Classification & physical attributes of wetlands

including pollutant impact in coastal zones

Georg Umgiesser and Roberto Zonta

National Research Council, Institute of Marine Sciences, Venice, Italy

curriculum in natural environmental science, vol. 2, 2010NEAR

Despite accounting for no more than 15% of the surface and 0.5% of the total volume of the ocean, about 90% of marine living resources come from the coastal waters.

On the other hand, more than 60% of the world’s population live within 60 km of the sea. Human activities are changing rapidly the fluxes of matter and related pollutants from the continent to the coastal zones.

Coastal zones are essential for the equilibrium of the whole marine life, since the large amount of organic matter that is produced in their waters is fundamental for the maintenance of the food chain.

These systems are therefore subjected to a very strong and increasing pressure that induces degradation phenomena, in term of loss of habitats for living organisms and biodiversity, water and sediment pollution, eutrophication and landscape deterioration.

European coasts, in particular, are most affected, with some 80% being at risk.

The global awareness related to the need of a comprehensive protection of the marine environment and coastal resources has increased since the 1972 United Nations Conference on Human Environment held in Stockholm.

International agreements were established and different policy actions were taken toward a sustainable and integrated coastal management.

Contemporarily, scientific investigations are needed to acquire an adequate understanding of coastal area conditions and related environmental processes, to counteract the effects of stressing factors, and to permit the forecasting of consequences of management options.

Freshwater discharged by rivers, municipal and industrial effluents, is quite generally the main source of pollutants for coastal zones, since most of the pollution load to the ocean derives from land-based activities, including the releases into the atmosphere.

This places the quality of freshwater delivered to the sea as a topic of world-wide importance.

Wetlands - Nature’s Filter

Wetlands are areas of land where the water table is usually at or near the surface, or the land is intermittently or permanently inundated by shallow water. Wetlands are not only attractive landscape features, but also provide habitats for native wildlife as well as potential sources of reusable water. There may also be the potential for sporting and recreational facilities, and agriculture and aquaculture farming within artificially managed wetland systems.

Wetlands are classified according to their depth, period of inundation and salinity. The role of wetlands as biological filters and treatment mechanisms to remove and convert catchment sourced contaminants in runoff is of prime importance.

LAND

sediment

coastal waters

SE

A

salt water

fresh water

sediment/particles

pollutants

The Players

Possible Classification Schemes of Wetlands

• The Ramsar Convention

• Functional Classification

• Salinity Stratification

• Flow rates

• Topographic Classification

• Lagoons and water exchange

The Ramsar Convention definition of “Wetland"

Under the Convention on Wetlands (Ramsar, Iran, 1971) "wetlands" are defined as shown below:– "For the purpose of this Convention wetlands are areas of

marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six metres.“

– Wetlands "may incorporate riparian and coastal zones adjacent to the wetlands, and islands or bodies of marine water deeper than six metres at low tide lying within the wetlands".

Marine/Coastal Wetlands 1/2

• A -- Permanent shallow marine waters in most cases less than six metres deep at low tide; includes sea bays and straits.

• B -- Marine subtidal aquatic beds; includes kelp beds, sea-grass beds, tropical marine meadows.

• C -- Coral reefs.

• D -- Rocky marine shores; includes rocky offshore islands, sea cliffs.

• E -- Sand, shingle or pebble shores; includes sand bars, spits and sandy islets; includes dune systems and humid dune slacks.

• F -- Estuarine waters; permanent water of estuaries and estuarine systems of deltas.

Marine/Coastal Wetlands 2/2

• G -- Intertidal mud, sand or salt flats.

• H -- Intertidal marshes; includes salt marshes, salt meadows, saltings, raised salt marshes; includes tidal brackish and freshwater marshes.

• I -- Intertidal forested wetlands; includes mangrove swamps, nipah swamps and tidal freshwater swamp forests.

• J -- Coastal brackish/saline lagoons; brackish to saline lagoons with at least one relatively narrow connection to the sea.

• K -- Coastal freshwater lagoons; includes freshwater delta lagoons.

• Zk(a) – Karst and other subterranean hydrological systems, marine/coastal

Inland Wetlands 1/2

• L -- Permanent inland deltas.• M -- Permanent rivers/streams/creeks; includes waterfalls.• N -- Seasonal/intermittent/irregular rivers/streams/creeks.• O -- Permanent freshwater lakes (over 8 ha); include large oxbow lakes.• P -- Seasonal/intermittent freshwater lakes (over 8 ha); include floodplain lakes.• Q -- Permanent saline/brackish/alkaline lakes.• R -- Seasonal/intermittent saline/brackish/alkaline lakes and flats.• Sp -- Permanent saline/brackish/alkaline marshes/pools.• Ss -- Seasonal/intermittent saline/brackish/alkaline marshes/pools. • Tp -- Permanent freshwater marshes/pools; ponds (below 8 ha), marshes and

swamps on inorganic soils; with emergent vegetation water-logged for at least most of the growing season.

• Ts -- Seasonal/intermittent freshwater marshes/pools on inorganic soils; includes sloughs, potholes, seasonally flooded meadows, sedge marshes.

Inland Wetlands 2/2

• U -- Non-forested peatlands; includes shrub or open bogs, swamps, fens.

• Va -- Alpine wetlands; includes alpine meadows, temporary waters from snowmelt.

• Vt -- Tundra wetlands; includes tundra pools, temporary waters from snowmelt.

• W -- Shrub-dominated wetlands; shrub swamps, shrub-dominated freshwater marshes, shrub carr, alder thicket on inorganic soils.

• Xf -- Freshwater, tree-dominated wetlands; includes freshwater swamp forests, seasonally flooded forests, wooded swamps on inorganic soils.

• Xp -- Forested peatlands; peatswamp forests.

• Y -- Freshwater springs; oases.

• Zg -- Geothermal wetlands

• Zk(b) – Karst and other subterranean hydrological systems, inland

Human-made wetlands

• 1 -- Aquaculture (e.g., fish/shrimp) ponds• 2 -- Ponds; includes farm ponds, stock ponds, small tanks; (generally below 8 ha).• 3 -- Irrigated land; includes irrigation channels and rice fields.• 4 -- Seasonally flooded agricultural land (including intensively managed or

grazed wet meadow or pasture).• 5 -- Salt exploitation sites; salt pans, salines, etc.• 6 -- Water storage areas; reservoirs/barrages/dams/impoundments (generally over

8 ha).• 7 -- Excavations; gravel/brick/clay pits; borrow pits, mining pools.• 8 -- Wastewater treatment areas; sewage farms, settling ponds, oxidation basins,

etc.• 9 -- Canals and drainage channels, ditches.• Zk(c) – Karst and other subterranean hydrological systems, human-made

Functional Classification of Wetlands

Inlet zone: This is a transitional zone between the waterways draining the catchment and the wetland . The purpose of this zone is to reduce the velocity of the inflowing water and to enable the larger particles to settle and sink. Aquatic plants can also be grown in the edges of this area as they will generally be able to withstand high velocity inflows during severe storm events.

Macrophyte zone: This area is usually occupied by emergent and submerged aquatic plants. The plants and the coatings of solids and micro-organisms on their stems plus the sediments in which these plants grow, take up or convert the nutrients and thus assist in the treatment of stormwater. There is decomposition of organic matter within this zone.

Open water zone: This is a deeper area that allows time for finer particles to settle and sink to the bed, plus allowing sunlight to kill bacteria. Due to sunlight and nutrient availability, periodic algal growth may also occur in this zone, which will also trap dissolved nutrients and allow them to either enter the food chain or settle to the bottom of the pond.

Definition of estuaries and classification

• An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land rainage. (Cameron and Pritchard, 1963)

Estuaries described by this definition are known as positive estuaries. Another definition that can be given eliminates this problem:

• An estuary is a narrow, semi-enclosed coastal body of water which has a free connection with the open sea at least intermittently and within which the salinity of the water is measurably different from the salinity in the open ocean. (Tomczak, 1996)

Examples of Estuaries

• Highly stratified estuaries (high flow, low tides)

• Slightly stratified estuaries (moderate flow and tides)

• Vertically mixed estuaries (low flow, high tides)

• Inverse estuaries (hypersaline lagoons)

• Salt-plug estuaries (temperature stratification)

• Sub-estuaries

Highly stratified estuary

Highly stratified: A highly stratified, salt wedge type estuary is one in which the outgoing lighter fresh water overrides a dense incoming salt layer. The dense salt wedge will advance along the bottom until the fresh-water flow forces can no longer be overcome

Well mixed estuary

Well-mixed: In estuaries where tidal flow is much larger than river flow and bottom friction large enough to mix the entire water column, a vertically homogeneous (well-mixed) estuary results. If the estuary is wide, Coriolis force may form a horizontal flow separation; and in the northern hemisphere, the seaward flow would occur on the right side (looking downstream), while the compensating landward flow would be on the left.

Partially mixed estuaryPartially mixed: A partially mixed estuary is one in which tidal energy is dissipated by bottom friction produced turbulence. These turbulent eddies mix salt water upward and fresh water downward with a net upward flow of saline water. As the salinity of the surface water is increased, the outgoing surface flow is correspondingly increased to maintain river flow plus the additional upward-mixed saline water. This causes a compensating incoming flow along the bottom. This well-defined, two-layer flow is typical of partially mixed estuaries

Inverse estuaries (hypersaline lagoons)

Inverse estuaries are a feature of hot arid climates. They include shallow regions with a large surface area such as hypersaline lagoons, but also gulfs of significant depth and extent. Evaporation becomes a dominant factor in the salt budget of the gulfs, and the salinity increases from the mouth to the head.

Stratification Numbers

• Simmons Ratio: Simmons (1955) related flow ratio (the ratio of riverflow per tidal cycle to the tidal prism) to estuary type. When the ratio is 1.0 or greater, the estuary is highly stratified. When the flow ratio is 0.2 to 0.5, the estuary is partially mixed, and when less than 0.1, a well-mixed condition exists.

• Ippen Number: Using the tidal properties of amplitude and phase, Ippen and Harleman (1961) developed a relationship between energy and estuary mixing. This stratification number is a measure of the amount of energy lost by the tidal wave relative to that used in mixing the water column. Increasing values of the stratification number indicate increasingly well-mixed conditions, and low numbers indicate highly stratified conditions.

• Hansen Parameter: Hansen and Rattray (1966) chose the parameters of salinity and velocity to develop a means of estuary classification and comparison. Two dimensionless parameters, stratification (ratio of the surface to bottom salinity difference divided by the mean cross-sectional salinity) and circulation (ratio of the net surface current to the mean cross-sectional velocity) are used to construct a diagram.

River Flow, Tides, and WavesDronkers (1988) proposed an estuarine classification that distinguished various types of estuarine ecosystems based on water exchange processes (e.g., river flow, tides, and waves) that greatly affect energy and material fluxes including mixing. This classification suggests that river flow in partially mixed estuaries is essentially neutral, but its variation relative to hydrodynamic residence time can be important in interpreting property-salinity diagrams (Cifuentes et al. 1990). River flow in the partially mixed mainstem of the Chesapeake Bay is seasonally important.

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Topographic classification

Pritchard (1952) has suggested a topographic classification in several groups:

• Coastal plain estuaries– Classical estuaries

– Salt marsh estuaries

• Fjords • Bar-built structures (lagoons)• Other (tectonically caused)

Coastal Plain Estuaries: Classical and Salt Marsh

Both subclasses are characterized by well-developed longitudinal salinity gradients that influence development of biological communities. Examples of the classical type include the Chesapeake Bay (the largest estuary of this type), Delaware Bay, and Charleston Harbor, SC. Vertically stratified systems with relatively long residence times (e.g., Chesapeake Bay) tend to be susceptible to hypoxia formation.

The salt marsh estuary lacks a major river source and is characterized by a well-defined tidal drainage network, dendritically intersecting the extensive coastal salt marshes (Day et al. 1989). Exchange with the ocean occurs through narrow tidal inlets, which are subject to closure and migration following major storms (e.g., Outer Banks, NC). Consequently, salt marsh estuarine circulation is dominated by freshwater inflow, especially groundwater, and tides. The drainage channels, which seldom exceed a depth of 10 m, usually constitute less than 20% of the estuary, with the majority consisting of subaerial and intertidal salt marsh.

Fjords and Fjordlike Estuaries

Classical fjords typically are several hundred meters deep and have a sill at their mouth that greatly impedes flushing. Hypoxia/anoxia is often a natural feature but anthropogenic nutrient loading can severely exacerbate the problem. Examples of classical fjords on the North American continent can be found in Alaska and Washington State (Puget Sound). Some other estuaries were also formed by glacial scouring of the coast, but in regions with less spectacular continental relief and more extensive continental shelves. Examples of these much shallower, fjordlike estuaries can be found along the Maine coast.

Lagoons

Lagoons are characterized by narrow tidal inlets and are uniformly shallow (i.e., less than 2 m deep) open-water areas. The shallow nature enhances sediment–water nutrient cycling. Flushing is typically of long duration. Most lagoonal estuaries are primarily wind-dominated and have a subaqueous drainage channel network that is not as well drained as the salt marsh estuary. Lagoons fringe the coast of the Gulf of Mexico and include the mid-Atlantic back bays; Pamlico Sound, NC; and Indian River Lagoon, FL. Although these systems are typically shallow, they may have pockets of hypoxic water subject to spatial variability because of freshwater pulsing and wind effects. Some lagoonal systems have relatively strong vertical stratifications near the freshwater river mouth and may be subject to hypoxia formation (e.g., Perdido Bay, AL/FL; Livingston 2001a).

Tectonically Caused Estuaries

Tectonically caused estuaries were created by faulting, graben formation (i.e., bottom block-faults downward), landslide, or volcanic eruption. They are highly variable and may resemble coastal plain estuaries, lagoons.

Classification of lagoons using water exchange

Lagoons are classified into 3 main types:  leaky lagoons, choked lagoons, and restricted lagoons (Kjerfve 1986).

– Leaky lagoons have wide tidal channels, fast currents and unimpaired exchange of water with the ocean. 

– Choked lagoons occur along high energy coastlines and have one or more long narrow channels which restrict water exchange with the ocean.  Circulation within this type of lagoon is dominated by wind. 

– Restricted lagoons have multiple channels, well defined exchange with the ocean, and tend to show a net seaward transport of water. Wind patterns in restricted lagoons can also cause surface currents to develop, thus helping to transport large volumes of water downwind

Lagoons

Tidal Amplitude - A Dominant Physical Factor

Tidal amplitude provides a means to broadly classify estuaries relative to their sensitivity to nutrient supplies. Monbet (1992) analyzed phytoplankton biomass in 40 estuaries and concluded that macrotidal estuaries (mean tidal range 2 m) generally exhibit a tolerance to nitrogen pollution despite high loadings originating from freshwater outflows. These systems generally exhibit lower concentrations of chlorophyll a than do systems with lower tidal energy, even when they have comparable concentrations of nitrogen compounds. Estuaries with mean annual tidal ranges 2 m seem more sensitive to dissolved nitrogen, although some overlap occurs with macrotidal estuaries.

Tide induced exchanges

• Simulate transport processes and dispersion of tracers and pollutants

• Estimate the renewal time of the basin

• Characterize water masses with the help of time dependent parameters

• Correlate physical, biological and chemical characteristics between each other

Water Residence TimeWater residence time, the average length of time that a parcel of water remains in an estuary, influences a wide range of biological responses to nutrient loading. The residence time of water directly affects the residence time of nutrients in estuaries, and therefore the nutrient concentration for a given loading rate, the amount of nutrient that is lost to internal processes (e.g., burial in sediments and denitrification), and the amount exported to downstream receiving waters (Dettmann in press, Nixon et al. 1996). Residence times shorter than the doubling time of algae will inhibit bloom formation because algal blooms are exported from the system before growing to significant numbers. Residence time can also influence the degree of recruitment of species reproducing within the estuary (Jay et al. 2000).

There are a number of definitions of water residence time, including freshwater residence time and estuarine residence time (Hagy et al. 2000; Miller and McPherson 1991), each with its own interpretation and utility.

Definition of flushing time

• Flushing time: tF = VF / R

• By salinity: tF = (fw V) / R

– fresh water volume fw is the integral of f

– fresh water fraction f = (S0 - S) / S0

• Tidal Prism method: tF = T V / (VT + VR)

• Knudsen: tF =  V / R (1 - Stop/Sbottom)

• Zimmermann: dtS

tSF

0

)(

Dealing with residence times

• Residence time is an indicator for the renewal capability of a basin

• Residence time is controlled through fresh water fluxes and exchange with the open sea

Dealing with residence times

Residence Time and Stratification

Residence Time

Estuaries that flush rapidly (i.e., have a short residence time) will export nutrients more rapidly than those that flush more slowly, resulting in lower nutrient concentrations in the estuary. Dettmann has derived a theoretical relationship between the mean residence time of freshwater in an estuary and the increase in the average annual concentration of total nitrogen in the estuary as a result of inputs from the watershed and atmosphere. In addition, estuaries with residence times shorter than the doubling time of algal cells will inhibit formation of algal blooms.

Stratification

Highly stratified systems are more prone to hypoxia than are vertically mixed systems. Stratification not only limits downward transport of oxygen from atmospheric reaeration, it also retains nutrients in the photic zone, making them more available to phytoplankton. In stratified systems, it may be more appropriate to estimate the dilution potential of the estuary using the volume above the pycnocline rather than the entire volume of the estuary.

When the freshwater of continental origin meets the sea water (mixing zone), a series of physical and physico-chemical processes occur (mixing processes), due to the different characteristics of the two waters, in term of density, temperature, ionic field, suspended particles concentration and so on.

In particular, finer suspended particles (and organic matter) transported by the fresh water flow tend to increase their weight by aggregating (flocculation, precipitation), so increasing their motion component toward the bottom.

On the other hand, turbulence occurring in the water column (and particularly in the lower layers close to the bottom) determines the re-suspension of particles. The overall result is quite often the presence of a (maximum) turbidity zones.

sediment

Sediment processes in estuaries

estuary sea river

salt water

(mainly)

strong influence of the

tide

fresh and salt water mixing

the extension of the middle

estuary varies with the fresh

water discharge and the strenght

of the tide

maximum turbidity zone

fresh water

tide still influences the

water dynamics

fresh water

here the water motion is

always toward the sea

salt water

low middle upper

Finer suspended particles (and organic matter) are the preferential carriers of pollutants from the land to the sea. As a consequence, high levels of pollutants are found in the zones where finer particles accumulate, i.e. slack dynamics zones such as shallow lateral areas of the estuary, lagoons, and bays.

This is for instance the example of the Cona Marsh, a shallow water area of the Venice Lagoon.

It is the lateral area of the Dese River (the main tributary of the lagoon) estuary, where pollutants accumulate, creating a zoning in the concentration distribution in surface sediments.

The Cona Marsh

Moreover, these locations where polluted sediments accumulate could be also subjected to the presence of other pollutant sources, such as cities, industrial districts, wastewater treatment plants, and so on.

Sediments can, in turn, behave as secondary sources of pollutants for the water column. As a consequence, the monitoring of sediment condition is important to describe and control the ecological status of the coastal zone.

Investigations in the estuarine environment furnish fundamental information on the evolution trends of pollutant processes, as well as for the maintaining of biodiversity, and the control of the impact of human activities on aquatic living organism.

By studying water circulation and matter transport in these environments we obtain basic indications for the management of the freshwater bodies and to plan actions for the safeguard of the coastal zone.

Sediments and pollution

Acknowledgements

Jonathan Sharples & Joanna Waniek & Cesar Ribeiro: Quantifying The Variability Of Vertical Turbulent Mixing And Its Role In Controlling Stability And Mass Transport In A Partially-Mixed Estuary http://www.soes.soton.ac.uk/research/groups/soton_water/index.html

EPA, US-842-B-06-003: Volunteer Estuary Monitoring A Methods Manual http://www.epa.gov/owow/estuaries/monitor/

Catherin Coriou: IFREMER

ftp://ftp.ifremer.fr/ifremer/delao/plazure/ENSAR2005/doc/

Tim Wool, U.S. Environmental Protection Agency Region 4: Water Quality Analysis Simulation Program (WASP)

http://www.cwemf.org/Agendas/tmdlmodelingwrkshp.htm

Matthias Tomczak: An Introduction to Physical Oceanography

http://www.es.flinders.edu.au/~mattom/IntroOc/index.html

We are grateful to the following authors for the possibility to use the material from their web sites: