What palaeoecology tells us about the

history of Poole Harbour

Introduction July 2019. Final version This note provides a non-technical summary of the research carried out by Dr Laura Crossley for her PhD

at Southampton University and is re-produced here with the kind approval of; Laura, her supervisors;

Professors Peter Langdon, David Sear and John Dearing; and Wessex Water. The original PhD report can

be obtained online (under licence) from; https://eprints.soton.ac.uk/429023/

Who will deliver projects?

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Poole Harbour

Poole Harbour estuary is one of the largest and shallowest natural harbours in the world. It is fed by two major rivers, the Frome and Piddle, two smaller rivers, the Sherford and Corfe, and several small streams. The western side is largely rural, with the town of Poole draining to the eastern shore, and the main sewage treatment works drain into Holes Bay, in the north of the harbour. The estuary connects to the English Channel via a narrow entrance, approximately 100 m wide (Figure 1). The Harbour is protected for its European importance to birds, known as a Special Protection Area, and is an internationally recognised wetland, or Ramsar site, with saltmarsh fringing mudflats that are exposed at low tide. It supports a significant shellfishery, as well as being a key recreation area for boating and fishing.

The rivers Frome and Piddle have been subject to extensive research leading to the multi-partner ‘Strategy for Managing Nitrogen in Poole Harbour Catchment to 2035’. This strategy was based on analysis of river water quality since the 1960s. Levels of nitrogen and phosphorus have risen from inputs from agriculture and treated sewage. This study aimed to see whether the historic trends within the harbour itself reflect those freshwater studies.

Background to the project

It is known that Poole Harbour has been subject to eutrophication, or over enrichment by nutrients, since the 1980s when mats of green algae were seen to be spreading over its mud flats. The levels of nitrogen and phosphorus have been well documented by scientists in the freshwaters that supply it; the rivers Frome and Piddle, and the Sewage treatment works (STW) of the Poole catchment. However, data on the harbour waters is less reliable due to the changing conditions with each tide. Historic estimates of sediment input from the rivers

have been made but based on infrequent samples or over short timescales. So, Wessex Water co-funded a 3-year PhD study in 2014 at the University of Southampton that used sediment cores and palaeoecology to;

Reconstruct the historic water quality and sediment changes within Poole Harbour.

Determine where key ‘tipping points’ in water quality had occurred and why.

This study would indicate what level of nutrient and sediment accumulation offers a ‘safe operating space’ for the protection and improvement of Poole Harbour for wildlife and people.

How the work was done Sediment records from estuary deposits can provide valuable archives with which to reconstruct many key ecosystem variables over hundreds of years, including nutrients, sediment loads and ecological responses of plants and animals.

Sediment cores, between 124 and 275 cm in length, were collected from four sites in 2015 and 2016 at the sites shown in Figure 1. Core sites from the channels of

Figure 1. Survey map of Poole Harbour (core sample sites are shown in red)

(cont.) the Rivers Frome and Piddle were selected to reflect the major freshwater sources, whilst Holes Bay receives the treated sewage from Poole. Arne was selected as a control site, but is also influenced by other local freshwater sources (Corfe and Sherford rivers), the sea and indirectly the larger freshwater sites. The cores were preserved intact in cold storage to maintain their chronology, until the analyses were carried out for a range of parameters, along their lengths. The parameters were chosen to indicate one of three areas;

• Chronology

To determine the timeframe of the core through its length. Spheroidal Carbonaceous Particles and radionuclides can be related to known dates, such as the burning of fossil fuels in the industrial revolution or the release of caesium from nuclear weapons. Both of these techniques were applied to the sediment cores to obtain a more reliable age-depth model.

• Sedimentology

To determine the nature of the sediment throughout the core, its makeup of mineral and organic matter. These can indicate the source of sediment and nutrients. The ratio of carbon and nitrogen in the sediment also provides indications of algal activity.

• Palaeoecology

To detect the remains of plants and animals through their chemical pigments, such as Chlorophyll, and the silica shells of diatoms (microalgae).

The four cores collected were dated back to between 1860 and 1880. From laboratory and statistical analysis, a picture of the past accumulation of sediment, water quality and algae was developed for each site and cross- referenced against historic local data on rainfall, sea level, temperature, river flows and records of population and agricultural practice.

This study also compared the results, and sediment accretion rates, with three similar studies in Poole Harbour.

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What the results show Sediment accumulation rates

Typically, estuaries accumulate sediment until the mudflats

rise above the tidal range. The rates vary with sediment

availability, sea level changes, erosion and urban

construction. Four cores provide a very small

representation of the whole harbour but sediment

accumulation rates at the Arne control site lie within the

range found in neighbouring south coast estuaries. All four

showed a rise in sediment accumulation rates between

1940 and 1970s (Figure 2) but show differences in initial

level and rates post 1970.

This period of sediment change coincides with

intensification of agriculture following the Agriculture Act

1947, government grants for drainage and the Common

Agriculture Policy in 1962, as well as land reclamation in

Holes Bay. The increasing calcium levels, changes in

organic matter and magnetic susceptibility (which reflects

erosion of top soils) in the cores at this time supports the

conclusion that agriculture intensified.

Figure 2. Graph of the different sediment accumulation rates for the four cores sites (Frome- FRM4, Piddle-PID1, Holes Bay-HB1, Arne- ARNE5). Note the different scales for each site. The grey shading indicates the period of higher sediment accumulation rates ca. 1940s-1970s.

Sediment accumulation rates (cont.)

Additional changes in magnetic susceptibility also appear in the Frome estuary core between 1910-1930 which may reflect an increased soil loss from the lower catchment, but no rise was evident in the core sediment accumulation rates until 1935. Other research suggests that the River Frome initially absorbed some of the fine sediments, narrowing its channel. There was a flooding incident (1909), increased ploughing and development of the Bovington army ranges that was offset afterwards by the new forestry plantations from the 1920s.

Sediment accumulation rates within the harbour will also be affected by vegetation. The saltmarsh changed to Spartina (cordgrass) after 1885, which then fell into widespread decline in 1930s, releasing sediment. At Holes Bay, sediment input would have been affected by land erosion on the adjacent heathlands (after WWII) and import of chalk for reclamation of land for the new power station. This reclamation reduced the area available for deposition of the mobilised sediment load. The sediment quality also reflects changes in air and water quality (lead), antifouling (copper) and chalk levels.

At the control site, Arne, a sediment rate rise was also influenced by sea level rises between 1945 and 1960, with associated salt and bromine rises. Although these are key, there is also evidence of the riverine sediment load increase, as seen from the River Frome core too. However, there is little sign though of the urban influence that was seen in the Holes Bay core.

Post 1970 the rate of sediment accumulation rates declined, as heathland erosion and agricultural soil erosion stabilised. However, whilst the control site at Arne reduced to pre-1940s accumulation rates, the other three did not. The Piddle site alone shows a reduction followed by a further raised sediment accumulation rate from 2000, from fine grained sediment. Although this may be linked to high flows during 2000, it does not follow the usual flow peak pattern so either the flood disturbed previously deposited fine silt (from agriculture) that has continued to be released from the channel, or there may be some other local farming changes in this smaller catchment.

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Nutrient patterns

These were determined by comparing the ratios of carbon and nitrogen isotopes, which reflect terrestrial and freshwater sources of productivity. The cores also show Silica, pigments and diatom shells from deposited algae, both micro and macro.

The earliest record was from the Frome core which showed a shift from external production (e.g. from the catchment) to in-river productivity from 1840-1880 which will represent anthropogenic changes.

The geochemical record shows that the nitrogen isotope 15N increased through the 1940-1970 period in Frome and Piddle Cores and continued afterwards, reflecting the time lag in nitrogen being released from groundwater after the farming practices had changed (as evidenced by sediment record above). Silica, usually from algae and land wash-off also increased from 1940 onwards so here, with the high N15 fraction, the source is probably increased agricultural nutrient inputs causing increased algae, rather than sewage derived nutrients despite the gradually rising rural population.

In Holes Bay, early records pre-1900, show a relatively more marine than freshwater response. However, it shows increased terrestrial influence, or productivity, until 1920, after which a decline occurred. This may reflect the impact of the growing population before the new treatment plant was built in 1922 at Poole STW. The 15N fraction increases steadily throughout the record supporting an increase in productivity, probably from sewage, reflecting the rise in population from 1950 onwards. Nitrogen was reduced within the sewage from 2008. The geochemical record changes abruptly post 2000 indicating that eutrophication may have started then.

At Arne, the productivity is primarily from marine algae. Again, the carbon and nitrogen data suggest a change toward terrestrially derived sources from 1920, with an increase in 15N which mirrors the river core sites. Rises in sea level would encourage mixing of the freshwater sources here.

Algae

Algae occur in freshwater and marine environments, as micro species (like diatoms) and macro (like the Ulva mats seen on mudflats) and may be attached to plants and substrates, or free floating. Their remains were identified through their chemical pigments and the remaining silica shells within the sediment. For many species of diatom, their preferred conditions are well documented, especially for river species and so the trends found within the core reflect the nutrient status there through time and the pigments can indicate the source of algal productivity.

Other factors influence algal communities, for example in the Arne core, Denticula subtillis, a species associated with the Spartina marsh, showed an increase until circa 1940, after which it and the Spartina declined. After 1990, three other species associated with nutrient rich waters increased and after 2000, the algal species present indicate eutrophication. However, the total productivity at Arne (as indicated by algal densities) was less than at the other sites.

The Frome, Piddle and Holes Bay cores also showed that algal productivity increased from the 1940’s but that eutrophication started circa 2000. This is evidenced by both the quantity and the species of diatoms present and the algal pigments. These arise primarily from freshwater species in the two river channel cores.

So, the algal evidence reflects the evidence of nutrients and sediment accumulation rates from the cores throughout the harbour, even at Arne. Unlike the changes in nutrient and sediment accumulation rates, algal productivity has not reduced post 1970s and a significant change, eutrophication, occurred post 2000.

Holes Bay has suffered similar eutrophication and plant changes, but this was influenced by sewage-derived nutrients, plus sediment changes from heathland erosion and in-harbour dredging. The treatment change at Poole STW in 2010 to reduce nitrogen in the effluent would not yet be visible in the sediment record.

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Implications for future management

To understand the key drivers of change in this estuarine system over time, a combined value for overall ‘water quality’ was calculated statistically for each core by combining normalised averages of four of the indicators (like the FTSE 100 index combines share prices):

• Nutrient loading (level of 15N) • Organic matter • Algal pigment scores • C:N ratio Carbon to nitrogen ratio

The trend for each core was compared with indicative parameters such as river nitrogen and phosphorus (River Frome) concentrations, plus calculated nitrogen fertiliser application rates (derived from national figures) and population of Poole. At each core site, nitrogen appears to be the key nutrient driver of the Water Quality Index trend from diffuse or sewage sources.

No significant relationship was found between the water quality index and temperature or rainfall records.

Figure 3. Graph showing trends in the Water Quality Index for Frome (FRM4), Piddle (PID1), Holes Bay (HB1) and Arne (ARNE5) sites. The red arrows show the trend direction and the dashed lines indicate the date where that trend changes visually.

The trend for the water quality shows a relatively steady state, after which the decline occurs, known as a ‘break’ or tipping point. The ‘break point’ indicates when a water quality limit has been exceeded and the system is no longer able to stabilise itself through natural environmental feedback loops.

The water quality break point appears to be between 1960-75 for the Frome and Piddle cores. This indicates that there is a time lag between earlier changes in sediment accumulation and recorded water quality in the rivers and so may reflect the combined effect of increasing sediment wash off (with its attached nutrient) and direct fertiliser run off.

The trend in Water Quality Index for each core was compared with different factors (population, river Frome phosphorus and nitrogen concentrations, nitrogen fertiliser usage, temperature and rainfall). The statistics indicate that the strongest relationship is with the nitrate inputs. There is a response to increasing phosphorus initially but no improvement is evident from the recent improvements in phosphorus levels within rivers and therefore harbour. Other research has shown that phosphorus may be recycled more easily in estuarine systems than in freshwater. Elsewhere declines in phosphorus within source rivers have resulted in increased algal blooms in the estuary downstream. No relationship was seen with temperature to date.

The greater decline in water quality index from 1960, suggests that water quality index will continue to fall until a new steady state is achieved. Arne site, whose break point was later, may in future follow the path seen in the Frome and Piddle cores.

For Holes Bay, the statistical break point in water quality is ca. 1970. Whilst population growth had a direct effect on increasing nutrient loads to Holes Bay, it increased steadily from 1970 to 2011 whereas the water quality index fell abruptly ca.2000. This suggests that the delayed shift to a eutrophic state may have been forced by the cumulative impact of earlier human pressures or a tipping point being reached in the nutrient and sediment levels.

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Safe operating space for Poole Harbour

Having established the statistical break points when the Poole Harbour failed to maintain a stable system, this study then looked at the measured water quality of the river and sewage inputs at that time.

Given that anthropogenic effects were present before 1880, when the sediment record began, it cannot be certain that Poole Harbour was in a good, or natural, state at that time. Changes have been evidenced here from the 1940s onwards but given that the sediment accumulation rates have now increased, it is unlikely that a return to 1940s steady state is achievable.

The water quality index was stable until ca.1960 in the Frome and Piddle cores, so the freshwater quality at that time is proposed as a safe ‘target’ for future inputs from the catchment’s rivers. The River Frome nitrate levels at East Stoke were ca. 2 milligrams per litre (mg/l-N) in 1960, reaching over 6mg/l-N by 2010. That river water concentration would not necessarily reflect the rise in nitrate loss to the ground-water and air that may have already started from agricultural intensification.

A Water Quality Index was calculated for the whole of Poole Harbour by averaging the analysis from the four cores. That provides a simplified view of the Poole Harbour system in three states (Figure 4).

Figure 4. Diagram showing the envelope of good water quality for Poole Harbour (dashed red lines) and transition to critical system state (red line). The colours represent the safe (green), cautious (yellow) and dangerous (red states). Note that the Water Quality Index shown is an average of the four cores

Conclusions

This study has provided clear evidence from sediments accumulated between 1880-2011 at four sites within Poole Harbour. This record provides clear evidence of changes in sediment and algal population over that time as well as an indication of water quality trends. Conclusions on the historic state of the harbour and causes of its changes were drawn from direct measurements, a water quality proxy and the history of the catchment.

Sediment accumulated faster during 1940-1970, which may reflect the intensification of farming and draining of land for the war. There was a further rise in the River Piddle core around 2000. The lack of response to earlier indications of soil loss (1910-20) in the Frome core shows that the river may have been able to absorb the extra sediment at that stage.

Increasing levels of nutrients coincide with rises in nitrogen and phosphorus measured in the rivers, and to historic farming and population records. The main drivers of water quality change appear to be the sediment accumulation rates and the nitrogen input. The nitrogen is primarily attributable to farming change (Frome and Piddle cores) and waste water from the Poole population (Holes Bay core) and the trends are less severe at Arne, which is more influenced by sea water dilution.

The historic algal communities changed during the 1940-1970s, reflecting the rising nutrient levels. The estuary becomes eutrophic from the 2000s, indicated by the diatom species present. This trend is also less apparent in the Arne core.

The four cores showed that despite rising levels of nitrogen, phosphate and sediment accumulation, the ecological system remained largely stable until 1960 or 1970 after which the general water quality has declined. It is recommended that returning nutrient input to the levels found just pre-1960/70s could return the system to its previous steady state, i.e. a freshwater concentration of around 2mg/l-N. The ground water and sediment will still contribute nutrients to the harbour for some decades.

Poole Harbour may experience further changes from climate change (rises in sea level, salinity and temperature), further sediment disturbance, and changes to the ecological system (fish, shellfish and bird stocks) or air quality. These may affect the final steady state achievable.

The study provides clear evidence that changes in agriculture and population growth in the catchment have

resulted in geochemical and ecological changes and, as a whole, a decline in the Water Quality Index

calculated for the harbour.

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Glossary of terms Lor-

Safe Operating Space’ and ‘tipping points’ in Poole Harbour

Nutrients are essential for all plant and animal life and are used to increase the production of food for ourselves. Whilst we understand that the harbour plant and algal community responds to nutrient levels as one of several ‘environmental drivers’, the system may respond in various ways. It could be a linear, step change or bi-fold relationship. The threshold of significant change is known as the ‘tipping point’ and once the plant/algal community has changed, it may not easily be reversed, despite reductions in nutrient level. By understanding at what level of nutrients the tipping point occurred, or may occur and keeping nutrient levels below that, we can agree what a ‘safe operating space’ is for the harbour. That is the level that we need to keep below to protect the wildlife, fisheries and recreation and they may differ from those set by National and European legislation or the Nutrient Management Plan which are used at present.

Palaeoecology samples

Remains of historic plant communities were identified within the sediment, through their biochemical pigments as well as the nutrient remains. Pigment and nutrient fractions can help to understand the sources of plant and algae production, whilst the preserved silica shells of diatoms provide a direct record of the historic community of these micro algae. Algae are a key indicator of nutrient status in both fresh and estuarine waters. They reflect the water quality at the time when they were deposited, along with other physical and biological factors. Changes in algal communities provides a proxy of changes within the environment over time.

Note: the extent of macro algal mat species are not directly measurable from the cores but contribute to the overall measures of plant productivity.

Markers of time

Sediment cores from estuaries have been found to provide a record of catchment system changes by other researchers. Changes within the catchment, e.g. increase in erosional activities and industrial discharges, produce signals that can be detected in the sedimentary record, e.g. increase in sediment accumulation rates and increases in elemental markers such as copper from antifoulant paint on the hulls of ships and lead from the use of leaded fuel. When these markers are deposited within the water they are incorporated into the sediments at the bottom of the water column. As time progresses, new sediment deposits on top of the old and over time this produces a sedimentary archive of past changes. Other proxies can be used to investigate historic environmental, nutrient and ecological change including:

Spheroid Carbonaceous Particles are a product of incomplete combustion of fossil fuels, primarily coal, and can provide a record of historic atmospheric pollution according to their accumulation profile in sediments. A peak in SCP within a sediment profile is a marker for 1970 in South and Central England, before the improvements due to the Clean Air Act 1968 and the Control of Pollution Act 1974.

Isotopes of lead and caesium and their ratios change in sediment during atmospheric fallout and reflect the start of atmospheric nuclear weapon testing.

Detection of productivity from nutrients

Estuaries receive their nutrients from many sources: as chemical or productivity from land and air in the harbour catchment and productivity within the rivers and the sea. Organic material deposited within the sediment core was analysed for carbon (C); its different Carbon-13 isotope and the carbon to nitrogen ratio to reflect the nature of the organic material. Thus, the proportion of freshwater and marine sources, and terrestrial and aquatic productivity can be estimated from the geochemical make up. Allowance is made for conflicting responses in carbon data from fossil fuel use too, post industrial revolution.

Similarly, the nitrogen isotope, 15N can be used as a tracer of nitrogen sources to differentiate between sewage and natural or aerial sources, and to indicate different productivity rates for algal production.

Analysis of C:N rations and the 13C and 15N isotopes through the cores can indicate both periods of raised algal production and the source of the organic matter laid down at the time.

Note: The reports uses the term nitrogen to represent dissolved inorganic nitrogen. This is made up of nitrate, plus nitrite and ammonium. Where a measurement was of that specific parameter, the term nitrate is used.

Glossary of terms