin et al. (2007)

proposed a phytoplankton index (IE) to classify and assess the UK marine waters

under the requirements of the WFD. An alternative method based on the use of

plankton lifeforms (assigning groups of species to plankton lifeforms summarises

large amounts of information on plankton species without losing information on

seasonal fluctuations in species abundance) and system state space theory was

introduced by Tett et al. (2008). Of several methods available, Gowen et al. (2011)

recommended the latter to Defra as the most useful for detecting long-term changes

in the status of the plankton in UK waters for the purposes of the MSFD.

9.2 The lifeform state space approach

The plankton community index (PIp) approach is based on the phytoplankton

community index (PCI) introduced by Tett et al. (2007; 2008). A detailed description

is also given by Gowen et al (2011). The approach builds on the idea of defining an

ecosystem state space in terms of values of state variables, in this case plankton

146

lifeforms (see Margalef, 1978) and mapping the abundance of lifeforms into a

multidimensional “state variable space”. The main features of the method are: the

grouping of species of planktonic organisms into lifeforms; the display of changes in

the abundance of each of these lifeforms using a state-space approach (Fig. 9.1);

calculating an index (PI) to quantify possible changes in the state of the plankton

relative to baseline or starting conditions (Fig. 9.1).

Figure 9.1: Mapping the abundance of lifeforms in state space and calculating an index to

quantify changes in the state of the plankton relative to a baseline. Note the term reference

envelope does not imply pristine conditions or that the plankton community is

representative of GES.

The choice of lifeforms is clearly important. In some cases a lifeform can be

based on biogeochemical or ecological function and can include organisms from

different taxa. Sieburth et al. (1978) proposed that the best way to place species into

useful groups was to ignore taxonomic hierarchies and to separate species into

groups based on the level of organisation and the mode of nutrition. Ryther &

Officer (1981) listed seven phytoplankter types which they ranked from the most

147

beneficial (centric diatoms) to the most undesirable (bluegreen algae or

cyanobacteria). Smetacek (1986) separated tychopelagic diatoms for the large heavily

silicified centric diatoms of shallow turbulent waters which are equally capable of

living on the sea bed. Riegman (1998) distinguished large diatoms, small diatoms,

haptophyta, dinoflagellates, mixotrophic algae and cyanobacteria by

ecophysiological properties shared with other members of the same taxonomic

group. Lee et al. (2003) distinguished microplankton on their silicate requirement i.e.

silicate users and non-silicate users.

Other developments in lifeform theory were reviewed by Tett & Wilson (2003)

based on function and taxonomy. Tett & Wilson (2003) distinguished groups of

factors that could identify and distinguish lifeforms in relation to ecosystem

sustainability. There are five examples:

• Their functionality in relation to biogeochemical cycling of bio-limiting

elements like C, N, P, Si, S, O and perhaps Fe and Co. There are two levels

that could explain variations in microplankton composition here.

• The first one is qualitative, and concerns the distinctions between algae that

require silica and those that do not. The second level is quantitative, and

concerns the idea of optimum ratios of nutrient elements required for growth

which may differ amongst lifeforms.

• The functionality of organisms in relation to the marine foodweb. Distinction

here was made between prey as primary producers (e.g. diatoms) and

predators (e.g. ciliates, some dinoflagellates and flagellates).

• The relationship to the physical environment (e.g. turbulence, velocity, light)

as considered by Margalef (1978).

• Taxonomy with differences between for example, organisms possessing thick

silical cell walls (e.g. diatoms) or cellulose theca (e.g. armoured

dinoflagellates) and those that lack these (e.g. naked dinoflagellates,

microflagellates).

The development of the state space plots is illustrated in Fig. 9.2. The first step

involves mapping each pair of data points for lifeform 1 (LF1) and lifeform 2 (LF2)

148

into the state space plot. The plot can be considered as a map created by co-ordinates

as LF1 and LF2 are independent from each other. An elliptical shape like a

`doughnut' appears due to the natural succession of lifeforms (see Fig. 9.2).

A geometric method known as Convex Hull (Sunday, 2004; Weisstein, 2006) is

applied to the cloud of the data points with a certain data exclusion (here 90% of the

data were considered), drawing an outer envelope. According to Tett and Mills

(2009) limitation theory suggests that the bundle of microplankton data points

should have a hollow centre. To create this hollow centre, an inner envelope is

established by applying the Convex Hull method to the centre points turning them

inside-out and once the envelope is also drawn around them, they are re-inverted

again. The procedure is illustrated in Fig. 9.2.

9.3 Assigning species to lifeforms

The selection of lifeform pairs relevant to the planktonic component of MSFD

descriptors D1 (Biological diversity), D4 (Food web), D5 (Eutrophication), and D6

(Sea floor integrity) was discussed by Gowen et al. (2011) and lifeform combinations

for each of the above descriptors were presented by Gowen et al. (2013) and are

shown in Table 9.1.

At present there are insufficient data from the seasonally stratifying region of

the western Irish Sea to prepare reference envelopes for all of the lifeform pairs in

Table 9.1. However, there are sufficient data to allow reference envelops to be

created for the following:

Biological diversity: Diatoms and dinoflagellates were chosen as lifeforms

because they are evolutionary distinct groups with different attributes and

general biology.

Food webs: Large (<20µm) and small (>20µm) phytoplankton were chosen

because the size of different phytoplankters reflect different pathways of

energy flow through the food web. Eutrophication: Three reference

envelopes have been created for this descriptor.

o Diatoms and auto/mixotrophic dinoflagellates: a shift in community

composition could appear towards potentially harmful groups.

149

o Microflagellates and ciliates: indicative for a shift from an autotrophic to

a more heterotrophic system.

o Pseudo-nitzschia spp. and potentially toxin producing dinoflagellates:

indicative of a shift in algal community towards harmful dinoflagellates.

Sea floor integrity: Pelagic and tychopelagic diatoms were chosen because

they can be indicative for seabed disturbance.

150

Figure 9.2: An illustration of the development of the state variable space plot in three steps

fitting an outer and inner envelope around the data points of the state variables (lifeform 1

and 2).

151

Table 9.1 A revised set of lifeform pairs for the MSFD Biodiversity, Food web, Eutrophication and Seabed integrity descriptors

Descriptor Lifeform pair 1 Lifeform pair 2 Lifeform pair 3

D1: Biodiversity Diatoms Dinoflagellates Gelatinous

zooplankton

Fish larvae Holoplanktonic

crustaceans

Non gelatinous and non

crustacean holoplankton

Lifeform

feature(s)

All diatoms All dinoflagellates Ctenophores &

Coelenterates

Excluding fish eggs Excluding eggs

Reasoning: Evolutionary distinct groups Indicators of alternative ecosystem states Evolutionary distinct groupings that capture all holoplankton

not included in Lifeform pair 2

Pressure(s): Nutrient enrichment; change in

hydrographic conditions

Fishing Fishing; Nutrient enrichment

D4:

Food-webs

Phytoplankton

Zooplankton Large

phytoplankton

Small phytoplankton Large copepods Small copepods

Lifeform

feature(s)

Chlorophyll (mg

m-3)

Abundance

(m-3)

> 20 μm < 19.9 μm > 2 mm <1.9mm

Reasoning: Energy flow Energy transfer from primary to

secondary producers

Benthic-pelagic coupling

Pressure(s): Fishing Fishing Fishing; Nutrient enrichment

D5: Eutrophication Diatoms Dinoflagellates Ciliates Microflagellates Pseudo-nitzschia

spp.

Toxin producing dinoflagellates

Lifeform

feature(s)

All diatoms Autotrophs and

mixotrophs

Including

tintinids

All species < 20 μm Excluding P.

delicatissima

All species on the Food Standards

Agency list (Table 4)

Reasoning: Shift in community composition

towards harmful groups

Shift from autotrophic to heterotrophic

system

Shift in algal community towards dinoflagellate HABs

Pressure(s): Nutrient enrichment Nutrient enrichment Nutrient enrichment

D6:

Sea floor integrity

Holoplankton Meroplankton Pelagic diatoms tychopelagic

diatoms

Lifeform

Feature(s)

Excluding fish larvae All species

Reasoning: Benthic-pelagic coupling Seabed disturbance

D1.7: Biodiversity Ecosystem Structure All lifeform pair combinations.

Reasoning: Changes in these lifeforms provide a comprehensive overview of the structure and functioning of the planktonic component of marine ecosystems.

Pressure(s): Fishing; nutrient enrichment; aquaculture, industrial spills (e.g. oil, contaminants); river damming; seabed disturbance (inc. contaminant re-suspension);

renewable energy; warm water outflows; ocean acidification

152

The reference envelopes for the seasonally stratifying region of the western Irish Sea

are presented in Fig. 9.3, 9.4, 9.5, and 9.6.

9.3.1 Biodiversity descriptor (D1)

Lifeform pair: diatoms and dinoflagellates

1 2 3 4 5 6 7

1

2

3

4

5

6


p: 0.90points: 153



log10


log

10(d

ino

flag

ell

ate

ab

un

dan

ce/L

)

1 2 3 4 5 6 7

1

2

3

4

5

6


log10



binom p: 0.0022chi-sq: 13.5 (df=1)

153

9.3.2 Food web descriptor (D4)

Lifeform pair: large (<20µm) and small (>20µm) phytoplankton.

1 2 3 4 5 6 7 8

1

2

3

4

5

6

7


p: 0.90points: 153



log10

(small phytoplankton (>20mu)/L)

log

10(l

arg

e p

hy

top

lan

kto

n (

<2

0m

u)/

L)

1 2 3 4 5 6 7 8

1

2

3

4

5

6

7


log10

(small phytoplankton (>20mu)/L)


binom p: 0.5990chi-sq: 0.0 (df=1)

154

9.3.3 Eutrophication descriptor (D5)

Lifeform pairs: i) Diatoms and auto/mixotrophic dinoflagellates; ii) microflagellates and ciliates;

iii) Pseudo-nitzschia spp. and potentially toxin producing dinoflagellates

1 2 3 4 5 6 7

1

2

3

4

5

6


p: 0.90points: 153



log10


log

10(a

uto

/mix

otr

op

hic

din

ofl

ag

ell

ate

ad

un

dan

ce/L

)

1 2 3 4 5 6 7

1

2

3

4

5

6


log10



binom p: 0.7079chi-sq: 0.2 (df=1)

1 2 3 4 5 6 7

1

2

3

4

5

6


p: 0.90points: 149



log10

(microflagellate abundance/L)

log

10(c

ilia

te a

bu

nd

an

ce/L

)

1 2 3 4 5 6 7

1

2

3

4

5

6


log10

(microflagellate abundance/L)


binom p: 0.5542chi-sq: 0.0 (df=1)

1 2 3 4 5 6 7

1

2

3

4

5

6


p: 0.90points: 87



log10

(Pseudo-nitzschia abundance/L)

log

10(t

ox

in p

rod

ucin

g d

ino

flag

ell

ate

ab

un

dan

ce/L

)

1 2 3 4 5 6 7

1

2

3

4

5

6


log10

(Pseudo-nitzschia abundance/L)


binom p: 0.7729chi-sq: 0.3 (df=1)

155

9.3.4 Sea floor integrity descriptor (D6)

Lifeform pair: Pelagic and tychopelagic diatoms

1 2 3 4 5 6 7

1

2

3

4

5

6


p: 0.90points: 117



log10

(pelagic diatom abundance/L)

log

10(t

ych

op

ela

gic

dia

tom

ab

un

dan

ce/L

)

1 2 3 4 5 6 7

1

2

3

4

5

6


log10

(pelagic diatom abundance/L)


binom p: 0.8355chi-sq: 0.6 (df=1)

156

10. Conclusions

The state of the microplankton in the seasonally stratifying region of the

western Irish Sea was assessed using an ecohydrodynamic region approach.

The physical and chemical oceanographic data show that the deeper offshore

region of the western Irish Sea is a distinct ecohydrodynamic region

(seasonally stratifying) which is subjected to a low level of anthropogenic

nutrient enrichment.

The start and duration of the microplankton production season is determined

by the sub-surface light climate and the level of seasonal production is

controlled by nutrient availability. The species that make up the

microplankton community are typical for a seasonally stratifying water body

and succession from diatoms and microflagellates in the spring to

dinoflagellates in the summer to diatoms in autumn is a recurrent event in

seasonally stratified regions in temperate shelf seas. We conclude that the

condition (state) of the microplankton in the western Irish Sea is

representative of GES. There has not been any obvious influence of top down

or bottom up pressure driven change in microplankton structure over the last

20 years.

Lifeform-state space reference envelopes have been created for the planktonic

component of MSFD descriptors: Biodiversity (diatoms and dinoflagellates),

Food webs (large (<20µm) and small (>20µm) phytoplankton), Eutrophication

(diatoms and auto/mixotrophic dinoflagellates; microflagellates and ciliates;

Pseudo-nitzschia spp. and potentially toxin producing dinoflagellates), and Sea

floor integrity (pelagic and tychopelagic diatoms).

These reference envelopes can be used to track future change in the

microplankton in the seasonally stratifying region of the western Irish Sea and

157

to provide reference conditions for other seasonally stratifying regions in UK

coastal and shelf seas.

158

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The status of the plankton community in Scottish waters: a preliminary assessment of the Stonehaven and Loch Ewe sentinel monitoring sites. Eileen Bresnan, Kathryn Cook, Sarah Hughes and Pam Walsham Introduction There is a pressing requirement to understand the dynamics of the plankton community to provide advice on the impacts of climate change, ocean acidification and eutrophication on the marine ecosystem as well as to meet the requirements of EU water quality directives such as the EU Water Framework Directive (WFD) and Marine Strategy Framework Directive (MSFD). The MSFD requires member states to assess if the plankton community in their regional waters are achieving ‘Good environmental status ‘ (GES) by 2020. To assist with meeting the requirements of this directive, UK waters are divided into 8 different ecohydrodynamic regions. This report provides a preliminary assessment of the plankton communities at two sentinel monitoring sites within UK waters; Stonehaven on the east coast of Scotland (Region 1) and Loch Ewe on the west coast of Scotland (Region 6). The summary presented here is based on Bresnan et al., (submitted). The coastal waters on the east and west coasts of Scotland are influenced by different oceanic regimes. On the west coast, a mix of coastal and Atlantic water (Innall et al., 2009) flows northward along the coast. The coastal waters eventually pass around the northern part of the mainland and enter the North Sea via the Fair Isle channel. In addition waters of a more Atlantic origin enter the North Sea to the east of Shetland (Turrell et al., 1996). The Scottish west coast has a complex coastline dotted with fjordic sea lochs and islands. The main source of nutrient input into the area comes from the Atlantic. The nutrient concentration of the freshwater inflow into many of the sea lochs is less than that of the coastal water flowing by (Smith et al., 2014). Intrusion of Atlantic water onto the shelf will increase the availability of nutrients in coastal region as these waters have higher nutrient concentrations (Hydes et al, 2004). The Clyde Sea area in the south west of Scotland is distinct and a review of its ecology is presented in McIntyre et al., (2012). In contrast to the west coast, the east coast of Scotland has a relatively smooth coastline with major river input from the Tay near Dundee and the Forth near Edinburgh. Since 1997 temperature miniloggers have been deployed by the Scottish Government in target areas around the Scottish coast, to complement existing long term temperature time-series (Hughes, 2007). These temperature data are valuable indicators of variability in the Scottish coastal region and have contributed to a number of ocean climate status reports (UK Marine Monitoring and Strategy 2010b, Dye et al., 2013). There has been a poor history of sustained plankton observations in Scottish waters. Loch Creran, on the west coast, has been extensively studied in the 1970s and more recently (Tett and Wallis 1978, Whyte 2012, Tett 2013). A series of mesocosm experiments were performed in Loch Ewe in the 1970s and 1980s (Gamble et al, 1977, Morris et al., 1983, Morris 1984) and a number of once off studies, describing the phytoplankton communities along the west coast were performed during this period and the early 1990s (Gowen et al., 1983, Jones et al., 1984, Joint et al., 1987, Savidge and Lennon, 1987). A transect, ‘the Ellet line’, has been in operation from Oban in the West coast of Scotland to Rockall since 1975 (Fehling et al., 2012), however there have only been two descriptions of the

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phytoplankton community from this area (Savidge and Lennon, 1987 and Fehling et al., 2012). Fehling et al. (2012) highlight mixing of water flows and associated differences in phytoplankton communities. Since the 1990s most phytoplankton studies in Scotland have focused on harmful species (Fehling et al., 2004, Fehling et al., 2005, Fehling et al., 2006, Bresnan et al., 2008, Brown and Bresnan, 2008, Collins et al., 2009, Davidson et al., 2009, Brown et al., 2010, Gowen et al., 2012). Historic zooplankton studies have focused on the North Sea (e.g. Hay et al., 1991), demography and production of individual copepod species (e.g. Cook et al., 2007, Heath et al., 1999, Invarsdòttir et al., 1999, Heath et al., 2000, Hill, 2009, Drif et al., 2010), effects of stressors such as ocean warming, ocean acidification (Mayor et al., 2009, 2012) and algal toxins (Cook et al., submitted). Until 2002 there were no sustained phyto and zooplankton time-series in operation simultaneously at coastal monitoring sites in Scottish waters. In the North East Atlantic, information about the plankton community over the last five decades comes from the Continuous Plankton Recorder (CPR) which has identified a number of signification changes in the 1970s, 1980s and 1990s (Edwards et al., 2002, Beaugrand 2003, Alvarez et al., 2012, Hinder et al., 2012). Some of these changes include an increase in phytoplankton biomass throughout the growing period, a decrease in dinoflagellates and increase in diatoms and a switch from Calanus finmarchicus to Calanus helgolandicus (Edwards et al., 2002, Beaugrand 2003, Hinder et al., 2012). Fixed point monitoring stations at L4 offshore from Plymouth (Harris, 2010) and Helgoland in the south east North Sea (Wiltshire et al., 2010) are also describing the variability and changes in the channel and southern North Sea plankton communities (Wiltshire et al., 2008, Widdecombe et al., 2010, Wiltshire et al., 2010, Loder et al., 2012, Shulter et al., 2012). CPR coverage is sparse in the waters to the west of Scotland, an area of particular importance for both fishing and aquaculture (Baxter et al., 2011). The requirement to maintain time-series, particularly with biological parameters, to meet scientific and policy requirements continues to be highlighted (Edwards et al., 2010, UK Monitoring and Marine Strategy, 2010a, Baxter et al., 2011, Koslow and Couture, 2013). Marine Scotland Science operates a coastal ecosystem monitoring programme around the Scottish coast to generate the baseline information to describe the physical and chemical environment as well as the plankton community. Within this programme, two monitoring stations (Stonehaven on the east coast and Loch Ewe on the west) are sampled weekly for temperature, salinity, nutrients, phytoplankton and zooplankton in order to describe baseline ecological conditions. Both of these sites have been selected by the Pelagic Descriptor Working Group for the MSFD to act as sentinel fixed point monitoring sites for the MSFD. Monitoring began in 1997 at Stonehaven and in 2003 at Loch Ewe. A preliminary assessment of the environmental status of these sites is presented here. Materials and Methods Sampling sites The Marine Scotland Coastal Ecosystem Monitoring Programme comprises a number of fixed point sampling stations located around the Scottish coast. Data from the two stations which collect phyto and zooplankton data in addition to temperature, salinity and nutrients (Stonehaven and Loch Ewe) are presented in this study (Figure 1).

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The Stonehaven monitoring site is represented by a single sampling location (56º 57.8´ N, 02 º 06.2´ W) located 5km offshore at a water depth of 48m, which has been visited weekly (weather permitting) since January 1997. Strong tidal currents mean that thermal stratification of the water column is weak and usually confined to neap tides during the summer months. Further information about the Stonehaven monitoring site can be found in Bresnan et al. (2009) and Heath et al., (1999b). The Stonehaven monitoring site is located in CP2 ecohydrodynamic region 1. The Loch Ewe monitoring site ( 57° 50.14' N, 5° 36.61' W) is 40 m in depth and located at the northerly face of a sea loch on the west coast of Scotland. Sampling is performed weekly – weather permitting. Some stratification is observed during the summer months, although a marked seasonal cycle in temperature still exists in the lower layers. Loch Ewe is one of the larger sea lochs on the west coast, ranked 9th of 110 by volume (Edwards and Sharples, 1986). Although sea-lochs in general are classified as regions of restricted exchange (Tett et al., 2003), Loch Ewe is not a typical long narrow sea loch. It is 12 km long and the entrance at the first sill is approximately 2.4km wide, giving it one of the smallest aspect ratios (length/width) of the Scottish sea lochs (ranked 102nd of 110). Over the whole loch, the flushing rate is estimated to be around 4 days and the salinity reduction as a result of freshwater input about 0.3. Although the sampling site is within the sill of the loch, the sill depth is 33m making the sampling site open to the coastal waters of the North Minch. More information about the Loch Ewe monitoring site can be found in O’Brien et al. (2013). Loch Ewe is located in CP2 ecohydrodynamic region 6.

Figure 1: Location of Loch Ewe ( ) and Stonehaven () monitoring sites.

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Temperature Niskin bottles fitted with digital reversing thermometers were deployed to surface and 45m depths to collect water for salinity and nutrient analysis at Stonehaven, and surface and 30/35 m depths at Loch Ewe on a weekly basis since 1997 at Stonehaven and 2002 in Loch Ewe. The temperature on the thermometers was recorded from each depth when they returned to the surface. Continuous records of sea surface temperature were obtained at each site using Vemco Minilogger temperature recorders. These small instruments were deployed in the near surface layer (approx. 5 m deep) attached to a fixed platform, and set to record at 20 min intervals, with an accuracy of 0.1°C. The minilogger at Loch Ewe, was attached to a mooring sited close to the monitoring station. At Stonehaven, the minilogger was sited at a coastal site (Findon), 54 km away from the sampling site. Calculation of the annual cycle from minilogger data was undertaken using a common time period (2003-2012). For the period 2010-2012, the minilogger at Findon was not operational and weekly surface (5m) temperature data collected using a digital thermometer from the Stonehaven site was substituted. Correlation analysis between weekly surface temperatures at the sampling site and the weekly subsampled data at Findon was significant (r=0.67, p<0.01), showing that the coastal site captures the same variability as the Stonehaven monitoring site further offshore and that the weekly sampling data are a suitable substitute for the periods when the minilogger at Findon was not operational. For comparison of surface and bottom conditions between sites, temperature data from the weekly bottle samples was used. However to examine in more detail the seasonal cycles and variability between sites, daily averaged data was calculated from the high resolution minilogger data. For analysis of broader regional patterns of sea surface temperature, seasonal averages (winter is Dec-Feb, spring is Mar-May, summer is Jun-Aug and autumn is Sep-Nov) were prepared using a climatological dataset, averaged at a resolution of 1/6 degree longitude and 1/10 degree latitude for the period 1971-2000 (Berx and Hughes, 2009). This data is freely available from the International Council for the Exploration of the Sea (ICES) website (http://ocean.ices.dk/Project/OCNWES). Data from 2003 – 2012 is presented from both sites. Salinity and nutrients Water samples for nutrients were collected at depths (1 m and 45 m at Stonehaven; 1 m and 35 m at Loch Ewe) using a Niskin Sampling bottle, which was also fitted with a digital reversing thermometer. The surface nutrient data only is presented here. Salinity samples were taken from surface and near bed depths sampled. Salinity samples were collected in glass bottles that are dried and sealed with a cap to prevent formation of salt crystals. The samples were analysed using a Guildline Portasal Salinometer Model 8410A. Before each analysis session, the salinometer was standardised using IAPSO standard seawater, this reference point is also checked after every crate (24 bottles). Following these procedures the analysis has an accuracy greater than 0.03. Data from 2003 – 2012 is presented from both sites. Water samples were analysed for total oxidised nitrogen (TOxN; nitrate and nitrite), phosphate and dissolved inorganic silicate (DSi). TOxN and DSi data are presented here. Samples for TOxN were stored in glass bottles at -20 oC and allowed to thaw for 24 hours before analysis. Samples for DSi analysis were stored in plastic bottles and either stored in a refrigerator maintained between 0 and 8 oC (1997 -2010) or at -20 oC (2011 onwards). Refrigerated samples were allowed to come to room

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temperature before analysis while frozen samples were thawed in the dark for 48 hours to allow for depolymerisation. Nutrients were measured using a continuous flow autoanalyser (CFA). From 2003 – 2006 a Bran and Luebbe Analyser (AA3) and from 2006 a Bran and Luebbe QuAAtro (SEAL Analytical, UK) was used. Nutrient analysis on the CFA is accredited to ISO 17025, details of the methodology can be found in Webster et al. (2007) and Smith et al. (2014). Analytical quality was assured through successful participation in the nutrient programme of QUASIMEME (Quality Assurance of Information for Marine Environmental Monitoring in Europe) and by running reference standards with each batch. Prior to 2010, QUASIMEME sea water samples were used as reference materials, whilst from 2010 onwards reference standards were prepared from standards procured from OSIL (Havant, Hampshire, UK). In both cases, measured concentrations were plotted on a Shewhart chart with warning and action limits at ±2 and ±3 standard deviations from the mean. Chlorophyll ‘a’’ Water for chlorophyll ‘a’ and phytoplankton community analysis was sampled using a 10 m Lund tube. For chlorophyll analysis the water sample was kept cool and dark and filtered on return to the laboratory (approx. 90 min later). 1 L aliquots were filtered onto GF/F glass fibre filters and stored at -80 o C until analysis. Samples were extracted in buffered acetone for 24 h. The fluorescence of the sample extract was measured on a 10-AU or TD-700 fluorometer (Turner Instruments, USA). The sample extract was acidified to convert chlorophyll ‘a’ to phaeophytin ‘a’. The acidified extract was mixed by inversion and the fluorescence was re-measured. The concentration of uncorrected chlorophyll ‘a’, corrected chlorophyll ‘a’ and phaeophytin ‘a’ in the sample extract was then calculated using fluorometric equations (Arar and Collins, 1992). The analysis is accredited to ISO 17025. Analytical quality was assured through successful participation in the chlorophyll programme of QUASIMEME and by running reference materials within each batch. Data from 2003 – 2012 is presented from both sites. Phytoplankton analysis A one litre subsample of the Lund tube water was preserved immediately with 0.5% acidic Lugol’s iodine (Throndsen, 1978) for phytoplankton community analysis on return to the laboratory. A 50 mL subsample was settled for 48h prior to analysis using a modified Ű termohl technique. Samples were analysed using an inverted microscope (Zeiss Axiovert 10, 100 or 200) at X200 magnification. An initial scan of the chamber was performed prior to analysis to ensure homogeneity of settling. To record the diversity of the sample, all phytoplankton species present were recorded and target species such as shellfish toxin producing dinoflagellates also enumerated. To enumerate the rest of the phytoplankton community, fields of view along a random transect in the sample chamber were counted. Sample log in and analysis of harmful species is accredited to ISO 17025. All phytoplankton analysts participate in BEQUALM interanalyst phytoplankton ring trails annually. Data from 1999 – 2013 is presented from Stonehaven and 2003 – 2013 for Loch Ewe. Zooplankton Zooplankton samples were collected by vertical 40cm diameter bongo (200 µm mesh) net hauls from 45m (Stonehaven) or 35m (Loch Ewe) to surface and were preserved in 4% buffered formaldehyde for community analysis in the laboratory under a Zeiss Stemi-11 stereomicroscope. Larger zooplankton categories (such as Calanus spp., chaetognaths, jellyfish, euphausiids etc.) were identified and

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enumerated from the whole sample. The remaining zooplankton categories were identified and enumerated from a series of subsamples (of variable volume depending on concentration of animals but minimum 2.5% of whole sample) so that at least 100 animals were recorded. All copepods were identified to the lowest taxonomic level possible, whilst others animals were grouped into categories. Zooplankton counts was converted to abundance using filtered volume estimated from vertical distance towed, net mouth area and 70% filtration efficiency. Zooplankton abundance was converted to biomass using historical dry weight data measured from the North Sea and North Atlantic (Hay et al., 1988, Hay et al., 1991, Marine Scotland Science, unpublished data). If there was no dry weight data for a particular species or stage, the dry weight of another similar animal was used (e.g. values for Eucalanus elongatus were used for Subeucalanus crassus). Cnidarians, ctenophores, hyperiid amphipods, chaetognaths, fish larvae, Tomopteris spp. and copepods of the genera Euchaeta and Candacia were classified as carnivorous zooplankton. All other animals were considered grazers. Zooplankton quality assurance follows the MSS joint code of practice and analysts participate in external identification ring trials as they arise. Data from 1999 – 2013 is presented from Stonehaven and 2003 – 2013 for Loch Ewe. Data quality All data within the Marine Scotland Science coastal ecosystem monitoring system operates within the data quality flag system described by Seadatanet (2010). Each point is reviewed and a quality flag (QF) value assigned based on the quality of the data. Data points which were given a QF value of 3 (probably bad) or 4 (bad) were not included in this assessment. Results Temperature and Salinity The average sea surface temperatures in Scottish waters are shown in Figure 2, and reveal strong spatial differences between east and west coast, particularly during the winter and spring. Although the North Atlantic is a source of heat in the winter months, during the summer, the region of inflowing Atlantic water east of Shetland and Orkney is cooler then the waters within the shallower North Sea (Figure 2).

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Figure 2: Seasonal pattern of sea surface temperature in coastal waters of Scotland. Figure prepared using a climatological dataset, averaged at a resolution of 1/6 degree longitude and 1/10 degree latitude for the period 1971-2000 (Berx and Hughes, 2009). Winter is Dec-Feb, Spring is Mar-May, Summer is Jun-Aug and Autumn is Sep-Nov. Temperature contours are presented at 0.5°C intervals. Black region on the western boundary is outwith the analysis area (waters deeper than 250m) Calculated from daily averaged data (not shown), the average annual temperature at the Loch Ewe site is 10.3°C, 0.7°C higher than that at Stonehaven. The annual temperature range at the Stonehaven (7.4°C) site is also larger than Loch Ewe (6.5°C). The same pattern is revealed in the weekly temperature data (Figures 3 and 4). The seasonal cycle of temperature at each site reflects the broader spatial patterns (Figure 2), where winter and spring temperatures are lower in the North Sea than on the west coast of Scotland. Spring time temperatures are a period critical for the initiation of the phytoplankton spring bloom and the average daily temperature reached by 15th March at Loch Ewe was 7.3°C. Examining the daily averaged values for a common time-period (2003-2012) at each site shows that, on average, this temperature is not exceeded until 23rd April (39 days later) at Stonehaven (data not shown). (A) (B)

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Month Figure 4: Seasonal cycle of (A) temperature and (B) salinity at near-surface (<5m) and (C) temperature and (D) salinity at near-bed (>40m) at the Stonehaven monitoring site from 2003 - 2012. Average salinity in the lower layers of Loch Ewe was 34.29 with a standard deviation of 0.28. In the upper layers the average salinity was 33.22 with a standard deviation of 1.47. The minimum salinity in the upper layer was 21.32 and salinity was below 30 for 2% (15) of the samples. Minimum salinities were observed during winter (December to January) and maximum salinities in the summer months (June and July). At Stonehaven the average salinity of the upper layer was 34.47 with a standard deviation of 0.25. The minimum salinity observed at the upper layer was 32.91. At the nearbed level the average salinity was 34.56 and the standard deviation was 0.17. The maximum salinity at Stonehaven is observed in September and the minimum in March. Winter nutrients The winter (Nov - Jan) concentrations of TOxN and DSi at Stonehaven and Loch Ewe between 2003 and 2012 are presented in Figure 5. Winter TOxN is variable ranging from 4.7– 8.1 µM in Loch Ewe and 4.4 – 9.7 µM at Stonehaven. Median winter TOxN concentrations are approximately 1 µM greater at Stonehaven than in Loch Ewe. DSi concentrations are similar at both sites with median winter values of 5.58µM in Loch Ewe and 4.97µM in Stonehaven. Nutrients are in the main depleted during the summer months with TOxN often at the limit of detection (LOD). A rapid decrease in nutrients is observed in the spring time, coincident with the start of the spring bloom. (A) (B)

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Year Figure 7: Box whisker plot of the annual chlorophyll ‘a’ concentration measured during the growing season (Feb – Oct) at (A) Loch Ewe and (B) Stonehaven during the phytoplankton growing season (Feb – Oct) since 2003. Phytoplankton community structure The seasonality of diatoms at Loch Ewe and Stonehaven is shown in Figure 8. Diatoms were more abundant in Loch Ewe than at Stonehaven, particularly during March and September, with cell densities exceeding 3.0 X 106 cells L-1. Analysis of the phytoplankton community shows that the spring phytoplankton community is dominated by a small number of diatom genera at both sites. These include Chaetoceros, Skeletonema, Thalassiosira, and Pseudo-nitzschia ‘delicatissima’ type cells. The autumn diatom bloom at Loch Ewe is more pronounced than at Stonehaven often exceeding 0.8 X 106 cells L-1. In contrast to the spring bloom, the autumn bloom is dominated by larger diatoms such as Rhizosolenia and Pseudo-nitzschia seriata ‘type’ cells. At Stonehaven, these species were present but did not achieve the same cell densities (0.2 X 106 cells L-1) observed in Loch Ewe.

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Zooplankton

Grazer biomass is, on average, slightly lower in Stonehaven than in Loch Ewe and a difference in the seasonality can be observed (Figure 10). Grazer biomass is low during the winter months at both sites. At Loch Ewe, grazer biomass begins to increase in March and the highest values are generally observed in June/July.

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Values remain high, but variable, until September/October. In contrast, at the Stonehaven site, although grazer biomass also starts to increase in March, the rate of increase is lower so that the highest grazer biomass is not observed until July/August.

Copepods dominate the zooplankton grazer communities at Loch Ewe and Stonehaven. Pseudocalanus spp., an important food species for fish larvae and planktivorous fish (Lynch et al. 2001, Casini et al. 2004, Heath & Lough 2007), is the most dominant copepod at both Loch Ewe and Stonehaven particularly during the spring bloom. Acartia clausi is the most important summer/autumn grazer. Oithona spp., Paracalanus parvus, and Temora longicornis and a mixed population of C. finmarchicus and C. helgolandicus are found at both Stonehaven and Loch Ewe. C. heloglandicus is more abundant than C. finmarchicus in most, but not all, years (O'Brien et al. 2013). Calanus juveniles make up a larger proportion of the spring and autumn population at Loch Ewe than at Stonehaven. C. finmarchicus overwinters at depths greater than 600m, and much of the spring population of adult C. finmarchicus in the North Sea is known to originate from animals that overwinter in the Faeroe-Shetland channel (Heath et al. 1999). The location of the overwintering Loch Ewe spring population is unknown, as is the role that the west coast Calanus population plays in seeding later populations via water flowing around the northern part of Scotland and entering the North Sea via the Fair Isle channel and to the east of Shetland (Turrell et al. 1996). The overwintering strategy of C. helgolandicus is poorly understood (reviewed by Bonnet et al. 2005) and never studied in a Scottish context. At Scottish latitudes, C. finmarchicus is close to the southern limit of its thermal niche and geographical distribution and C. helgolandicus is close to its northern limit (Bonnet et al. 2005). These Calanus species may therefore be more sensitive to extremes in environmental variables at both Stonehaven and Loch Ewe.

Appendicularians, bivalve larvae and cirripede nauplii are also important components of the grazer communities at both sites. These zooplankton have been found to respond quickly to changes in environmental conditions (see review by Deibel & Lowen 2012), and be synchronised with phytoplankton concentrations (Starr et al. 1991), in some instances over large areas (Philippart et al. 2012). Cladocerans are important components of the zooplankton grazer community at the Loch Ewe site only. Ekvall & Hansson (2012) found that cladocerans benefit from higher temperatures more than copepods, possibly through increased recruitment from resting eggs in the sediment. At Stonehaven, echinoderm and polychaete larvae comprise a larger part of the grazer community compared to Loch Ewe which may simply reflect the composition of the benthos at the different sites. There has been little study on the composition of the benthic community at either Loch Ewe or Stonehaven.

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Figure 10: Box whisker plot of grazer biomass from (A) Loch Ewe (2002-2013) and (B) Stonehaven (1999 – 2013).

The seasonality of planktonic carnivore biomass is similar at Stonehaven and Loch Ewe, although both the amount of, and variability in, planktonic carnivore biomass is higher at Loch Ewe (Figure 11). This variability in planktonic carnivore biomass is mainly driven by fluctuating abundance of cnidarians although chaetognaths and ctenophores also increase in abundance during autumn at both sites. Planktonic carnivore biomass is low during the winter months at both sites. At Loch Ewe, planktonic carnivore biomass begins to increase a month earlier than at Stonehaven; however the spring maximum is seen in April at both sites. Values then decrease to a summer minimum in June, before increasing to an autumn maximum in September. The size of this autumn peak is higher than the spring peak for both sites.

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Data from Loch Ewe reveals considerable interannual variability in the occurrence of the cnidarians (Figure 12) with variation in the species achieving these high abundances. Species of cnidarians observed at high densities at the Loch Ewe site include the hydrozoans Obelia spp., Clytia hemisphaerica, Lizzia blondina, Euphysa spp., Rathkea octopunctata, Bougainvillia spp., Sarsia spp., Hydractinia spp., Phialella quadrata, and the anthozoan Cerianthus spp. although the dominant species varies from year to year. These species have also been recorded at Stonehaven but with a much lower abundance. Siphonophores are important cnidarians at both sites. The occurrence of jellyfish blooms is known to be highly variable, and the mechanisms behind them unknown, although local variation in physical and anthropogenic processes are thought to be major factors (reviewed by Graham et al. 2001, Purcell 2012).

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In the North East Atlantic, information about the zooplankton community over the last five decades comes from the Continuous Plankton Recorder (CPR), although CPR coverage is sparse in the waters to the west of Scotland, an area of particular importance for both fishing, and aquaculture (Baxter et al. 2011). The CPR survey has identified a number of changes in the plankton community of the North East Atlantic (e.g. Beaugrand et al. 2001, Edwards et al. 2002, Beaugrand et al. 2010, Alvarez-Fernandez et al. 2012, Edwards et al. 2013). Some of these changes include biogeographical shifts northwards in a number of copepod and calcifying plankton species (Beaugrand et al. 2002, Beaugrand et al. 2013), including a switch from Calanus finmarchicus to Calanus helgolandicus in the northern North Sea (Helaouët et al. 2013), an increase in the occurrence of cnidarians (Licandro et al. 2010) and the appearance of introduced species (Jha et al. 2013). Fixed point monitoring stations at L4 offshore from Plymouth (Harris 2010) and Helgoland in the south east North Sea (Wiltshire et al. 2010) are also describing variability and changes in the channel and southern North Sea plankton communities (Eloire et al. 2010, Schlüter et al. 2012). A 3-D coupled physical-biogeochemical model to explore ecosystem responses to climate change predicted that, in the North Sea, increased ocean stratification would cause zooplankton biomass to decrease in response to a warming climate (Chust et al. 2014) but this has not been seen within the timeframe of monitoring at Stonehaven or Loch Ewe (Figure 13).

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Considerable interannual variability has been observed in the abundance of both C. finmarchicus and C. heloglandicus at the Stonehaven and Loch Ewe monitoring sites (Figure 14 and 15) with no significant linear trends (O'Brien et al. 2013). In 2009, due to a combination of high C. finmarchicus and low C. helgolandicus abundances, C. finmarchicus became more dominant at these sites than C. helgolandicus for the first time since monitoring began. In 2010 extremely low numbers of both species were recorded and C. helgolandicus was again the dominant of the two species. Calcifying zooplankton (Clione limacina, Limacina retroversa, gastropod larvae, bivalve larvae and echinoderm larvae) regularly make up a large proportion of the summer zooplankton at Stonehaven and Loch Ewe and shows considerable interannual variability in their abundance since monitoring began at these sites (Figure 16). The introduced copepod, Pseudodiaptomus marinus, seen in the CPR survey has not been recorded in the zooplankton at either Stonehaven or Loch Ewe.

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Figure 15: Box whisker plot of Calanus helgolandicus stage CV-VI abundance from (A) Loch Ewe and (B) Stonehaven. Earlier stages cannot be distinguished from C. finmarchicus.

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Figure 16: Box whisker plot of calcifying zooplankton abundance from (A) Loch Ewe and (B) Stonehaven.

Energy flow through the food web

There have been few studies of energy flow through the pelagic food webs at the specific Stonehaven and Loch Ewe monitoring sites. Measurements of primary production made between 2007 and 2008 at Stonehaven indicated annual primary production at the site of around 48.6 - 79.8 gCm-2y-1 (Heath & Rasmussen In prep). Concurrent secondary production measurements estimated annual carbon specific production by zooplankton between 18 - 50 gCm-2y-1 with between 9 - 40 % of this annual production being produced in spring (Cook et al. In prep).

Heath & Beare (2008) estimated the annual primary production in ICES rectangle VIa (northwest coast of Scotland) to be about double that of IVb (central North Sea). Heath (2005b) estimated average secondary production by omnivorous zooplankton to be 35 gCm-

2y-1 in the North Sea and 23 gCm-2y-1 in the West of Scotland and fishery demands for zooplankton represented, on average, 5% of zooplankton gross production in these areas. Heath (2005a) estimated that demand for secondary production by fish in the North Sea has declined from about 20 gCm-2y-1 in the 1970s to 16 gCm-2y-1 in the 1990s and the proportion of demand provided by zooplankton production has increased from around 70% to 75%.

Initial assessment

Does the assessed region represent a distinct hydrodynamic region?

Physics, chemistry and biology examined for both Stonehaven and Loch Ewe suggest both sites are representative of the hydrodynamics in the region based on expert opinion.

0

1

2

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-3 +

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184

Is the seasonal pattern of dissolved inorganic nutrient concentrations in the assessed region consistent with current understanding of biogeochemical cycling in shelf seas?

Smith et al., 2014 provide a comprehensive review of the nutrient dynamcs in Scottish waters and data collected at the Stonehaven and Loch Ewe monitoring sites fall within the concentrations described within. Thus the data collected agrees with the question above.

Is the seasonal cycle of plankton production and biomass consistent with current understanding of the processes controlling the microplankton biomass and production in the shelf seas.

The seasonal cycle of plankton in Stonehaven and Loch Ewe is consistent with current understanding of processes in many temperate coastal seas (Figure 6 - 10). Subtle differences between both sites suggest a difference in the plankton dynamics between the two ecohydrodynamic regions where these sites are located. Phytoplankton growth begins in early March is followed by a period over the summer when dinoflagellates and sometimes diatoms can also dominate. An autumn diatom bloom is also observed in Loch Ewe between August and Sept. The community diversity is similar to the phytoplankton community at Stonehaven described by Bresnan et al. (2009). In contrast, at Stonehaven the phytoplankton growth period begins later, reflecting the cooler water temperature, and maximum cell densities are reached a month later. The intensity of the spring bloom varies between years and some years the highest concentration of chlorophyll is observed during the autumn bloom (Bresnan et al., 2009). A decreases in the abundance of thecate dinoflagellates such as Ceratium has been observed in keeping with the trend observed by the CPR (Hinder et al., 2012). Transport of phytoplankton in the coastal current from the west coast to the east was observed during the Karenia mikimotoi bloom in 2006 (Davidson et al., 2009).

The seasonal cycles in grazer and carnivore biomass observed at the Loch Ewe and Stonehaven monitoring sites (Figure 10 and 11) are similar to that observed in many temperate coastal seas and are consistent with the widely accepted theory that a pronounced spring bloom is followed by a summer period of relative stability of zooplankton stocks (Colebrook 1986, Greve et al. 2004, Eloire et al. 2010, O'Brien et al. 2013). Grazer biomass in Loch Ewe begins to increase in March and the highest values are generally observed in June/July (values ranging up to 260 mg DW m-3). Values remain high, but variable, until September/October where grazer biomass values can dip to 5 mg DW m-3. At Stonehaven, although grazer biomass also starts to increase in March, the highest grazer biomass is not observed until July/August (values ranging up to 300 mg DW m-3). Planktonic carnivore biomass is low during the winter months (usually <2 mg DW m-3), and the spring maximum is in April at both sites (values ranging up to 25 mg DW m-3 at Loch Ewe and 9 mg DW m-3 at Stonehaven). Values then decrease to a summer minimum in June, before increasing to an autumn maximum in September. The size of this autumn peak is higher than the spring peak at both sites, with values ranging up to 180 mg DW m-3 at Loch Ewe and 80mg DW m-3 at Stonehaven. ). As irradiance concentrations decline and turbulence increases in the water column, phytoplankton growth rates decrease at the end of autumn at both sites. Zooplankton growth rates and biomass also decrease during this period. .

185

A study of CPR data (Hinder et al., 2012) has shown an decrease in the number of thecate dinoflagellates in the North East Atlantic coincident with an increase in summer surface wind intensity since the late 1990s and this pattern has also be observed in the Loch Ewe phytoplankton data with Ceratium abundances decreasing. Previous long-term studies of the seasonality of zooplankton in the North Sea and CPR data from the wider west of Scotland area have stated that the traditional autumn bloom in zooplankton has become earlier, merged with the spring bloom and created a seasonal cycle where abundance remains high between spring and autumn (Greve et al., 1996, Bailey et al., 2011) which agrees with the patterns found in grazer biomass at Loch Ewe and Stonehaven in this study. The seasonal cycle of total zooplankton found at L4 in the western channel (Eloire et al. 2010) is comparable to that found at Loch Ewe, although there are species differences. CPR data from the west of Scotland and North Sea show seasonal patterns in calanoid copepod diversity and total copepod abundance that reflects the seasonal pattern in grazer biomass seen at Loch Ewe and Stonehaven (Fransz et al. 1991, Beaugrand et al. 2001, Bailey et al. 2011).

Is the succession of species consistent with what is expected for temperate coastal waters?

The plankton succession at both sites is consistent with that expected for temperate waters. The phytoplankton species observed at this site is typical of that observed in temperate waters. A spring bloom of small diatoms (Skeletonema, Thalassiosira and Chaetoceros . Dinoflagellates are more abundant during the summer. In the autumn, high cell denisites of larger diatoms such as Pseudo-nitzschia ‘seriata – type’ cells and Rhizosolenia species occur. Harmful algal species are observed, with DInophysis and Pseudo-nitzschia recorded at both sites (Cook et al., submitted). Alexandrium is more prevelant on the east coast at the Stonehaven monitoring site (Bresnan et al. 2009) while high densities Karenia mikimotoi have been recorded more frequently on the west coast (Davidson et al., 2009). A strict seasonal succession of particular species is not expected in North Atlantic zooplankton communities (Fransz et al. 1991, Tommasi et al. 2013a). A trend of predominantly herbivores early in the year with an increase in carnivores later in the season is expected (Fransz et al. 1991, Bode & Alvarez-Ossorio 2004) and is seen (Figure 10 and 11), with the particular species varying depending on hydrographical conditions, and timing and structure of the phytoplankton communities (Tommasi et al. 2013b).

Appendicularians, bivalve and cirripede larvae as well as the copepods Acartia clausi, Oithona spp., Paracalanus parvus, Pseudocalanus spp. and Temora longicornis are present at both sites. Pseudocalanus spp., which is an important food species for fish larvae and planktivorous fish (Lynch et al. 2001, Casini et al. 2004, Heath & Lough 2007), is the most abundant grazer at both sites, particularly during the spring bloom. Acartia clausi was the most important summer/autumn grazer. The broad seasonal patterns in copepod species and meroplankton types are similar to those seen at L4 in the English Channel (Eloire et al. 2010, Highfield et al. 2010). Although Calanus helgolandicus and C. finmarchicus are present at both

186

sites, Calanus juveniles make up a larger proportion of the spring and autumn population at Loch Ewe than at Stonehaven, as do cladocerans. In contrast, echinoderm and polychaete larvae comprise a larger part of the grazer community at Stonehaven than at Loch Ewe. Appendicularians, cladocerans, bivalve larvae and cirripede nauplii have been found to respond quickly to changes in environmental conditions (see review by Deibel & Lowen 2012), and synchronised with phytoplankton concentrations in some instances over large areas (Starr et al. 1991, Philippart et al. 2012). Ekvall & Hansson (2012) found that cladocerans benefit from higher temperatures more than copepods, possibly through increased recruitment from resting eggs in the sediment.

C. helgolandicus is more abundant than C. finmarchicus in most, but not all, years at both sites (O'Brien et al. 2013). C. finmarchicus overwinters at depths greater than 600m, and much of the spring population of adult C. finmarchicus in the North Sea is known to originate from animals that overwinter in the Faeroe-Shetland channel (Heath et al. 1999). The location of the overwintering Loch Ewe spring population is unknown, as is the role that the west coast Calanus population plays in seeding later populations via water flowing around the northern part of Scotland and entering the North Sea via the Fair Isle channel and to the east of Shetland (Turrell et al. 1996). The overwintering strategy of C. helgolandicus is poorly understood (reviewed by Bonnet et al. 2005) and never studied in a Scottish context. At Scottish latitudes, C. finmarchicus is close to the southern limit of its thermal niche and geographical distribution and C. helgolandicus is close to its northern limit (Bonnet et al. 2005). These Calanus species may therefore be more sensitive to extremes in environmental variables at both Stonehaven and Loch Ewe. Data from Loch Ewe reveals considerable interannual variability in the occurrence of the cnidarians (Figure 12A) with high interannual variation in the species achieving these high abundances. The occurrence of jellyfish blooms is known to be highly variable, and the mechanisms behind them unknown, although local variation in physical and anthropogenic processes are thought to be major factors (reviewed by Graham et al. 2001, Purcell 2012).

Does the microplankton support higher trophic levels?

The data suggests that microplankton do support higher trophic levels. Zooplankton grazer biomass at both sites follows the distinctive pattern in chlorophyll concentration. There have been few studies of energy flow through the pelagic food webs at the specific Stonehaven and Loch Ewe monitoring sites. Measurements of primary production made between 2007 and 2008 at Stonehaven indicated annual primary production at the site of around 48.6 - 79.8 gCm-2y-1 (Heath & Rasmussen in prep). Concurrent secondary production measurements estimated annual carbon specific production by zooplankton between 18 - 50 gCm-2y-1 with between 9 - 40 % of this annual production being produced in spring (Cook et al. In prep).

Heath & Beare (2008) estimated the annual primary production in ICES rectangle VIa (northwest coast of Scotland) to be about double that of IVb (central North Sea). Heath (2005b) estimated average secondary production by omnivorous zooplankton to be 35 gCm-

2y-1 in the North Sea and 23 gCm-2y-1 in the West of Scotland and fishery demands for

187

zooplankton represented, on average, 5% of zooplankton gross production in these areas. Heath (2005a) estimated that demand for secondary production by fish in the North Sea has declined from about 20 gCm-2y-1 in the 1970s to 16 gCm-2y-1 in the 1990s and the proportion of demand provided by zooplankton production has increased from around 70% to 75%.

Has there been a long-term change in zooplankton phenology and biomass?

The time series duration at this site is too short to reveal robust changes in the plankton phenology and biomass over a multidecadal scale. A high degree of interannual variability can be observed in the plankton data at this site. In the North East Atlantic, information about the plankton community over the last five decades comes from the Continuous Plankton Recorder (CPR), although CPR coverage is sparse in the waters to the west of Scotland, an area of particular importance for both fishing, and aquaculture (Baxter et al. 2011). The CPR survey has identified a number of changes in the plankton community of the North East Atlantic (e.g. Beaugrand et al. 2001, Edwards et al. 2002, Edwards and Richardson 2004, Edwards et al., 2006, Beaugrand et al. 2010, Alvarez-Fernandez et al. 2012, Hinder et al., 2012, Edwards et al. 2013). Some of these changes include a change in the distribution of harmful algal bloom species in the North Sea (Edwards et el. 2006), a change in the phenology of dinoflagellate species (Edwards and Richardson 2004), decrease in the abundance of summer dinoflagellates (Hinder et al., 2012), biogeographical shifts northwards in a number of copepod and calcifying plankton species (Beaugrand et al. 2002, Beaugrand et al. 2013), including a switch from Calanus finmarchicus to Calanus helgolandicus in the northern North Sea (Helaouët et al. 2013), an increase in the occurrence of cnidarians (Licandro et al. 2010) and the appearance of introduced species (Jha et al. 2013). Fixed point monitoring stations at L4 offshore from Plymouth (Harris 2010) and Helgoland in the south east North Sea (Wiltshire et al. 2010) are also describing variability and changes in the channel and southern North Sea plankton communities (Eloire et al. 2010, Schlüter et al. 2012). A 3-D coupled physical-biogeochemical model to explore ecosystem responses to climate change predicted that, in the North Sea, increased ocean stratification would cause zooplankton biomass to decrease in response to a warming climate (Chust et al. 2014) but this has not been seen within the timeframe of monitoring at Loch Ewe or Stonehaven.

Considerable interannual variability has been observed in the abundance of both C. finmarchicus and C. heloglandicus at the monitoring sites (Figure 14 and 15) with no significant linear trends (O'Brien et al. 2013). In 2009, due to a combination of high C. finmarchicus and low C. helgolandicus abundances, C. finmarchicus became more dominant than C. helgolandicus for the first time since monitoring began. In 2010 extremely low numbers of both species were recorded and C. helgolandicus was again the dominant of the two species. Calcifying zooplankton (Clione limacina, Limacina retroversa, gastropod larvae, bivalve larvae and echinoderm larvae) regularly make up a large proportion of the summer zooplankton at Loch Ewe and Stonehaven but there are no obvious trends in their abundance since monitoring began (Figure 16). The introduced copepod, Pseudodiaptomus marinus, seen in the CPR survey has not been recorded in the zooplankton at Loch Ewe or Stonehaven. However, identifying significant trends in a 12-15 year timeseries is difficult (Edwards et al. 2010), and particularly so when the decade has been characterized by higher variability and unusual hydrographic regimes compared to the years prior to 1999 (Hughes et al. 2012).

Is there evidence that bottom up and top down pressure has altered phytoplankton phenology and production over the last 20 years?

188

This has yet to be investigated.

Does the state of the microplankton in the assessed region represent GES?

Table 1

Table of agreement and disagreement over initial assessment at the

Stonehaven monitoring site

Assessment question Agreement Disagreement

1. Does the assessed region

represent a distinct

hydrodynamic region?

Only one site but data is in

keeping with region - Yes

2. Is the seasonal pattern of

dissolved inorganic nutrient

concentrations in the

assessed region consistent

with current understanding

of biogeochemical cycling in

shelf seas?

Yes

3. Is the seasonal cycle of

microplankton production

and biomass consistent with

current understanding of the

processes controlling

microplankton biomass and

production in shelf seas?

Yes

4. Is the succession of species

in the assessed region

consistent with what is

expected for a seasonally

stratifying temperature shelf

sea?

Yes

5. Does the microplankton

support higher trophic

levels?

Yes

6. Does the concentration of

anthropogenic nutrient

enrichment stay below the

OSPAR threshold (15µm for

N)?

Yes

7. Is there evidence that

bottom up and top down

pressure has altered

phytoplankton phenology

and production over the last

Not enough info at this site.

High degree of interannual

variability.

189

20 years?

8. Does the state of the

microplankton in the

assessed region represent

GES?

Can we see the doughnuts

before we answer this?

Table 2

Table of agreement and disagreement over initial assessment Loch Ewe

Assessment question Agreement Disagreement

1. Does the assessed region

represent a distinct

hydrodynamic region?

Limited data suggests yes

MarCRF Ph.D. studenship is

investigating this.

2. Is the seasonal pattern of

dissolved inorganic nutrient

concentrations in the

assessed region consistent

with current understanding

of biogeochemical cycling in

shelf seas?

Yes

3. Is the seasonal cycle of

microplankton production

and biomass consistent with

current understanding of the

processes controlling

microplankton biomass and

production in shelf seas?

Yes

4. Is the succession of species

in the assessed region

consistent with what is

expected for a seasonally

stratifying temperature shelf

sea?

Yes

5. Does the microplankton

support higher trophic

levels?

Yes

6. Does the concentration of

anthropogenic nutrient

enrichment stay below the

OSPAR threshold (15µm for

N)?

Yes

7. Is there evidence that

bottom up and top down

pressure has altered

phytoplankton phenology

and production over the last

Not enough data to say with

certainty owing to high

degree of interannual

variability.

190

20 years?

8. Does the state of the

microplankton in the

assessed region represent

GES?

Don’t we need to see the

doughnuts?

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SEPA


community of the inner Firth of Clyde, Scotland

Malcolm Baptie and Kirsty Barclay

August 2014

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Summary This document describes an assessment of a SEPA monitoring location in the Firth of Clyde, designated ‘Firth of Clyde at CMT7, NW of Cloch Point’. It is a single location which is monitored at monthly frequency for the principal purpose of reporting chemical and phytoplankton determinands in support of assessment of ecological status for the Water Framework Directive (WFD). It is the only location in the Firth of Clyde that is monitored continually, though other sites in the Clyde basin are monitored on a 1 year in 3 basis. The aim of the study was to determine whether the state of microplankton at the site is representative of Good Ecological Status (GES) for the purposes of the Marine Strategy Framework Directive (MSFD). Determination was made by expert judgement as recommended by the MSFD pelagic subgroup. This document follows the approach of Scherer & Gowen (2013). All data reported are held by SEPA, with the exception of Atlantic nutrient values in Table 1, which are from Scherer & Gowen (2013). Weak tidal currents and large volumes of fresh water cause near permanent haline stratification, with an additional thermal component to stratification in summer when air temperatures are higher and river flows are lower. The Firth of Clyde is a region of freshwater influence due to the input of the Clyde River in particular, but also other rivers and sea lochs. Haline stratification distinguishes the Clyde Sea from the North Channel of the Irish Sea, which is vertically mixed by stronger tidal currents. Anthropogenic nutrient enrichment is a feature of the firth through domestic, agricultural and industrial inputs. Silicate input from the Clyde River contributes to high abundance of diatoms at CMT7, growth beginning in March-April. Zooplankton grazes on phytoplankton in summer, terminating the spring bloom phytoplankton species, which are succeeded by different species that are more resistant to grazing. Nutrient replenishment in the inner Clyde in winter is achieved more by river input than vertical mixing. The firth of Clyde microplankton supports a variety of fish, birds and mammals.

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Physical oceanography of CMT7 Introduction The SEPA sampling site ‘Firth of Clyde at CMT7, NW of Cloch Point’, hereon referred to as ‘CMT7’, is in the inner Firth of Clyde, to the south of the Clyde estuary and Holy Loch (Figure 1). The Firth of Clyde covers 3671km2, occupying a volume of 179km3 and with a depth range of 3m in Loch Ryan to over 200m in parts of Loch Fyne (McIntyre et al., 2012). The Firth of Clyde is bathymetrically complex, its sea floor has been formed by prehistoric volcanism and repeated glaciation events leading to a 40-50m deep region termed the Great Plateau (Tivy, 1986; Edwards et al., 1986), which divides the North Channel of the Irish Sea from the 100m+ deep waters of the Arran basin. A front develops along this boundary between the Mull of Kintyre and the Rhinns of Galloway which largely separates the deep water of the Irish Sea from the Firth of Clyde (Edwards et al., 1986).

Figure 1: Firth of Clyde at CMT7 sampling site (black star).

The River Clyde is the major source of fresh water inputting to the Clyde basin. The small tidal range of 1.8-3.1m (Ross et al., 2009) combined with the weak tidal velocities encountered in the area means stratification through salinity gradient is present year round. An additional thermal component to stratification develops in spring and summer (Rippeth & Jones, 1997). Mean salinity at CMT7 is 30.8 indicating water sampled is diluted by the Clyde estuary, containing 13.24% fresh water compared to fully marine water.

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Figure 2: 1-10m depth average salinity with increasing distance from the Clyde estuary transitional water body. Red lines are 95% confidence intervals.

Flow River inputs are highest in winter and lowest in summer. The Clyde basin receives 60-700m3 s-1 freshwater, and the main source of this is the Clyde River and its tributaries, though the sea lochs of the northern part of the firth, and other estuaries along the Ayrshire coast are also important (Poodle, 1986). This affects surface salinity across the Firth of Clyde, as seen from the presence of fresh water in samples taken 100km from the mouth of the Clyde estuary (Figure 2). When rivers are in spate the salinity at CMT7 can reach as low as 17, as fresh water inundates the upper mixed layer and displaces more saline water. Tidal excursions in the inner Firth are in the range of 1-4km (Townson & Collar, 1986) Seasonal cycle of temperature and salinity and stratification A mooring at CMT7 has been in operation continuously since 2009. This has recorded temperatures ranging from 3.5 to 19°C and typically temperatures have been between 5-15°C (Figure 3). Salinity decreases in winter as flow from the estuary increases, and increases in summer as estuary flow decreases (Figure 4). A short period of vertical mixing appears to occur in October-November, before haline stratification becomes established over winter.

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Figure 3: Temperature from the mooring at CMT7. Values taken at 30 minute intervals parsed to 15 minutes.

Figure 4: Salinity from the mooring at CMT7. Values taken at 30 minute intervals parsed to 15 minutes.

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Figure 5: Temperature, Salinity and Chlorophyll a profiles at CMT7 in 2011. Stratification is present year round, with salinity the main mode in winter and temperature the main mode in summer. Chlorophyll peaks initially in May and reaches its highest surface concentrations in July. Gaps are due to instrument failures and cancelled surveys.

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Figure 6: Temperature, Salinity and Chlorophyll a profiles at CMT7 in 2012. Stratification is present year round; this year both salinity and temperature are contributors in summer. Chlorophyll peaks three times in March, May and September. Gaps are due to cancelled surveys.

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Figure 7: Temperature, Salinity and Chlorophyll a profiles at CMT7 in 2013. Stratification is present year round; again salinity is the main gradient of stratification with temperature as an additional factor in summer. Chlorophyll peaked in May and August. Gaps are due cancelled surveys.

The near permanent salinity stratification in the inner Firth of Clyde confirms that it is situated in a stratified ‘Region of Freshwater Influence’ (ROFI; Connor et al., 2006). The frontal zone which develops in summer and demarcates the mixed North Channel and stratified Great Plateau restricts input from the Irish Sea to the surface layer, and the weak tidal velocities indicate that tidal stirring is generally not sufficient to break down vertical density gradients (Edwards et al., 1986). Storm surges can interrupt tidal circulation and may at times affect stratification; however Northern Ireland, the Kintyre Peninsula and the Isle of Arran shield the Firth of Clyde from the prevailing south-westerly winds (Ross et al., 2009). Sub-surface light climate Secchi depth profiles indicated water clarity in the range of 2-6m. Light attenuation coefficient Kd

was estimated from Secchi depth (Devlin et al., 2008), and euphotic zone depth as 4.6/Kd (Kirk, 1994; Figure 8). The chlorophyll a signatures in the profiles of figures 5-7 were all within the euphotic zone, indicating light limitation in summer is not a frequent occurrence, due to stratification. The euphotic

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zone depth is, however, likely to be an overestimate due to the presence of yellow substance from the Clyde estuary (Bowers et al., 2000).

Figure 8: Secchi depth at CMT7 from 2000 to 2014, and estimated euphotic zone depth.

Dissolved Inorganic Nutrients Total oxidised Nitrogen (TOxN) drawdown defined as the time at which nutrient concentration in the upper mixed layer (here assumed to be top 10 metres) reaches 50% of winter (DJF) maximum concentrations generally occurs in April and persists until October. Uninterrupted stratification means there is a continued supply of TOxN from fresh water sources and the surface concentration does not decrease below 2µMol L-1 even in summer (Figure 9). This indicates CMT7 to have elevated levels of nutrients relative to the wider Clyde Sea (Rippeth & Jones, 1997). Similar patterns in drawdown are observed for DIP (Figure 10) and Silicate (Figure 11). The long periods of the year when the water column is stratified and nutrients are transported into the firth leads to phytoplankton blooms early in spring and late in summer (Figures 5-7). The drawdown in summer appears to be the period when phytoplankton demand begins to exceed river supply, as flows are at their lowest. The relatively short period of lower nutrient concentrations reflects the onset of greater river flows following summer.

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There was no upward trend in TOxN data (Figure 12) over time, rather a decline. Average winter TOxN concentrations have varied between approximately 10 and 15µMol L-1 between 2005 and 2014.

Figure 9: Averaged TOxN concentrations in µMol L

-1 at CMT7 (2005-2014). Gaps are missing data.

Figure 10: Averaged Orthophosphate concentrations in µMol L


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Figure 11: Averaged Silicate concentrations in µMol L


Figure 12: Time series of 1-10m average TOxN at CMT7. Trend is a linear model.

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Nutrient concentration from the upper Clyde estuary where salinity was equal to or less than 2, and the tributary rivers Black Cart, White Cart, Leven and Gryfe was listed in table 1, compared to estimates of Atlantic nutrient concentrations in the Malin shelf break region (Scherer & Gowen, 2013). The mouth of the Kelvin was in the same vicinity as low salinity estuarine samples so was not included. Table 1: Comparison of mean marine and estuarine contributions to 1-10m averaged winter nutrient concentrations at CMT7 from 2005-2014. Malin shelf break values from Scherer & Gowen (2013). Estuarine data restricted to samples in Clyde estuary where salinity was less than or equal to 2, and the contribution of the Black Cart, White Cart, Gryfe and Leven rivers, discharging into a higher salinity stretch of the estuary.

Concentration Contribution CMT7

Clyde estuary Oceanic Estuarine (13.2%)

Marine (86.8%)

Predicted Measured

TOxN 79.35 7.15 10.47 6.20 16.67 13.61

DIP 2.55 0.45 0.34 0.43 0.77 1.02

Silicate 104.24 2.65 13.75 2.30 16.05 14.57

TOxN:DIP 21.64 13.34

TOxN:Si 1.04 0.93

TOxN and Silicate appeared to be overestimated by the prediction, while DIP was underestimated. The difference between predicted and measured TOxN:Si ratios were small, but the difference between predicted and measured TOxN:DIP ratios were large. Ratios from 1982-1999 were 10.08 and 0.97 for TOxN:DIP an TOxN:Si respectively. Permanent stratification prevents mixing of surface and deep water, leaving river input as the main mechanism by which TOxN and Silicate is replenished in winter. Deposition of dredging spoil causes short term local nutrient enrichment (Jones & Lee, 1981). Efforts to deepen the Clyde River date to at least the 18th century (Tivy, 1986). Since the Second World War, 272000m3 of sediment is suction dredged annually from the Clyde estuary and is transported by barge to a deep water spoil ground near Cloch Point, which is in the vicinity of CMT7 (Allen, 1995). This is normally done in summer (Clydeport, pers. comm.), and with zooplankton grazing contributes to the increase in nutrients in June (Figure 13). Most of this material however is transported northward into Loch Long (Allen, 1966), and so nitrification in most of the dredged estuary sediments that would otherwise have led to increased winter levels of TOxN at CMT7 occurs away from the site. In winter, occasional plough dredging is done and the spoil is removed by the flow of the estuary, rather than deposition at Cloch Point. In both summer and winter cases, sediment sinking below the surface mixed layer is the likely reason for the discrepancy between predicted and measured TOxN and Silicate.

There are several possible reasons for underestimation of predicted DIP. Phosphorous moves from particulate to dissolved phase with increasing salinity, but is retained by the presence of manganese and iron oxyhydroxides, which bind phosphorous to particles in sediment. The exception is in anoxic conditions, where metal oxyhydroxides are converted to metal sulphides by sulphate reduction and no longer bind to phosphorous, which is released into sediments as DIP, particularly as pH rises above 7 (Roden & Edmonds, 1997). If the anoxic layer is close to the surface, DIP enters the water column. Benthic surveys of the Clyde estuary have discovered anoxic sediments beneath a shallow layer of recently deposited sediment (Balls, 1991), particularly along the north bank of the lower estuary (A. Moore, pers. comm.). Downstream nutrient input not accounted for in the selected freshwater and estuarine data from one of several waste water treatment plants is another possible source. A peak in DIP in the outer estuary in that is visible in September (Figure 13) would be transported into the outer estuary & retained in the upper mixed layer, as day length begins to limit phytoplankton uptake. Using the Atlantic (salinity 35.5) and upper Clyde estuary (mean salinity 0.16) nutrient concentrations in table 1 to construct a theoretical salinity nutrient relationship line

12

for each nutrient visualised the differences between theoretical and measured nutrient and salinity relationships (Figure 14).

Figure 13: 2005-2014 Averaged TOxN, Silicate and DIP from 1-10m depth at monitoring stations in the Lower Clyde Estuary (blue), CMT7 (red) and the Outer Clyde Sea, south of Arran (green).

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Figure 14: Comparison of theoretical salinity nutrient mixing relationships (dashed lines) and measured winter

concentrations (blue points) averaged over 1-10m depth at CMT7 (2005-2014). Summary At CMT7 there is a seasonal cycle of TOxN, DIP and Silicate at 1-10m depth. Maximum monthly mean concentrations of TOxN (14.31µMol L-1) and Silicate (15.79µMol L-1) occur in January, and for DIP (1.14µMol L-1), in December. The inner firth is elevated in nutrients compared to the outer firth, as a result of input from the Clyde estuary, which maintains continual stratification & nutrient input to the surface mixed layer. Nutrient drawdown begins in April and continues until October. TOxN and Silicate concentrations remain above 2 µMol L-1 in summer as a result of input from the Clyde estuary, including a summer campaign of dredging of the main channel. Loss of TOxN and Silicate released from dredge spoil below the surface mixed layer is the likely explanation for the tendency for both of these nutrients to be found at winter concentrations below the theoretical mixing line. Particular effects of salinity and oxygen in sediments on the balance between particulate and dissolved phosphorous in the Clyde estuary are likely to explain the higher than expected winter DIP concentrations measured at CMT7. Winter TOxN has not increased since the 1980s.

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Microplankton The seasonal cycle of biomass at CMT7 The spring bloom begins in April, and continues until August or September. The productive period is characterised by several successive blooms, occurring during summer when river flow is lower (Figure 15). Optical estimates of chlorophyll a were not calibrated with field samples so are only a guide, and absolute values are not reliable estimates of biomass. Fluorometric determination of acetone extracted chlorophyll a shows mean chlorophyll a concentration peaks in May at 9µg L-1 and remains at approximately 6µg L-1 until October, when the concentration declines to 3µg L-1.

Figure 15: Chlorophyll a from the YSI6600 mooring at CMT7. Green points are optically estimated chlorophyll a, and blue line is flow rate from a river gauge at Daldowie on the Clyde.

CMT7 has high levels of diatom abundance, due to the silicate input from the Clyde estuary, and the spring bloom begins early in the year as a result of permanent stratification. The long period of high chlorophyll a concentration in surface water even as abundance of diatoms declines is indicative of a shift from small species (spring is dominated by Skeletonema sp.) to large species after May. Typical forms found in spring and summer are the same as those described by Boney (1986). Microflagellates achieve numerical dominance by May, and will also contribute to chlorophyll a concentration, but SEPA do not possess an epifluorescence microscope with which to make the distinction between autotrophic and heterotrophic forms. Biomass by functional group is not determined at SEPA. Dinoflagellates reach their peak in late summer (Figure 16). Continued nutrient supply from the Clyde estuary combined with permanent stratification means light and zooplankton grazing intensity are the two factors that limit phytoplankton populations.

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Figure 16: Seasonal cycle of microplankton at CMT7, mean monthly abundances between 2007 and 2013.

Boney (1986) described a short 2 week ‘window’ in which conditions are suitable for bloom onset in the inner firth, and the time of this window determines which species will be dominant in the spring bloom assemblage. This betrays the problem of monthly resolution sampling, which is inadequate to detect events of this resolution, and means the variability of species found at CMT7 (Table 2) is difficult to interpret. The two spring genera most consistently observed at high abundance are Skeletonema and Chaetoceros.

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Table 2: Top ten most abundant diatoms (cells L-1

) during spring (Mar-May) from 0-10m depth at CMT7. Species 2007 2008 2009 2010 2011 2012 2013 Asterionellopsis glacialis - 360 - - - - -

Cerataulina pelagica - - - - - 345187 -

Chaetoceros (Hyalochaete) 280 4700 74100 734660 - 217957 272687

Coscinodiscus >50µm - - 2200 - - - -

Cylindrotheca closterium/Nitzschia longissima 920 4600 - 1740 200 18531 -

Dactyliosolen fragilissimus - - - - 402768 - 860

Ditylum brightwellii - 120 - - 500 - -

Guinardia delicatula - - 3300 - - - -

Gyrosigma/Pleurosigma sp. 80 40 - - - - -

Indet. araphiated pennate diatom >50µm - - - 2600 - - -

Indet. araphiated pennate diatom 20-50µm - - - - 7496.46 - -

Indet. centric <20µm - - - - - 402655 7727

Indet. centric diatom >50µm - - - - - 5153 -

Indet. centric diatom 20-50µm - - 2300 1000 200 77010 -

Indet. raphiated pennate diatom <20µm - - - - - 4813 2208

Indet. raphiated pennate diatom >50µm - 80 - - - - -

Indet. raphiated pennate diatom 20-50µm 800 - - 1800 - - 5168

Leptocylindrus danicus 291000 28200 6900 - 67128 - -

Leptocylindrus mediterraneus - - 3400 - - - -

Other diatoms - - - 1000 - - 27600

Paralia sulcata 400 - - - - - -

Pseudo-Nitzschia <5µm - - 2100 - - 78242 -

Pseudo-Nitzschia >5µm 1080 - 3800 1200 - - 7728

Rhizosolenia imbricata - - - - 200 - -

Rhizosolenia setigera - 80 - - 4100 - -

Rhizosolenia sp. 2080 - - - - - -

Skeletonema sp. 2931360 4782150 7813700 994620 550594 2959476 2097231

Thalassiosira <10µm - 945000 - - - - 77267

Thalassiosira 10-50µm 3880 - - - - 1612643 57398

Long term trends With no upward term trend in winter TOxN from the Clyde estuary, the levels of chlorophyll a observed in 1-10m depth samples have not increased compared to the seasonal envelopes constructed from historic data measured at CMT7 by the Clyde River Purification Board, precursor body to SEPA (Figure 17). The values recorded in January to March 2005-2014 were substantially higher than the minimum values from 1982-1999 but were, with one exception, still within the envelope.

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Figure 17: Comparison of maximum and minimum monthly 1-10m averaged chlorophyll a concentrations from 1982-1999 and 2005-2013 data at CMT7.

Zooplankton Planktonic copepod abundance peaks in June and again to a subordinate degree in October (Figure 18). Calanoid copepods are the most abundant group at CMT7, with cyclopoid copepods, harpacticoid copepods, appendicularians, and the larvae of polychaetes, bivalves, barnacles and echinoderms also abundant groups (Figure 19). Gelatinous plankton species are often found in summer and may be an important fraction of biomass, but they were numerically subordinate to copepods and other zooplankton. Calanoid copepods found at CMT7 are dominated by the genera Calanus, Acartia, Pseudocalanus, Paracalanus, Microcalanus, Centropages and Temora. Cyclopoid copepods are mostly Oithona spp. and harpacticoid copepods are mostly Microsetella norvegica. Zooplankton monitoring has only recently been undertaken at monthly intervals in the context of the marine monitoring programme at SEPA, so long term trends are not calculable at present. The groups found in abundance at CMT7 in 2013-14 were the same groups found by Adams (1986) to dominate in 1970-74, though there was a difference in seasonal composition. March abundance in the samples of Adams (1986) nearest to CMT7 was dominated by Pseudocalanus spp., but here at CMT7 in 2013-14 were dominated by Oithona spp. and Microsetella norvegica. August abundance in samples of Adams (1986) nearest to CMT7 was dominated by Acartia spp., and this remained the case at CMT7 in 2013-14 (Figure 20).

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Figure 18: Total monthly abundance of planktonic copepods (excluding nauplii) at CMT7, data composite from 2013-14.

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Figure 19: The 8 most abundant groups in the zooplankton at CMT7 (excluding eggs and nauplii) as a proportion of total zooplankton abundance. Data composite of 2013-14.

Figure 20: Seasonal succession of copepod genera at CMT7. Data composite of 2013-14.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

J F M A M J J A S O N

Microsetella

Oithona

Temora

Pseudocalanus

Paracalanus

Microcalanus

Centropages

Calanus

Acartia

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Assessing the state of the microplankton Does the Firth of Clyde represent a distinct ecohydrodynamic region? The answer to this question is yes. The shallow sill at the south of the Clyde Sea restricts exchange between the Clyde Sea and North Channel of the Irish Sea. This means the large flow of fresh water into particularly the inner firth results in haline stratification throughout the year, distinguishing it from the mixed North Channel. Is the seasonal pattern of dissolved inorganic nutrients consistent with understanding of biogeochemical cycling in shelf seas? The answer to this question is yes. Although it is primarily river supply, and not deep water mixing, which replenishes nutrients in winter, a similar pattern of drawdown is visible. Nutrients are at their maximum in winter. Low river flow in summer diminishes input, and concentrations in the surface mixed layer rapidly decrease in spring as increasing day length permits phytoplankton production to exceed nutrient replenishment. Is the seasonal cycle of microplankton production and biomass consistent with current understanding of the processes controlling microplankton biomass in shelf seas? The answer to this question is yes. The onset of the spring bloom is not governed by stratification, which is permanent, but by day length, and the initial absence of grazing pressure. The bloom begins in April, and chlorophyll a concentration reaches 9µg L-1 in the surface mixed layer in May. Replenishment of nutrients from the Clyde River sustains a chlorophyll a concentration of 6µg L-1 until September. The productive season ends by November, when day length limits phytoplankton production. Is the seasonal cycle of species consistent with what is expected for a seasonally stratifying temperate shelf sea? The answer to this question is yes. The silicate input of the Clyde River promotes high abundance of diatoms such as Skeletonema, Thalassiosira and Chaetoceros. This results in growth in zooplankton, primarily copepod, populations in May and grazing down of phytoplankton in June. Chlorophyll concentration remains high through summer and reflects a shift to lower numbers of larger diatoms such as Rhizosolenia and the beginning of the peak in dinoflagellates in later summer. Does the microplankton support higher trophic levels? The answer to this question is yes. The Firth of Clyde has been described as a distressed ecosystem, suffering “ecological meltdown” (Thurstan & Roberts, 2010). Heath and Speirs (2011) used less disparate language to demonstrate that the biomass of commercial fish species in the Firth of Clyde was double that prior to trawling in the 1960s, but with a size distribution skewed to small fish. Basking sharks and minke whales visit the Firth of Clyde in summer (McIntyre et al., 2012) Is the Firth of Clyde enriched with anthropogenic nutrients? The answer to this question is yes. TOxN, Silicate and DIP are all enriched compared to shelf break concentrations from the Malin Sea, and DIP is above predicted concentration for the area, taking into account nutrient input from fresh water. DIP enrichment may occur as a result of release of phosphate from particulate phase by increasing salinity and sulphate reduction in sediments of the

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lower estuary. TOxN and Silicate are lower than expected at CMT7 given the input from the Clyde River. Dredging removes a large volume of sediment from the estuary in summer and winter, and the absence of vertical mixing in winter means most of the nutrients liberated from these sediments are probably lost to the sea loch system to the north of CMT7. Has there been a long term change in phytoplankton phenology and biomass? The answer to this question is no. CMT7 has been monitored for chlorophyll a since 1982 and data from the period 2005-2014 is largely within the maxima and minima from 1982-1999. Does the state of the microplankton at CMT7 represent good environmental status (GES)? The phytoplankton productive season in the inner firth of Clyde is long, as a result of permanent stratification and continuous nutrient supply from the Clyde River. It is not however free from controlling factors. Light limitation dictates the onset and suspension of phytoplankton production, and a diverse zooplankton community grazes upon phytoplankton in summer. A long time series of nutrient and chlorophyll data points to a steady improvement in nutrient enrichment, and no evidence of up-rating of the sustainable level of phytoplankton biomass at CMT7. Using expert judgement it is concluded that the seasonal cycle of plankton observed at the site is indicative of GES, though continued work on how to objectively define GES may affect this conclusion. References

Adams, J.A. (1986). Zooplankton investigations in the Firth of Clyde. In Allen, J.A., Barnett, P.R.O., Boyd, J.M., Kirkwood, R.C., Mackay, D.W. and Smyth, J.C. (Eds). The Environment of the Estuary and Firth of Clyde. Proceedings of the Royal Society of Edinburgh 90: 239-254.

Allen, J. H. (1966). On the hydrography of the River Clyde. Coastal Engineering Proceedings 1: 1360-1374.

Balls, P. FRV Clupea Cruise 3/91 report. 3pp. Boney, A.D. (1986). Seasonal studies on the phytoplankton and primary production in the inner Firth of Clyde. In Allen, J.A., Barnett, P.R.O., Boyd, J.M., Kirkwood, R.C., Mackay, D.W. and Smyth, J.C. (Eds). The Environment of the Estuary and Firth of Clyde. Proceedings of the Royal Society of Edinburgh 90: 203-222. Bowers, D. G., Harker, G. E. L., Smith, P. S. D., and Tett, P. (2000). Optical properties of a region of freshwater influence (the Clyde Sea). Estuarine, Coastal and Shelf Science, 50: 717-726.

Connor, D.W., Gilliland, P.M., Golding, N., Robinson, P., Todd, D. and Verling, E. 2006. UKSeaMap: the mapping of seabed and water column features of UK seas. Joint Nature Conservation Committee, Peterborough. 28pp. Devlin, M.J., Barry, J., Mills, D.K., Gowen, R.J., Foden, J., Sivyer, D. and Tett, P. Relationships between suspended particulate material, light attenuation and Secchi depth in UK marine waters. Estuarine, Coastal and Shelf Science. 79: 429-439. Dooley, H. D. (1979). Factors influencing water movements in the Firth of Clyde. Estuarine and Coastal Marine Science 9: 631-641

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Edwards, A., Baxter, M.S., Ellett, D.J., Martin, J.H.A., Meldrum, D.T. and Griffiths, C.R. (1986). Clyde Sea hydrography. In Allen, J.A., Barnett, P.R.O., Boyd, J.M., Kirkwood, R.C., Mackay, D.W. and Smyth, J.C. (Eds). The Environment of the Estuary and Firth of Clyde. Proceedings of the Royal Society of Edinburgh 90: 67-84. Jones, R.A. and Lee, F.R. (1981). The significance of dredging and dredged material disposal as a source of nitrogen and phosphorous for estuarine waters. In Neilson, B.J. and Cronin, L.E. Estuaries and Nutrients. Humana Press, Clifton, New Jersey. 517-530. Kirk, J. T. O. (1994). Light and photosynthesis in aquatic ecosystems (2nd ed.). Cambridge University Press. McIntyre, F. Fernandes, P.G., and Turrell, W.R. (2012). Clyde Ecosystem Review. Scottish Marine and Freshwater Science 3. 123pp. Poodle, T. (1986). Fresh water inflows to the Firth of Clyde. In Allen, J.A., Barnett, P.R.O., Boyd, J.M., Kirkwood, R.C., Mackay, D.W. and Smyth, J.C. (Eds). The Environment of the Estuary and Firth of Clyde. Proceedings of the Royal Society of Edinburgh 90: 55-66. Rippeth, T.P. and Jones, K.J. (1997). The seasonal cycle of nitrate in the Clyde Sea. Journal of Marine Systems 12: 299-310. Roden, E.E., and Edmonds, J.W. (1997). Phosphate mobilization in iron-rich anaerobic sediments: microbial Fe (III) oxide reduction versus iron-sulfide formation. Archiv für Hydrobiologie, 139: 347-378. Ross, D., Thompson, K.R., and Donnelly, J.E. (2009). The State of the Clyde: Environment Baseline Report. SSMEI Clyde Pilot Project. 100pp. Scherer, C. and Gowen, R. (2013). Determining the status of the microplankton community in the western Irish Sea. EFF Project: Ecosystem Based Management of Irish Fisheries and other resources. Work Package 4. (CA/033766/11) Tivy, J. (1986). The geography of the Estuary and Firth of Clyde. In Allen, J.A., Barnett, P.R.O., Boyd, J.M., Kirkwood, R.C., Mackay, D.W. and Smyth, J.C. (Eds). The Environment of the Estuary and Firth of Clyde. Proceedings of the Royal Society of Edinburgh 90: 7:24. Townson, J.M. and Collar, R.H.G. (1986). Water movement and simulation of storm surges in the Firth of Clyde. In Allen, J.A., Barnett, P.R.O., Boyd, J.M., Kirkwood, R.C., Mackay, D.W. and Smyth, J.C. (Eds). The Environment of the Estuary and Firth of Clyde. Proceedings of the Royal Society of Edinburgh 90: 85-96.

SEPA


community in the Firth of Forth, Scotland

Malcolm Baptie

March 2014

Summary This document describes an assessment of a SEPA monitoring location in the Firth of Forth, designated ‘Gunnet Ledge, South of Kinghorn’. It is a single location monitored at monthly frequency for the principal purpose of reporting chemical and phytoplankton determinands in support of assessment of ecological status for the Water Framework Directive (WFD). It is the only location in the Firth of Forth that is monitored continually, though other sites in the outer firth are monitored on a 1 year in 3 basis. The aim of the study was to determine whether the state of microplankton at the site is representantive of Good Ecological Status (GES) for the purposes of the Marine Strategy Framework Directive (MSFD). Determination was made by expert judgement as recommended by the MSFD pelagic subgroup. This document follows the approach of Scherer and Gowen (2013). All data reported are held by SEPA, with the exception of North Sea data in table 2, which is from the European Environment Agency ‘waterbase’ coastal nutrient dataset (http://www.eea.europa.eu/data-and-maps/data/waterbase-transitional-coastal-and-marine-waters-8). Stratification in the vicinity of Gunnet Ledge begins in spring and continues into summer. Salinity and tidal state regulate stratification in spring, and temperature becomes and important factor in summer. The Firth of Forth is a region of freshwater influence due to the proximity of the Forth Estuary and the enclosed nature of the firth. Anthropogenic nutrient enrichment is a feature of the firth, given the high concentration of domestic and industrial activity with its catchment. Both Silicate and Phosphorous concentrations in winter above predicted values. Microplankton growth begins in spring when water column stability matches the euphotic zone depth. Diatoms dominate in spring, before dinoflagellate abundance increases in summer. Autumn bloom phytoplankton is dominated by diatoms but of different species to those found in summer.

http://www.eea.europa.eu/data-and-maps/data/waterbase-transitional-coastal-and-marine-waters-8

http://www.eea.europa.eu/data-and-maps/data/waterbase-transitional-coastal-and-marine-waters-8

Physical oceanography of Gunnet Ledge Introduction The Firth of Forth is an inlet covering 670km2 from Queensferry to the Isle of May on the east coast of Scotland, which connects to the North Sea. In the inner firth between Queensferry and Kirkcaldy the depths in the northern channel reach 20-30m below chart datum, while the larger area covering waters to the south of Inchkeith island between Edinburgh and North Berwick is generally 5-15m below chart datum. The outer firth off East Lothian and to the south of the Isle of May reaches depths greater than 50m. The monitoring location, Gunnet Ledge, is situated near to navigation buoy 10 in approximately 25m of water on the north channel used by merchant vessels to access Grangemouth refinery. Figure 1 illustrates its location.

Figure 1: Gunnet Ledge sampling site (red star) in the Firth of Forth. Blue boundaries are SEPA coastal water bodies. The river Forth and Forth estuary transport fresh and brackish water into the firth and influence salinity, and tidal mixing in the inner region of the firth affects the extent to which seasonal stratification can occur. Figure 2 illustrates salinity with increasing distance from the transitional water body that is located to the west of the Forth Rail Bridge. The range of salinities encountered at Gunnet Ledge inidicates the freshwater influence as well as the tidal influence given the maximum salinity at the site does on occasion nearly approach the average salinity of coastal sampling locations outside the firth. Mean salinity at Gunnet Ledge indicates it contains 6.48% freshwater, compared to oceanic salinity of 35.5.

Figure 2: 1-10m depth average salinity at selected locations with increasing distance from the Inner Forth transitional water body. Red lines are minimum and maximum. First data point minimum salinity is off y-axis scale of graph at 17.97. Flow The mean low water volume of the firth of forth from Stirling to the Isle of May is 1.7*1010 m3 and the mean high water volume is 2.1*1010 m3 meaning there is daily tidal transport in and out of the firth of ~0.4*1010 m3 (FRPB, 1978). Flow rates of 10-300m3 s-1 (average 63m3 s-1) from the Forth river into the estuary demonstrate the substantial volume of fresh water which enters the firth (Balls, 1992). There is a gradient of spring tide excursion distances from 8km in the transitional zone to the west of the rail bridge to 3km offshore of Edinburgh reflecting the dissipation of tidal energy across the greater volume of the firth with increasing distance from the mouth of the river. Seasonal cycle of temperature and salinity and stratification A mooring at Gunnet Ledge has been in operation since 2003, which has recorded temperatures ranging from 3.5°C in the cold winters of 2010 and 2011 and 2013 to a high of 18.5°C in August 2008 (this pre-dates routine QC of thermometers on YSI6600 CTDs so may be doubtful). More usual minima and maxima are on the order of 5-15°C (Figure 3). The influence of the tidal cycle and its interplay with river flow is obvious on salinity (Figure 4). In spring salinity is the most important stabilising factor, and by summer, temperature becomes more important. The spring-neap cycle of the tides either disrupts or promotes stabilisation. Figures 5, 6 and 7 illustrate the progression of temperature and salinity vertical variability through the years 2011, 2012 and 2013.

Figure 3: Temperature from the mooring on Forth Navigation Buoy 10 near to Gunnet Ledge sampling site. Values taken at 30 minute intervals parsed to 15 minutes.

Figure 4: Salinity from the mooring on Forth Navigation Buoy 10 near to Gunnet Ledge sampling site. Values taken at 30 minute intervals parsed to 15 minutes. Salinity data sparse before 2007.

Figure 5: Temperature, Salinity and Chlorophyll a profiles at Gunnet Ledge in 2011. Change in salinity is the dominant mode of stratification, but tidal effects are disguised in these monthly plots. Chlorophyll a in 2011 reached its maximum in August, following a period of thermal stratification.

Figure 6: Temperature, Salinity and Chlorophyll a profiles at Gunnet Ledge in 2012. As observed in 2011, thermal stratification was not present in spring, but developed a modest gradient in July and August. Maximum chlorophyll a was observed in May, and a subordinate bloom was observed in October, both associated with haloclines.

Figure 7: Temperature, Salinity and Chlorophyll a profiles at Gunnet Ledge in 2013. Chlorophyll maximum in May (April sonde failure) was associated with halocline, weak thermal stratification in summer. The roles of salinity and tidal cycle in regulating stratification confirms that Gunnet Ledge is situated in a ‘Region of Freshwater Influence’ (ROFI). The Forth Estuary – Firth of Forth – North Sea gradient in salinity reflects the hydromorphological characteristics of the firth. Sub-surface light climate Photosynthetically active radiation (PAR) profiles of the upper 10 metres at the site indicate a variable light regime (Figure 8), which is likely to be as a result of transport by river and tide of suspended particulate matter. Light attenuation coefficient, Kd, estimates varied considerably, as did the corresponding euphotic zone depth (Table 1).

Figure 8: Profiles of photosynthetically active radiation expressed as a percentage of irradiance lost compared to on deck reading. Table 1: Estimates of light attenuation coefficient of PAR, with corresponding estimate of euphotic zone depth.

Month Kd (m-1) Euphotic Zone Depth (m)

Jun12 0.4219 10.9

Jul12 0.3404 13.3

Oct12 0.3698 12.4

Jan13 1.2482 3.7

May13 0.4538 10.1

Jun13 1.1522 4.0

Aug13 0.5991 7.7

Sep13 0.6512 7.1

Seawater density (σT) calculated from pressure, salinity and temperature profiles at three sites in the Firth of Forth indicates variability in the onset of stratification, and the depth of the upper mixed layer. With increasing distance from Queensferry, the effect of tide and estuarine water on the stability of the water column diminishes (Figure 9).

Figure 9: Seawater density profiles at Gunnet Ledge (Black lines), Fairway Navigation Buoy (11km further towards North Sea, red lines) and south of the Isle of May (39km further towards North Sea, blue lines). Profiles are from 2011.

Dissolved Inorganic Nutrients Gunnet Ledge has a seasonal cycle of total oxidised nitrogen intermediate between that observable in the outer firth to the south of the Isle of May, and in the estuary to the west of the bridges (Figure 10). Nutrient drawdown defined as the time at which nutrient concentrations in the upper mixed layer reaches 50% of winter (DJFM) maximum concentration has occurred between April and May in the period 2007-2013. The majority of winter TOxN was removed from the top 10 metres between May and August. Nutrient increase occurs at the same time as autumn halocline development, suggesting that the difference in typical TOxN concentration in September and October when comparing Gunnet Ledge with the Isle of May is more likely to be as a result of increased flow from the estuary after summer than from mixing following the breakdown of summer stratification, which can be variable in its extent (Figures 5-7). This likely also explains the marginally higher concentration in the outer firth in August (though data for this site are compared to Gunnet Ledge sparse).

Figure 10: Averaged TOxN from 1-10m depth at the Forth Estuary 5km west of the bridges (2000-2013), Gunnet Ledge (2005-2013), and south of the Isle of May (2003-2013).

Freshwater TOxN in has increased since the 1980s (Balls et al, 1996) but saltwater concentrations showed no trend to the 1990s (SEPA, 2000). A polynomial model fitted to the time series of TOxN data at Gunnet Ledge indicated a trend was present, which is suggestive of a cyclical component to interannual variation (Figure 11). If there is such a component, the time series is too short to describe a full cycle. The period 2011-2013 appears to show year on year increase in winter TOxN from approximately 6 to 9µMol L-1, though a concentration of this order was observed in 2008. There was no obvious drop in salinity that would indicate conspicuously higher river influence at Gunnet Ledge over the winters of 2011-13 (Figure 4).

Figure 11: Time series of 1-10m average TOxN at Gunnet Ledge. Trend is random walk model from which seasonality has been subtracted. The seasonal cycle of DIP drawdown and replenishment indicated as with TOxN that Gunnet Ledge is intermediate in the extent to which DIP declines between the estuary and the outer firth (Figure 12). The drop in concentration in October is likely occurring because DIP in the surface depths is utilised by autumn bloom phytoplankton (e.g. Figure 6). A similar feature is observable in the seasonal cycle of silicate, though concentrations remain above 2µmol L-1 in summer (Figure 13). The very much higher concentrations of all three nutrients in the Forth estuary serve to explain the gradient in concentration as sampling moves further from Queensferry. Gunnet Ledge can be composed of 3.2% to short periods of up to 33.5% fresh water depending on tide and river flow, which is why nutrient concentrations are higher here than in the outer firth. In the outer firth, river influence persists but is dispersed by the greater volume of water.

Figure 12: Averaged DIP from 1-10m depth at the Forth Estuary 5km west of the bridges (2001-2013), Gunnet Ledge (2005-2013), and south of the Isle of May (2003-2013).

Figure 13: Averaged Silicate from 1-10m depth at the Forth Estuary 5km west of the bridges (2001-2013), Gunnet Ledge (2005-2013), and south of the Isle of May (2002-2013). 90th percentile winter nutrient concentrations aggregated across all sampling locations in the Forth estuary where salinity was equal to or less than 1 are listed in table 2, compared to estimates of nutrient concentrations in the North Sea (1980-2011 data from EEA waterbase dataset within 57-60°N, 1 to -1°E). Table 2: Comparison of marine and estuarine contribution to winter nutrients at Gunnet Ledge. Concentrations are 90th percentile. North Sea values from EEA waterbase dataset. Estuarine data restricted to samples where salinity was less than or equal to 1.

Concentration Contribution Gunnet Ledge

Forth Estuary North Sea Estuarine (6.5%)

Marine (93.5%)

Predicted Measured

TOxN 51.97 9.9 3.37 9.25 12.62 7.61

DIP 2.94 0.8 0.19 0.75 0.94 1.42

Silicate 75.41 4.8 4.90 4.49 9.39 19.0

TOxN:DIP 13.42 5.35

TOxN:Si 1.34 0.40

What is apparent from the comparison of predicted versus measured is supply of TOxN is not adequately described by measurements from the top 10 metres of the water column. Balls (1994) suggested that the sinks in a slow flushing estuary such as the Forth means the riverine component of the coastal nutrient load is too variable to be estimated accurately. Both DIP and Silicate concentrations at Gunnet Ledge exceed predicted values, suggesting an underestimate of the

freshwater contribution. DIP is removed to particulate phase in low salinity, and resorbed to dissolved phase in high salinity (SEPA, 2000), so the concentrations from the upper estuary are likely to be an underestimate. Losses through dilution in the Forth Estuary are abrupt, but Firth of Forth values are in line with those reported by Balls (1992), suggesting the estuarine input is an underestimate. Measured Silicate was 102% greater than predicted, which suggests either major underestimation of the input from the river, where silt transported out of the estuary may act as a sink to dissolved concentrations within the estuary that is subsequently available in the firth, or temperature dependent dissolution may reduce measured concentration in the upper estuary where winter water temperature of 1.5°C is not uncommon.

Using the North Sea (mean salinity 34.57) and estuarine (mean salinity 0.24) nutrient concentrations in table 2 to construct a theoretical salinity nutrient relationship line for each nutrient visualises these differences. TOxN was concentrations lower than predicted, Silicate concentrations higher than predicted, and estimation close to the theoretical line for DIP (Figure 14).

Figure 14: Comparison of theoretical salinity nutrient mixing relationships (dashed lines) and measured concentrations (blue points) averaged over 1-10m depth at Gunnet Ledge (2005-2013 data).

Summary At Gunnet Ledge there is a seasonal cycle of TOxN, DIP and Silicate at 1-10m depth. Maximum monthly mean TOxN (6.25µmol L-1) and Silicate (10µmol L-1) occur in March and maximum monthly mean DIP (1.05µmol L-1) occurs in October. Rapid removal (<0.7µmol L-1 TOxN, <0.5µmol L-1 DIP, <3µmol L-1 Silicate) occurs in May and persists until August. Concentrations of all three nutrients increase by the end of summer. Silicate and DIP concentrations are elevated compared to near North Sea concentrations. The most plausible source of additional nutrients is the river Forth and Forth Estuary. Winter TOxN concentrations have not increased since the 1980s. Microplankton The season cycle of biomass at Gunnet Ledge There is a recurrent spring bloom at Gunnet Ledge in April and May. A feature of the location is the repeated interruption of growth in spring by the action of the tidal cycle, which can be observed as successive peaks within the period of elevated chlorophyll in spring (Figure 15). The tidally agitated environment of the mooring makes optical estimates of chlorophyll valuable only as a guide to seasonal change and the absolute values recorded are not reliable estimates of biomass. Chlorophyll measured by acetone extraction and fluorometry indicates mean concentration peaks at 5µg L-1 in May, and declines to 2µg L-1 by August.

Figure 15. Chlorophyll a from the YSI6600 CTD mooring at Forth Navigation Buoy 10, near to Gunnet Ledge sampling location. Microplankton species abundance and composition Peak chlorophyll in May is reflected in peak phytoplankton abundance in April, with a subordinate peak in June. Including microflagellates indicates a summer bloom in this group. It is not currently possible at SEPA to resolve these into autotrophic and heterotrophic groups. Excluding this group, diatoms were the most abundant group in spring (Figure 16), though even in the short time for which phytoplankton sampling has been undertaken at this location there has been considerable

variability in the constitution of the top ten most abundant taxa within this group (Table 3). Dinoflagellate abundance peaks in summer with occasional high abundance in spring of indeterminate small unarmoured forms. Biomass is not determined by functional groups at SEPA. Table 3. Top ten most abundance diatoms during spring (April-May) from 0-10m depth at Gunnet Ledge. Values are cells L-1.

Species 2010 2011 2012 2013

Asterionellopsis glacialis 4300 - 660 -

Cerataulina pelagica 680 - 2424 -

Chaetoceros subgenus Hyalochaete 33960 146580.5 - -

Ceratoneis closterium/Nitzschia longissima 15826.67 7333 3345 6713.54

Dactyliosolen fragilissimus - - 2051 -

Guinardia delicatula - - 19131.5 7832.46

Gyrosigma/Pleurosigma sp. - - 720 2024.5

Indet. araphiated pennate diatom 20-50µm - 400 - 2070

Indet. centric diatom 20-50µm 1480 - - 3449

Indet. raphiated pennate diatom <20µm 1420 999 - 3916.23

Indet. raphiated pennate diatom >50µm - - 812.5 -

Indet. raphiated pennate diatom 20-50µm 8680 5666 - 26143.465

Leptocylindrus danicus - 600 - 9510.84

Paralia sulcata - - - 20696

Pseudo-nitzschia sp. <5µm 107546.7 6766.5 6040 -

Pseudo-nitzschia sp. >5µm - 450 800 -

Rhizosolenia setigera - - - 675

Skeletonema sp. 473546.7 2666 20800 -

Thalassiosira 10-50µm 38360 29266 - -

Figure 16. Seasonal cycle of microplankton at Gunnet Ledge, average monthly abundances between 2010 and 2013. Long term trends Gunnet Ledge monitoring began relatively recently at monthly resolution, however a time series of chlorophyll data in the 1990s was collected by SEPA and its precursor body the Forth River Purification Board at a sampling station, CSP04A, 700 metres from Gunnet Ledge, focusing on the period between April and October (Dobson et al, 2001). Seasonal maximum and minimum envelopes were constructed from these data and contemporary chlorophyll a concentrations from Gunnet Ledge were overlaid on these. From this it was concluded the chlorophyll a concentrations observed at Gunnet Ledge between 2008 and 2013 were largely in line with the range of values encountered in the 1990s (Figure 17). Of note was the small number of recent points outside the envelope in October, suggesting the productive season may have lengthened in the 2000s and 2010s. This would require further monitoring to confirm, and the conclusion from this comparison is that there has been no long term change in the concentration of chlorophyll in the vicinity of Gunnet Ledge since the 1990s.

Figure 17: Comparison of chlorophyll a concentrations over the period April to October from the nearby ‘CSP04A’ sampling station (dark lines represent maximum and minimum monthly concentration) and contemporary data from Gunnet Ledge (green points). Zooplankton Planktonic copepod abundance peaked in May and again in August in 2013 which coincided with the periods of highest chlorophyll concentration (Figure 18). Copepods in total numerically dominate the zooplankton at Gunnet Ledge, with appendicularians, barnacle larvae, gastropods, bivalve larvae and polychaete larvae also being abundant species. Gelatinous plankton are often found in summer but in low numbers and as biomass is not estimated, do not appear in Figure 19. The zooplankton community in August is more diverse than in May when it is chiefly composed of calanoid copepods. Calanoid copepods found often at Gunnet Ledge are Acartia clausi, Acartia discaudata, Pseudocalanus miinuts-elongatus, Paracalanus parvus, Calanus finmarchicus, Calanus helgolandicus, Centropages hamatus and Temora longicornis. The brackish water species Eurytemora affinis is occasionally observed in samples but is not thought to survive in the Firth of Forth east of Queensferry. Cyclopoid copepods are mostly Oithona spp. Harpacticoid copepods are mostly Microsetella norvegica.

Zooplankton monitoring has only recently been undertaken at monthly intervals in the context of the marine monitoring programme at SEPA, so long term trends are not calculable at present. The species found are similar to those found by Taylor (1983), though this study focused on the estuary rather than the firth.

Figure 18: Total abundance of planktonic copepods at Gunnet Ledge in 2013.

Figure 19: The 8 most abundant groups in the zooplankton at Gunnet Ledge in 2013 as a proportion of total zooplankton abundance.

Assessing the state of the microplankton Does the Firth of Forth represent a distinct ecohydrodynamic region? The answer to this question is yes. The input of the Forth estuary and the action of the tide results in a stratification regime that is distinct from the seasonally stratifying North Sea, to which the Firth is adjacent. Stratification is governed by the interaction between tide and river flow and occurs between 5-15m depth. The euphotic zone corresponds with this depth range. Gunnet Ledge, SEPA’s long term monitoring location is situated in the inner firth, and is more obviously subject to river influence than periodically monitored outer firth locations. Is the seasonal pattern of dissolved inorganic nutrients consistent with current understanding of biogeochemical cycling in shelf seas? The answer to this question is yes. Nutrients are at their maximum in later winter, and there is a rapid drawdown in nutrient concentrations in spring, which remain low in surface waters until late summer. The seasonal pattern of nutrient follows the expected pattern of uptake by phytoplankton in spring, isolation of bottom water in summer, and replenishment as mixing restocks surface waters at a rate that exceeds uptake by phytoplankton in autumn. At Gunnet Ledge, both TOxN:DIP and TOxN:Si ratios are approximately 40% of Redfield ratio, suggesting the area is N limited. Is the seasonal cycle of microplankton production and biomass consistent with current understanding of the processes controlling microplankton biomass and production in shelf seas? The answer to this question is yes, however the freshwater influence of the Forth estuary and the narrowing of the Firth towards Queensferry affects the degree and duration of stratification to an extent that would not be observed on an unenclosed stretch of coastal water. Nonetheless, a spring bloom in phytoplankton permitted by the combination of suitable subsurface light climate, water column stability and nutrient availability is a recurrent feature observed each year through data from a monitoring buoy near to the location of Gunnet Ledge. Additional mixing late in summer deepens the surface mixed layer and affords an autumn phytoplankton bloom. Chlorophyll concentrations are modest, at approximately 5µg L-1 in May. This increase in chlorophyll is coincident with the decrease in surface nutrients observed in summer. Is the seasonal succession of species consistent with what is expected for a seasonally stratifying temperate shelf sea? The answer to this question is yes. Diatoms dominate the spring bloom and are composed chiefly of genera such as Skeletonema, Thalassiosira, Pseudo-nitzschia and Chaetoceros that are the normal components of a temperate shelf sea spring bloom. Dinoflagellates reach greater abundance in summer. These are mostly small indeterminate unarmoured forms, though representatives of Prorocentrum, Protoperidinium, Gyrodinium, Dinophysis and Ceratium are also seen in greater abundance at this time of year. In late summer and autumn, larger diatoms such as Rhizosolenia, Cerataulina are in greater abundance. This follows the succession of phytoplankton as described by Margalef (1967).

Does the microplankton support higher trophic levels? The answer to this question to this question is yes. Since the 1980s there has been an increase in the diversity and abundance of fish species found in trawls (SEPA, 2002). Is the Firth of Forth enriched with anthropogenic nutrients? The answer to this questions is yes. Both DIP and Silicate are enriched relative to both near North Sea waters, and to the predicted concentration taking into account the nutrient load of the Forth Estuary and its contribution to water at the Gunnet Ledge sampling location. DIP enrichment may occur as a result of removal to particulate phase in the estuary and subsequent return to DIP in the higher salinity of the Firth, however this is balanced against dilution in the greater volume. Silicate enrichment is less easily explained. It is possible that as water enters the Firth its temperature increases and so does the solubility of silicate, or it may be the case that sedimentary input from estuary to firth makes available additional silicate that is not dissolved at sites in the upper estuary where nutrient samples are taken. Has there been a long term change in phytoplankton phenology and biomass? The answer to this question is likely to be no. Looking at data from the 1990s from a nearby sampling station indicates that there is no evidence of an up-rating of the normal maxima and minima of chlorophyll at Gunnet Ledge. There may be evidence of an increase in chlorophyll towards the end of the productive season, but more data are required to confirm this to be the case. Does the state of the microplankton at Gunnet Ledge represent good environmental status (GES?) While there is obvious enrichment of nutrients as a result of the proximity of the sampling site to the Forth river, it does not appear to have resulted in perturbation to the expected seasonal cycle of microplankton in a seasonally stratified temperate sea. Using expert judgement it is concluded that the seasonal cycle of plankton observed at the site is indicative of GES, though continued work on how to objectively define GES may affect this conclusion. Lifeform state space plots Life form pairs according to Scherer and Gowen (2013) are presented below for data from Gunnet Ledge. Data from 2010-13 used for phytoplankton lifeform pairs and 2012-13 for zooplankton lifeform pairs, as these periods have the most continuity in sampling for the respective groups.

Biodiversity descriptor (D1) Life form pair: diatoms and dinoflagellates (2010-2013).

Biodiversity descriptor (D1) Lifeform pair: Holoplanktonic crustaceans and non gelatinous, non crustacean holoplankton (2012-2013).

Food-webs descriptor (D4) Lifeform pair: Chlorophyll and Zooplankton (2012-2013).

Food-webs descriptor (D4) Lifeform pair: Large (>20µm) and small (<20µm) phytoplankton (2010 - 2013).

Food-webs descriptor (D4) Lifeform pair: Large (>2mm) and small (<2mm) copepods (2012-13).

Eutrophication descriptor (D5) Lifeform pair: Diatoms and autotrophic, mixotrophic dinoflagellates (2010-2013).

Eutrophication descriptor (D5) Lifeform pair: Pseudo-nitzschia spp. excluding P. delicatissima and toxin producing dinoflagellates (2010-2013).

Sea floor integrity descriptor (D6) Lifeform pair: Holoplankton and meroplankton, excluding fish larvae (2012-2013).

Sea floor integrity descriptor (D6) Lifeform pair: Pelagic diatoms and tychopelagic diatoms (2010-2013).

References

Balls, P.W., Brockie, N., Dobson, J., Johnston, W. (1996). Dissolved Oxygen and Nitrification in the Upper Forth Estuary During Summer (1982–92): Patterns and Trends. Estuarine, Coastal and Shelf Science 42(1), 117-134.

Balls, P.W. (1992). Nutrient behaviour in two contrasting Scottish Estuaries, the Forth and Tay. Oceanologica acta, 15(3).

Balls, P.W. (1994). Nutrient inputs to estuaries from nine Scottish east coast rivers; influence of estuarine processes on inputs to the North Sea. Estuarine, Coastal and Shelf Science. 29(4), 329-352.

Dobson, J., Edwards, A., Hill, A., & Park, R. (2001). Decadal changes in the Forth Estuary and Firth of Forth in relation to the North Sea 1980–2000. Senckenbergiana maritima, 31(2), 187-195.

Forth River Purification Board (1978). The Physical dimensions of the Firth of Forth and Forth estuary. Report ES 3/78.

Margalef, R. 1967. Some concepts relative to the organisation of plankton. Oceanography and Marine Biology, 5: 257-289.

SEPA (2000). Water Quality in the Forth Estuary 1980-1999. Report TW07/00. 26pp.

Taylor, C.J.L. (1983). The zooplankton of the Forth. Report ES 3/83, 40pp.

Assessment of current environmental status of the Plymouth L4 site (Western

English Channel), a MSFD sentinel site for seasonally stratified shelf waters:

March 2014

Angus Atkinson, Claire Widdicombe, Rachel Harmer, Andrea McEvoy, Elaine Fileman,

Penelope Lindeque

Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, Devon, UK

Summary assessment

The plankton at the “L4” time-series site, approximately 16 km SW of Plymouth, has

been sampled typically weekly since 1988. This forms an ongoing monitoring

programme supported by NERC National Capability. The status of the site (physics,

chemistry and biology) has been described in detail in several recent reports, and this

document is a highly summarised version of these. The site is sufficiently far offshore

to be stratified and nutrient stressed in summer, although it receives variable

freshwater influence from the coast. The seasonal cycle of the plankton is

characterised by spring blooms of diatoms and in some years Phaeocystis, followed by

later summer and autumn blooms of autotrophic and heterotrophic dinoflagellates,

plus coccolithophores in some years. The zooplankton also shows spring and autumn

increases, being dominated by holoplankton (mainly suspension feeding copepods).

Merozooplankton, however, are also important, comprising ~40% of metazoans during

spring. While these general patterns among functional groups can be described, their

species composition and abundance vary greatly from year to year. This is possibly

related to the dynamics of the site, with variable riverine nutrient input and larger scale

influxes of more oceanic waters with the prevailing SW winds. These dynamics are

illustrated by two “extreme” weather events in the last few years. Firstly, the very wet

summer of 2012 increased nutrient inputs from flood waters, and the diatom bloom was

exceptionally long lasting that season. Secondly the stormy 2013/2014 winter led to an

unusual incursion of the warm water, oceanic nitrogen-fixer Trichodesmium. This great

natural variability between years tends to mask more gradual changes associated with

a warming climate, although some shifts, such as increasing magnitude of the autumn

blooms compared to those in spring, have been recorded. All of the above changes

have been interpreted in the context of a variable and changing climate, and we cannot

detect any evidence of superimposed pertubations, such as influences from pollution

events or introduced species. Our expert interpretation is that the L4 site, while

perhaps not pristine, nevertheless represents “Good Environmental Status” for

transitionally stratified waters of the Western English Channel.

L4 sampling

Sampling of the L4 site (50°15′N 4°13′W) is ongoing, with weekly sampling by Plymouth

Marine Laboratory, PML since 1988 (Harris, 2010 and other papers in that issue;

http://www.westernchannelobservatory.org.uk/). Samples are taken every Monday morning

(weather permitting) at the site ~16 km southwest of Plymouth. The Status of the L4

plankton, including seasonality, natural variability and long term changes, have been

described by Harris (2010) and other papers in that Special Issue, Widdicombe et al. (2012)

and Atkinson et al. 2013). This report contains only a brief summary, update and synthesis

of these findings.

Weekly zooplankton and surface temperature sampling at L4 started in March 1988 and an

increasing array of measurements have been added since then. Copepod egg production

measurements, chl a, plus microplankton composition (from 10 m depth only) started in

1992. Full water column profiling with additional variables, including nutrients, began in the

2000s. These profiles included flow cytometric measurements of nano- and picoplankton

from 2007 (Widdicombe et al. 2012).

For the microplankton cell count data, water samples are collected at 10 m depth and lugols-

preserved aliquots are allowed to settle before counting under a microscope (Widdicombe et

al. 2010, 2012). The counts are done in a consistent manner by a skilled analyst, with quality

control measures in place. For the zooplankton, two replicate tows are made using a WP-2

net (56 cm diameter, 200 μm mesh) towed vertically from the seabed at ca. 50-m depth to

the surface. Mesozooplankton are identified and counted from catch fractions (currently by

two trained analysts, each enumerating one net haul and then averaging the results). For

some taxa, particularly copepods such as Calanus helgolandicus, identification is to species

level, with additional information on sex and life stages. All L4 samples are analysed at PML

and the data are available through BODC and the above Western Channel Observatory

website.

Environmental setting

Station L4 is continually affected by the tide, which is associated with an interplay of regular

estuarine outflow from Plymouth Sound and oceanic waters coming in with the dominating

southwesterly winds. The water column is weakly stratified from mid-April to September and

mixed during winter (Smyth et al. 2010); the minimum and maximum surface temperatures

occur in March (9.1°C) and August (16.4°C), respectively.

In line with observations around the UK shelf seas, the western English Channel has

warmed by ~0.6°C per decade over the past 20 years. The greatest temperature rises

followed a period of reduced wind speeds and enhanced surface solar irradiation during the

2000’s (Smyth et al., 2010).

http://www.westernchannelobservatory.org.uk/

Seasonality

The seasonal cycle of the phytoplankton community is characterized by spring diatom and

autumn dinoflagellate blooms, but there is high interannual variability in abundance and

floristic composition (Widdicombe et al., 2010). For example in some years there are

pronounced spring blooms of Phaeocystis or autumn blooms of the coccolithophore

Emiliania huxleyi. The nanoplankton also increase during the summer months, although they

have a much lower seasonal amplitude of change (5-fold) than more variable taxa suich as

autotrophic dinoflagellates or Phaeocystis (over 1000-fold). Microzooplankton also show a

transition from low abundance in winter months to a peak in the summer, with ciliates often

peaking earlier (May) than autotrophic dinoflagellates (July).

The seasonal cycle of mesozooplankton abundance is characterized by high values from

April right through to October, and in common with phytoplankton there is often a spring and

an autumn peak. Interestingly, both holo and meroplankton often start increasing in numbers

in spring before the increase in their microplankton food. The mesozooplankton community

at L4 is dominated by copepods, which typically form around 90% of total during the winter.

In summer this percentage drops to ~50% when meroplanktonic larvae and non-crustacean

groups peak strongly (Eloire et al., 2010). Meroplankton larvae play an important role at L4

right through the productive season. Cirripedes are particularly abundant in March and April

(Highfield et al., 2010). By contrast other groups, such as echinoderms, bivalves and

gastropods reach maximum abundance in late summer or autumn, when the contribution of

predatory taxa is also highest. These predators contribute significantly to the non-crustacean

holoplankton component, with chaetognaths and siphonophores particularly numerous and

peaking often during late summer.

Decadal scale changes

The pronounced inter-annual variability at L4 tends to obscure clear evidence for longer term

change. Among the phytoplankton, there is some evidence that in recent years the

importance of the autumn bloom has increased, relative to that of the spring bloom. Also

there is some evidence for a phenological shift among the microzooplankton during the

warmer (predominantly later) years. Between 1992 and 2005, the peak in average monthly

microzooplankton abundance varied from June - August whereas since 2006, the peak in

average abundance has occurred in May.

Over the whole sampling period, trends in total mesozooplankton reflect those of the

dominant component, the copepods. These show typically an irregular pattern, often of 2-5

year periods of successive negative and positive anomalies. The last couple of years of data

seem to reflect the start of the latest downturn in this cycle. While the 23 years of data may

be too short to reveal firm evidence of multi-decadal or longer-term trends, the fine resolution

weekly sampling of zooplankton, phytoplankton, and nutrients at L4 captures well the

complex variation in phenology as well as inter-annual and sub-decadal periodicity.

Inter-annual variability

These long term trends tend to be obscured by pronounced inter-annual variability. The

various functional groups can broadly be categorised into 3 classes according to the extent

of their interannaual variability. Most constant are the nanoplankton and the predominantly

herbivorous holozooplankton, whose annual mean abundance over the last 20 years has

varied less than fourfold. A large intermediate category of functional groups have varied from

6-11 fold in the same time period. This group comprises Diatoms, cocolithophores, ciliates,

heterotrophic dinoflagellates, meroplankton (excluding fish larvae) and carnivorous

zooplankton. Most extreme are Phaeocystis and autotrophic dinoflagellates, which have

varied over 20 fold. Among these groups the extent of this inter-annual variation relates very

strongly and positively to the amplitude of their seasonal variability in abundance.

This inter-annual variability has been well illustrated during the last couple of seasons. The

2012 summer was exceptionally wet and the L4 site received regular signals of flood water

discharge from the rivers Tamar and Plym which discharge at Plymouth (these signals were

best documented by the autonomous buoy at the site). The diatom bloom during this season

was also exceptionally large and long lasting, over twice the average biomass of any value

in a previous year. A likely explanation is the increased availability of nutrients injected into

the system in the freshwater inflow. Another example is the exceptionally stormy winter of

2013/2014 with prolonged periods of strong S or SW airflow rapidly transporting surface

waters large distances. This was associated with increased incidence of the nitrogen fixing

Trichodesmium, traditionally thought to be a warmer water, oceanic species.

Status of the L4 site

These examples of inter-annual variability are all explainable as “natural events” (i.e. they

can be set in the context of unusual/extreme weather patterns or a changing climate). There

has been no evidence of invasive, potentially deleterious species such as the Ctenophore

Mnemiopsis leidyi becoming established at the L4 site, to the possible detriment of other

species. Very rare species are likely to be missed by the microscopic methods that we have

used now for decades to maintain the necessary continuity in monitoring. This was

investigated by a recent study comparing zooplankton taxonomic richness based on the

standard microscopic time series analysis and Next Generation sequencing (Lindeque et al

2013). Although this molecular study revealed over twice as many taxa as the microscope

counts, the extra taxa were either very rare, parasitic, or difficult to identify under a

microscope, as is the case for many meroplanktonic larvae.

In summary, there is no evidence of stressors on the plankton that cannot be assigned to

climatic or weather events, we assign the Plymouth L4 site as being in “Good Environmental

Status”.

References

Atkinson A, Fileman E, Widdicombe C, Harmer, R, McEvoy A, Harris R, Smyth T (2013)

Plymouth L4 (Site 48) In O’Brien TD, Wiebe PH, Falkenhaug T (eds) ICES Zooplankton

Status Report 2010/2011. ICES cooperative Research Report No. 318, pp 127-131

Eloire D, Somerfield PJ, Conway DVP, Halsband-Lenk C, Harris R (2010) Temporal

variability and community composition of zooplankton at station L4 in the Western Channel:

20 years of sampling. J Plankton Res 32: 657-679

Fileman, E, Petropavlovsky, A., and Harris, R. (2010) Grazing by the copepods Calanus

helgolandicus and Acartia clausi on the protozooplankton community station L4 in the

Western English Channel. Journal of Plankton Research, 32, 709-724

Harris RP, The L4 time series: the first 20 years. Journal of Plankton res 32: 577-583

Highfield JM, Eloire D, Conway DVP, Lindeque PK, Attrill MJ, Somerfield PJ (2010) seasonal

dynamics of meroplankton assemblages at station L4.

Lindeque PK, Parry HE, Harmer RA, Somerfield PJ, Atkinson A (2013) Next Generation

Sequencing reveals the hidden diversity of zooplankton assemblages. PLoS One 8 iss 11

e81327

Smyth TJ, Fishwick JR, Al Moosawi L, Cummings DG, Harris C, Kitidis V, Rees A, Martinez-

Vicente V, EMS Woodward (2010) A broad spatio-temporal view of the Western English

Channel observatory. J. Plankton res 32: 585-601

Widdicombe, CE, Eloire D, Harbour D, Harris RP, Somerfield PJ. (2010) long-term

phytoplankton community dynamics in the Western English Channel. J Plankton res 32: 643-

655.

Widdicombe C, Tarran G, Smyth T (2012) 6.6 Plymouth L4 (Site 36). In O’Brien TD, Li WKW

and Morán XAG (eds) ICES Phytoplankton and Microbial Staus Report 2009/2010 ICES

Cooperative research report No. 313, pp 86-91

Annex B: An example of an EMECO report

3.5.5 First results for reference envelopes for lifeform pair 1 presented with the EMECOreporting toolThe following is a demonstration of a report written using the text editor within EMECO. It shows some examples of the results ofthe lifeform and state space and demonstrates its operational readiness.

The lifeform and state space approach has been integrated into the EMECO datatool and is operational. Figure 3.7 shows thereference envelopes (2008 - 2010) for lifeform pair 1 for descriptor 1 (diatoms and dinoflagellates) from CPR1, the mooringstation 38A in the western Irish Sea, the fixed point station Stonehaven and the Bristol Channel (IBS001P).

Figure 3.7. Reference envelopes (2008-2010) for the lifeform pair 1 descriptor 1 from CPR1 data, the western Irish Sea,Stonehaven and the Inner Bristol Channel.

The reference envelopes from the other sites that are Loch Ewe, Firth of Clyde, Loch Linnhe, Firth of Forth, PML, and WestGabbard are to follow. This report is an example to demonstrate how reporting can take place when assessments on monitoringthe state of the plankton are required by the MSFD for the first time in 2016.

Figure 3.8 illustrates the reference envelope (2008-2010) and the new points for 2012 for lifeform pair 1 of descriptor 1 for theCPR1 data. The comparison shows that 73% of the new 51 points fall into the reference conditions indicating that the PCI value0.73 is significant (p<0.05).

Figure 3.8. Reference envelope and comparison for lifeform pair 1 descriptor 1 for the CPR1 data.

For the comparison of the data from the western Irish Sea (Fig. 3.9) there is currently only one point available for 2012 and thePCI value of 1.00 is not significant (p=1.00).

Figure 3.9. Reference envelope and comparison for lifeform pair 1 descriptor 1 for the fixed point station 38A in the western IrishSea.

The comparison for the Stonehaven monitoring site in 2012 delivers a PCI value of 0.93 indicating that 7% of the 29 new pointsfall outside the reference envelope with p>0.05.

Figure 3.10. Reference envelope and comparison for lifeform pair 1 descriptor 1 for the Stonehaven monitoring site.

Since the reference envelopes are based on recent years (2008-2010) it is not yet possible to estalish a time series plot to trackchange away from these conditions. However, fig. 3.11 illustrates how an individual PI or the holistic PI time-series can look like.

Figure 3.11. The beginning of a time-series plot for the PI. the data points derive from the CPR1 comparison in 2011 and 2012.

Individual lifeform time-series plots can also be established within EMECO which may sometimes help to better understand thelifeform pair results. An example is given in Fig. 3.12 from the Stonehaven datasets for diatoms and dinoflagellates.

Figure 3.12. Abundance for lifeform one (diatoms) and lifeform two (dinoflagellates) for 2011 to 2013.

Fig. 3.13 shows a map with sampling/monitoring stations and the abundance of lifeform 1 (diatoms).

Figure 3.13. A map showing the sampling/monitoring stations and the abundance of lifeform 1 (diatoms).

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