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Estuaries, or transitional waters, represent the transition between freshwater and marine environments and are influenced by both aquatic realms. Salinity levels are indicative of the position within the mixing zones of an estuary. The upper limit of an estuary is referred to as its head, while the lower limit is called the mouth of the estuary. Between the freshwater head and the saline mouth of the estuary lie a number of zones marked by intermediate salinity values, each with distinct characteristics pertaining to light penetration and type of substratum, thus hosting different communities of organisms.
The challenges of estuarine ecosystems
Estuaries are peculiar yet challenging ecosystems. The main challenge and at the same time the most important feature governing species diversity of transitional waters is the variable salinity regime. Salt dissolved in water dehydrates living organisms by exerting what is called osmotic pressure on the cell walls. Organisms living in the sea are equipped with buffering mechanisms allowing them to retain their body fluids in the presence of salt. In most cases this mechanism is adjusted to a particular salinity and yields a distinction between freshwater and marine species. Most aquatic organisms fall into one of these categories. In turn very few species can withstand variable salinity, which has implications for the biodiversity of estuaries.
Lowland reaches of rivers are characterised by high levels of suspended solids, such as silt, clay and organic detritus, inducing high turbidity. In the fresh water – sea water transition zone these particles become effectively ‘trapped’ due to flocculation and converging suspended sediment fluxes  (see also Dynamics of mud transport). These convergences result in pronounced estuarine turbidity maxima (ETM).
Turbidity limits the depth of the photic zone (light penetration zone), thus limiting the photosynthesis and primary production. Much of the primary production in estuaries occurs on the seaward side of the ETM (Swart, 2007). Macrophyte vegetation and benthic algae are often limited to the periodically exposed (intertidal) part of the estuary, while the growth of phytoplankton is restricted to a thin uppermost layer of the water column (Cloern, 1997.) As a result, estuaries are heterotrophic systems, where more energy is consumed than produced. Most estuarine species are detritivores, meeting their energy intake requirements from organic matter contained either in the sediment (deposit feeders), in suspension (filter feeders) or both. Benthic invertebrates play a major role in energy transfer and circulation in estuaries (Wilson & Parkes, 1998).
Particulate matter eventually settles out and the substratum of estuarine mid reaches tends to be composed of fine fractions, typically creating extensive sand and mudflats. These conditions hinder organisms with an affinity for hard substratum but constitute important habitats for a range of burrowing invertebrate species, of which bivalve molluscs and polychaete worms are usually dominant in terms of numbers and biomass. These in turn provide a rich feeding opportunity for a range of higher consumers.
Industrialised and urbanised river catchments and estuaries receive anthropogenic input from various sources (both point sources and diffuse sources) and often carry a range of contaminants. Sheltered, low-energy areas such as intertidal mudflats in enclosed bays or estuaries are most susceptible to these pollutants due to slow dispersion. The finer substrata in these areas act as a sink for contaminants making estuarine benthic fauna susceptible to pollution (Kausch & Michaelis, 1996).
See also: Coastal and marine sediments
Biodiversity of estuarine ecosystems
Truly estuarine species are those that complete their whole life cycle within the transitional waters. Species permanently dwelling there are mostly hardy, stress-tolerant species able to handle salinity shifts and high suspended solid levels. Not many species can perform well under such conditions. Estuarine ecosystems are thus typically characterised by relatively low species diversity comparing to freshwater or full salinity conditions. Freshwater species are becoming less abundant with increasing salinity and are gradually replaced by marine organisms moving down the estuary with some truly estuarine species found at intermediate salinities. This pattern is reflected by the overall species richness, where the least diverse fauna is found at salinity levels of between 5 and 18 PSU.
Apart from the permanent dwellers, estuaries are host to a number of visitors. Some of them have to travel through estuaries on their migratory route, being either anadromous (spawn in freshwater and feed and grow at sea) or catadromous (spawn at sea and feed and grow in freshwater). The absence of many of the marine predators and rich particulate food supply offer attractive spawning and nursery grounds for many species that normally live under full salinity conditions. Even though estuarine ecosystems are usually species-poor, they maintain a high productive throughput of invertebrate fauna. This productivity provides rich feeding opportunities for a range of higher consumers. Some marine predators are well equipped to cope with reduced salinity and frequently enter estuaries in search of food. Ebb tide makes estuarine beds available to terrestrial predators, of which birds take the greatest share. Estuarine sand and mudflats that are periodically exposed to air support high densities of a diverse bird fauna. This value of estuaries has been long recognized and many estuarine sand and mudflats have been designated as Special Protection Areas (SPA) under the EU Birds Directive (79/409/EEC).
Estuarine ecosystems are usually dominated by stress-tolerant organism, able to withstand a relatively wide range of environmental factors. However, they to face some threats from anthropogenic activities.
- Estuaries are preferred locations for human settlement due to their high productivity and availability of natural connections between maritime and inland waterways. Residential, recreational and industrial developments (such as marinas, harbours or ports) are usually located right at the waterfront with supporting structures such as embankments impacting on the upper shore communities. Estuaries are often challenged by land development; land reclamation is particularly detrimental in this respect as it results in a permanent loss of habitat.
- Rivers discharging into their estuaries carry various constituents depending on the landuse of the drainage area (catchment). This means that various contaminants introduced at any point in the catchment ultimately end up in the estuary. Although estuarine organisms are typically hardy, these pollutants and excess nutrients impede their overall performance (including growth and reproduction). Pollution from densely populated or heavily industralised catchments has detrimental effects on life in estuaries. See: Eutrophication in coastal environments, Coastal pollution and impacts.
- Climate change impacts of concern for estuaries are the overall temperature rise and elevation of the sea level. The first one is likely to induce latitudinal migration with more warm-water species being increasingly established and possibly outcompeting the native species in a long run. Sea level rise would result in a shift of the salinity zonation landwards (see also Effects of global climate change on European marine biodiversity). However, coastal areas, including estuaries, are usually heavily populated and developed. In such developed areas the vertical shift of salinity zonation can be hindered by flood defence structures and river and shore embankments resulting in estuarine squeeze.
- Estuarine and Coastal Sciences Association http://www.ecsa-news.org/
- Joint Nature Conservation Comitee (JNCC) - Estuaries http://www.jncc.gov.uk/page-2160
- US Environmental Protection Agency - Estuarine Science http://omp.gso.uri.edu/ompweb/doee/science/intro.htm
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