Estuaries and tidal rivers
This article sets out to introduce the reader to the concept of an estuary and the relationship with tidal rivers. Estuaries are highly complex and dynamic ecosystems . At their simplest estuaries can be defined in relation to their form and function . The definition of an estuary used here encompasses both these aspects.
"Estuaries" is one of the habitat sub-categories within the section dealing with biodiversity of coastal and marine habitats and ecosystems. This forms part of the ENCORA Theme 7.
Estuaries, or transitional waters, represent the transition between freshwater and marine biomes and as such are influenced by both aquatic realms. Salinity level marks the position within the mixing zones of an estuary. The uppermost limit of an estuary is referred to as its head, while the southernmost limit is called the mouth of the estuary. Between the freshwater head and the saline mouth of the estuary lie a number of distinctive zones marked by intermediate salinity values, each with distinct characteristics pertaining to the water clarity and type of substratum, thus hosting different communities of organisms.
The physical form provides a basis for discussing the importance of an individual estuary, including:
1 Drowned river valleys. They develop where the land was inundated by the sea when sea levels rose rapidly (about 20mm per year) towards the end of the last glaciation between 15,000 and 7,000 years ago . At its height around 8,000 years ago, when a catastrophic melting of the ice cap took place in northwest Europe, it may have been as much as 75mm per year . The ‘pulse’ of sediment mobilised and moved landward by these events has helped create the extensive low-lying coastal plains where sediment movement plays an important part in the development of the estuary structure.
2 Bar-built estuaries. Similar to (1) above but where tidal influence and hence sediment penetration is restricted. This may be caused by the nature of the hinterland but usually stems from the presence of spits, bars or barrier islands which surround the river mouth and help create embayments. Because of the restricted opening to the sea, some of these may more closely resemble lagoons.
3 Rias. Tidal inlets which are closely related to drowned river valleys though here the steep-sided nature of the land results in a much narrower profile. Depending on the elevation of the adjacent land, these estuaries may reach some distance inland with a series of side arms and are developed on rocky coasts.
4 Fjords and fjards which occur in glaciated, rocky coastal areas. Fjords with very steep walls occur in areas of high relief. Fjards occur in areas of low relief and may include shallow marine waters and small islands (skerries). For both the hard rock nature of the land results in a limited sediment supply.
Some of these estuary types may also provide a link with deltas and lagoons and it is sometimes difficult to distinguish between them. They appear to occur mostly on meso-tidal coasts where wave energy is relatively high (Pethick 1984) and in micro-tidal zones, such as the deltas of the Mediterranean.
The challenges of estuarine ecosystems
Estuaries are very 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 applies what is called osmotic pressure on the cell walls of living organisms, dehydrating it. Organisms living in saline media such as the sea water 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 dissolved salt level and yields a distinction between freshwater and marine species, into which most organisms of aquatic life fall. 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 and organic detritus, inducing high turbidity (water cloudiness). When the water carried with a river collides with the seawater, these particles become effectively ‘trapped’. Estuaries are naturally turbid environments and among the factors shaping the diversity of estuaries particulate matter claims much of the relevance . In many estuaries and tidal channels particles are trapped at specific locations due to converging suspended sediment fluxes. These convergences result in pronounced estuarine turbidity maxima (ETM) and the majority of primary production in estuaries occurs on the seaward side of the ETM 
Turbidity limits the depth of photic zone (light penetration zone), thus limiting the photosynthesis and primary production. 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 . 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 and benthic invertebrates play a major role in energy transfer and circulation in estuaries (e.g.).
The 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 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 and diffuse 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 as dispersion is low and the finer substrata in these areas act as a sink for contaminants making estuarine benthic fauna susceptible to pollution .
Biodiversity of estuarine ecosystems
Truly estuarine species are those that complete their whole life cycle within the transitional waters. Those 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 thus the estuarine ecosystems are 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 form the permanent dwellers, estuaries play 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 is what offers 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 that in turn provides rich feeding opportunities for a range of higher consumers. Some marine predators are well equipped to cope with reduced salinity and frequently penetrate the estuaries in search of food. In turn, retreating tides make the estuarine beds available to terrestrial predators, of which birds take the greatest share. Estuarine sand and mudflats that are periodically exposed to air support a variety of bird fauna in high densities. 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 Bird Directive.
Effects of sea level rise on estuaries
Sea level rise will result in saltwater intrusion that wreaks havoc with freshwater ecosystems, including rivers, freshwater marshes and coastal lowland farm acreage. Changes in strength and seasonality can affect the retention-dispersion mechanism of planktonic larvae. A change in precipitation will affect estuaries through enhanced river runoff and changes in nutrients and salinity. Al these physical disturbances affect the ecosystem functions and the species assemblages.
In an estuary there is a large amount of detritus. Detritus may be consumed directly by deposit feeders or in suspension by various benthic suspension feeders Where the bottom is rocky, benthic communities are dominated by sessile filter feeders such as bivalves, sponges, barnacles, and bryozoans. Mud and sand substrates support an epifauna and an infauna of suspension and deposit feeders. The large numbers of suspension feeders and deposit feeders are consumed by a number of predators like fish and grow rapidly under the relatively low predation pressure there.
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