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.
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.
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, . 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, .
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. 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.
What is estuary ?एश्चुअरी क्या होती है?
See also: Coastal and marine sediments. 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. 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. Log in.
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Article reviewed by. Job Dronkers See the discussion page. Ecology of estuarine benthic invertebrates: a perspective, p. Lauff ed. The Estuarine Ecosystem, 2nd Ed. Sedimentology — Deriving fluxes of suspended particulate matter in the Humber Estuary, UK, using airborne remote sensing.
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Continental Shelf Research 7: Network analysis of the energy flow through the Dublin Bay ecosystem. Suspended particulate matter in rivers and estuaries - Proceedings of an International Symposium held at Reinbek near Hamburg, Germany.
Advances in Limnology, Die Brackwasserfauna. This page was last edited on 3 November , at Search taxa.
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Browse taxa. Online sources. The main author of this article is Penk, Marcin Please note that others may also have edited the contents of this article. Citation: Penk, Marcin : Estuarine ecosystems.