Estuaries

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Estuaries

Estuaries have for long been important to mankind, either as places of navigation, or as locations on their banks for towns and cities. Nowadays they are under pressure, either as repositories for the effluent of industrial processes and domestic waste, or as prime sites for land-claim to create sites for industry or urban development. Against this background the biologist has been attracted to other functions of estuaries: Vital feeding areas for many species of birds, especially waders and wildfowl, like the locations of coastal fisheries or as fascinating areas present challenges to our understanding of how animals and plants adapt to their environment.

An estuary is a partially enclosed body of water formed where freshwater from rivers and streams flows into the oceans, mixing with the seawater. Estuaries and the lands surrounding them are places of transition from
land to sea, and from fresh to saltwater. Although influenced by tides, estuaries are protected from the full force of the ocean waves, winds and storms by reefs, barrier islands or fingers of land, mud, and sand that
define an estuary’s coastal boundary. Estuaries come in all shapes and sizes and can be called by many different names—bays, lagoons, harbors, and inlets or sounds. When considering any estuarine habitat
worldwide there are many generalizations that can be perceived, for example, common features being the gradient of conditions from the open sea into the sheltered estuary, and on to the freshwater river. Along this gradient there are clear changes in salinity ranging from full strength seawater decreasing to freshwater. Associated changes in sedimentary conditions from coarse sediment (sand or gravel) without the estuaries to fine sediments (muds) within the estuaries are invariably found.

Most modern-day estuaries were formed during the Holocene epoch by the flooding of river-eroded or glacially-scoured valleys when sea level began to rise about 10,000-12,000 years ago.  Estuaries are amongst the most heavily populated areas throughout the world, with about 60% of the world’s population living along estuaries and the coast. As a result, estuaries are suffering degradation by many factors, including sedimentation from soil erosion from deforestation; overgrazing and other poor farming practices; overfishing; drainage and filling of wetlands; eutrophication due to excessive nutrients from sewage and animal wastes; pollutants including heavy metals, PCBs, radionuclides and hydrocarbons from sewage inputs; and diking or damming for flood control or water diversion.

The estuarine environment is characterized by having a constantly changing mixture of salt and freshwater, and by being dominated by fine sedimentary material carried into the estuary from the sea and from rivers, which accumulates in the estuary to form mudflats. The mixtures of salt and freshwater present challenges to the physiology of the animals, which few are able to adapt to. Estuaries have been claimed to be amongst the most productive natural habitats in the world, and we shall attempt to explain why they are so productive, and how the energy produced is utilized by succeeding trophic levels. Estuaries are transition zones between rivers and the sea, which differ from both in abiotic and biotic conditions. Temperature, salinity, and turbidity fluctuate on a daily basis and reach more extremes in estuarine waters than they do at sea or in rivers. From a biotic viewpoint, estuaries are highly productive ecosystems ranking at the same level as coral reefs and mangrove swamps. An elevated productivity is maintained because of high nutrient levels in both sediment and water column.

Parts of an estuarine ecosystem

Within an estuary ecosystem certain characteristic zones emerge, each with typical sediments and salinity. In particular estuaries one zone may occupy a large proportion of the area, and the other zones may be compressed, but a clear sequence can always be seen:

1. Head. Where freshwater enters the estuary, and river currents predominate. Tidal but very limited salt penetration. FSI salinity 5. Sediments become finer downstream.

2. Upper reaches. Mixing of fresh and saltwater. Minimal currents, especially at high tide, leading to turbidity maxima. Mud deposition. Salinity 5–18.

3. Middle reaches. Currents due to tides. Principally mud deposits, but sandier where currents faster. Salinity 18–25.

4. Lower reaches. Faster currents due to tides. Principally sand deposits, but muddier where currents weaken. Salinity 25–30.

5. Mouth. Strong tidal currents. Clean sand or rocky shores. Salinity similar to adjacent sea (30).

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Types of estuaries

Fjords. Drowned glacial troughs. Fjord-type estuaries occur where valleys have been deeply eroded by glaciation. Characterized by deep inner basins linked to the sea by shallow entrance sills, for example, Sea lochs in West of Scotland, Fjords in Norway, Sweden, Alaska, British Columbia, New Zealand, such as the coasts of Norway, Western Scotland, Alaska, and New Zealand.

Fjards. Typical of glaciated lowland coasts. More complex than fjords, with a more open and irregular coastline, for example, Solway Firth England/Scotland, eastern Canada, and New England.

Rias. Drowned river valleys, formed by subsidence of land and/or a rise in sea level. Deep, narrow channels with a strong marine influence, for example, Estuaries of Cornwall, England and Brittany, France.

Coastal plain estuaries. Formed by the flooding of pre-existing valleys. Unlike Rias, these estuaries are often very shallow and filled with sediment so that extensive mudflats and saltmarshes occur. Commonest type of estuary in United Kingdom, for example, Severn, Dee, Humber, Thames, England. Chesapeake Bay, Charleston Harbor, Delaware Bay, USA.

Bar-built estuaries. Also drowned river valleys, but recent sedimentation has kept pace with their drowning so that they have a characteristic bar across their mouths, for example, Alde, England; Ythan, Scotland. Barnegat Bay, New Jersey, Laguna Madre, Texas, Albufeira, Portugal, and most estuaries of North Carolina—Florida coast. In many estuaries in South Africa and Australia the bar may seasonally close the estuary, creating closed or blind estuaries.

Complex estuaries. Drowned river valleys of complex origin, typically a mixture of glaciation, river erosion, and sea level rise, for example, Scottish Firths: Solway, Moray, Dornoch, Tay, and Forth. San Francisco Bay is a complex estuary created by tectonic activity (land movement due to faulting).

Barrier beaches. Open coast system where a bar or barrier develops offshore, and an estuary is thereby created behind the barrier. For example, North Norfolk Coast, Lindisfarne, England.

Linear shore sites. Formed where the shore is sheltered, for example, by barrier islands. Usually considered as a subdivision of a complex estuary. For example Essex and North Kent coast, England.

Embayments. Large natural areas formed between rocky headlands that naturally fill with soft sediments. For example, Carmathen bay, Wales, Morecambe bay, The wash, England.

Estuary Wildlife

Estuaries are a hostile environment for the animals that live there, the changing tide requires less mobile species to be specially adapted to survive under water at high tide and at low tide when exposed. Birds use different parts of the estuary depending on the position of the tide; feeding on the estuary mudflats at low water and roosting on higher ground (such as Bowling Green) at high-water.

The following are examples of species that depend on coastal and floodplain grazing marsh, some of which are of conservation concern:

There is more to an estuaries wildlife than first meets the eye. The vast mudflats are home to an abundance of invertebrate species making the estuary as rich in biodiversity as a tropical rainforest. A cross section through the sand and mud reveals the wealth of life that thrives below and on the surface of this rich habitat.

All life on an estuary starts with organisms that are too small for us to even see. Microscopic bacteria and algae, which get their energy from the sun or non living materials, are fed upon or filtered from the water by species of invertebrates such as snails, worms and clams.

The reason why so many wading birds flock there in the winter is that the invertebrates are a vital food source. Each cubic metre of estuary mud contains the same calorific value as 14 Mars bars.

Threats to Estuaries

Human

As ecosystems, estuaries are under threat from human activities such as pollution and overfishing. They are also threatened by sewage, coastal settlement, land clearance and much more. Estuaries are affected by events far upstream, and concentrate materials such as pollutants and sediments. Land run-off and industrial, agricultural, and domestic waste enter rivers and are discharged into estuaries. Contaminants can be introduced which do not disintegrate rapidly in the marine environment, such as plastics, pesticides, furans, dioxins,phenols and heavy metals.

Such toxins can accumulate in the tissues of many species of aquatic life in a process called bioaccumulation. They also accumulate in benthic environments, such as estuaries and bay muds: a geological record of human activities of the last century.

For example, Chinese and Russian industrial pollution, such as phenols and heavy metals, in the Amur River have devastated fish stocks and damaged its estuary soil.

Estuaries tend to be naturally eutrophic because land runoff discharges nutrients into estuaries. With human activities, land run-off also now includes the many chemicals used as fertilizers in agriculture as well as waste from livestock and humans. Excess oxygen depleting chemicals in the water can lead to hypoxia and the creation of dead zones. It can result in reductions in water quality, fish, and other animal populations.

Overfishing also occurs. Chesapeake Bay once had a flourishing oyster population which has been almost wiped out by overfishing. Historically the oysters filtered the estuary's entire water volume of excess nutrients every three or four days. Today that process takes almost a year, and sediment, nutrients, and algae can cause problems in local waters. Oysters filter these pollutants, and either eat them or shape them into small packets that are deposited on the bottom where they are harmless.

Climate Change

Climate change may decrease or increase precipitation, thereby altering coastal and estuarine ecosystems. Decreased precipitation and delivery of fresh water alters food webs in estuaries and affects the amount of time required to flush nutrients and contaminants from the system. Although reduced river flow would decrease nutrient input in estuaries with relatively uncontaminated watersheds, there could be different effects in polluted watersheds that contain point sources of nutrients and contaminants that are not a function of river flow.

The combined effects of human development and reduced river flow would degrade water quality conditions, negatively affecting fisheries and human health through such changes as increased presence of harmful algal blooms and accumulation of contaminants in animals and plants. Increased rainfall and resultant freshwater runoff into an estuary would increase stratification of the water column, leading to depleted oxygen concentrations in estuaries with excess nutrients. It would also change the pattern of freshwater runoff in coastal plain watersheds, such as along the southern Atlantic coast and in the Gulf of Mexico. In those regions where water resources are managed by humans, the effects of increased flooding would depend on how managers controlled regional hydrology.

Because water expands and glaciers melt as temperatures warm, higher temperatures would raise sea levels, inundating coastal lands and eroding susceptible shores. In salt marsh and mangrove habitats, rapid sea-level rise would submerge land, waterlog soils, and cause plant death from salt stress. If sediment inputs were limited or prevented by the presence of flood-control, navigational, or other anthropogenic structures, marshes and mangroves might be starved for sediment, submerged, and lost.

These plant systems can move inland on undeveloped coasts as sea levels rise on sedimentary shores with relatively gentle slopes, but seaside development by humans would prevent inland migration. Marshes and mangroves are critical contributors to the biological productivity of coastal systems and function as nurseries and as refuges from predators for many species.

Thus their depletion or loss would affect nutrient flux, energy flow, essential habitat for a multitude of species, and biodiversity. Some organisms might thrive (e.g., shrimp, menhaden, dabbling ducks, some shorebirds), at least over the short term as marshes break up and release nutrients or become soft-bottom habitat. Other organisms would be lost from affected areas if their feeding or nesting grounds disappeared and they could not use alternative habitats (e.g., Black and Clapper Rails, some terns and plovers).

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