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An estuary is a semi-enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries are thus subject to both marine influences, such as tides, waves, and the influx of saline water; and riverine influences, such as flows of fresh water and sediment. As a result they may contain many biological niches within a small area, and so are associated with high biological diversity.

Estuaries are typically the tidal mouths of rivers (aestus is Latin for tide), and they are often characterized by sedimentation or silt carried in from terrestrial runoff and, frequently, from offshore. They are made up of brackish water. Estuaries are often given names like bay, sound, fjord, etc. The terms are not mutually exclusive.

As ecosystems, many estuaries are under threat from human activities such as pollution and overfishing. Due to their suitability to human settlement, estuaries typically have a heavy human presence and of the 32 largest cities in the world, 22 are located on estuaries.

Estuary circulation

Estuary mouth
Estuaries are marine environments whose pH, salinity, and water levels vary, depending on the river that feeds the estuary and the ocean from which it derives its salinity (oceans and seas have different salinity levels). The time it takes an estuary to completely cycle is called its flushing time.

  • Estuarine circulation is common in estuaries; this occurs when fresh or brackish water flows out near the surface, while denser saline water flows inward near the bottom.

  • Anti-estuarine flow is its opposite, in which dense water flows out near the bottom and less dense water circulates inward at the surface.

These two terms, however, have a broader oceanographic application that extends beyond estuaries proper, such as in describing the circulation of nearly-closed ocean basins.

Estuaries are more likely to occur on submerged coasts, where the sea level has risen in relation to the land; this process floods valleys to form rias and fjords. These can become estuaries if there is a stream or river flowing into them.

Large estuaries, like the Gulf of St. Lawrencemarker and Chesapeake Bay often have many streams flowing into them and can have complex shapes. Where an enormous volume of river water enters the sea (as, for example, from the Amazon into the South Atlantic) its estuary could be considered to extend well beyond the coast.

The Baltic Seamarker can be considered as the common estuary of all rivers flowing in it.

Types of estuary

Estuaries can be grouped by circulation.
  • Salt wedge. River output greatly exceeds marine input; there is little mixing, and thus a sharp contrast between fresh surface water and saline bottom water.

  • Highly stratified. River output and marine input are more even, with river flow still dominant; turbulence induces more mixing of salt water upward than the reverse.

  • Slightly stratified. River output is less than the marine input. Here, turbulence causes mixing of the whole water column, such that salinity varies more longitudinally rather than vertically.

  • Vertically mixed. River output is much less than marine input, such that the freshwater contribution is negligible; longitudinal salinity variation only.

  • Inverse estuary. Located in regions with high evaporation, there is no freshwater input and in fact salinity increases inland; overall flow is inward at the surface, downwells at the inland terminus, and flows outward subsurface.

  • Intermittent estuary. Estuary type varies dramatically depending on freshwater input, and is capable of changing from a wholly marine embayment to any of the other estuary types.

Grouped by structure rather than circulation, there are other types of estuaries.

  • Coastal plain estuaries are flooded river valleys, and fjords are submerged glacier-eroded valleys.

Implications for marine life

Estuaries provide habitats for a large number of organisms and support very high productivity. Estuaries provide habitats for many fish nurseries, depending upon their locations in the world, such as salmon and sea trout. Also, migratory bird populations, such as the black-tailed godwit, Limosa limosa islandica make essential use of estuaries.

Two of the main challenges of estuarine life are the variability in salinity and sedimentation. Many species of fish and invertebrates have various methods to control or conform to the shifts in salt concentrations and are termed osmoconformers and osmoregulators. Many animals also burrow to avoid predation and to live in the more stable sedimental environment. However, large numbers of bacteria are found within the sediment which have a very high oxygen demand. This reduces the levels of oxygen within the sediment often resulting in partially anoxic conditions, which can be further exacerbated by limited water flux.

Plankton are key primary producers in estuaries. They move with the water bodies and can be flushed in and out with the tides. Their productivity is largely dependant upon the turbidity of the water. The main plankton present are diatoms and dinoflagellates which are abundant in the sediment.

It is important to remember that a primary source of food for many organisms on estuaries, including bacteria, is detritus from the settlement of the sedimentation.

Human impacts

Of the 32 largest cities in the world, 22 are located on estuaries. For example, New York Citymarker is located at the orifice of the Hudson River estuary.

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 impacted 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 Rivermarker 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 zone. It can result in reductions in water quality, fish, and other animal populations.

Overfishing also occurs. Chesapeake Bay, North America's largest estuary , 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.

Notable examples

See also


  1. Pritchard, D. W. (1967) What is an estuary: physical viewpoint. p. 3–5 in: G. H. Lauf (ed.) Estuaries, A.A.A.S. Publ. No. 83, Washington, D.C.
  2. Ross, D A (1995) Introduction to Oceanography. New York: Harper Collins College Publishers. ISBN 978-0673469380
  3. Tomczak, M (2000) Oceanography Notes Ch. 15: The flushing time. Retrieved 17 August 2008.
  4. Tomczak, M (2000) " Oceanography Notes Ch. 12: Estuaries. Retrieved 30 November 2006.
  5. " Types of Estuaries: Based on Geology" . Retrieved on 1 December 2006.
  6. Bronwyn M. Gillanders, Evidence of connectivity between juvenile and adult habitats for mobile marine fauna: an important component of nurseries. 2003. Marine Ecology Progress Series
  7. Jennifer A. Gill, The buffer effect and large-scale population regulation in migratory birds. 2001. Nature 412, 436-438
  8. NOAA Estuaries tutorial Revised March 25, 2008
  9. G.Branch, Estuarine vulnerability and ecological impacts, TREE vol. 14, no. 12 Dec. 1999
  10. "Indigenous Peoples of the Russian North, Siberia and Far East: Nivkh" by Arctic Network for the Support of the Indigenous Peoples of the Russian Arctic]
  11. Gerlach: Marine Pollution, Springer, Berlin (1975)

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