Estuary
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Coastal habitats |
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Open ocean |
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An estuary is a partially enclosed
Most existing estuaries formed during the Holocene epoch with the flooding of river-eroded or glacially scoured valleys when the sea level began to rise about 10,000–12,000 years ago.[3] Estuaries are typically classified according to their geomorphological features or to water-circulation patterns. They can have many different names, such as bays, harbors, lagoons, inlets, or sounds, although some of these water bodies do not strictly meet the above definition of an estuary and could be fully saline.
Many estuaries suffer degeneration from a variety of factors including soil erosion, deforestation, overgrazing, overfishing and the filling of wetlands. Eutrophication may lead to excessive nutrients from sewage and animal wastes; pollutants including heavy metals, polychlorinated biphenyls, radionuclides and hydrocarbons from sewage inputs; and diking or damming for flood control or water diversion.[3][4]
Definition
The word "estuary" is derived from the Latin word aestuarium meaning tidal inlet of the sea, which in itself is derived from the term aestus, meaning tide. There have been many definitions proposed to describe an estuary. The most widely accepted definition is: "a semi-enclosed coastal body of water, which has a free connection with the open sea, and within which seawater is measurably diluted with freshwater derived from land drainage".[1] However, this definition excludes a number of coastal water bodies such as coastal lagoons and brackish seas.
A more comprehensive definition of an estuary is "a semi-enclosed body of water connected to the sea as far as the
Classification based on geomorphology
Drowned river valleys
Drowned river valleys are also known as coastal plain estuaries. In places where the sea level is rising relative to the land, sea water progressively penetrates into river valleys and the topography of the estuary remains similar to that of a river valley. This is the most common type of estuary in temperate climates. Well-studied estuaries include the Severn Estuary in the United Kingdom and the Ems Dollard along the Dutch-German border.
The width-to-depth ratio of these estuaries is typically large, appearing wedge-shaped (in cross-section) in the inner part and broadening and deepening seaward. Water depths rarely exceed 30 m (100 ft).
Lagoon-type or bar-built
Bar-built estuaries are found in a place where the deposition of sediment has kept pace with rising sea levels so that the estuaries are shallow and separated from the sea by sand spits or barrier islands. They are relatively common in tropical and subtropical locations.
These estuaries are semi-isolated from ocean waters by barrier beaches (barrier islands and barrier spits). Formation of barrier beaches partially encloses the estuary, with only narrow inlets allowing contact with the ocean waters. Bar-built estuaries typically develop on gently sloping plains located along tectonically stable edges of continents and marginal sea coasts. They are extensive along the Atlantic and Gulf coasts of the U.S. in areas with active coastal deposition of sediments and where tidal ranges are less than 4 m (13 ft). The barrier beaches that enclose bar-built estuaries have been developed in several ways:
- building up of offshore bars by wave action, in which sand from the seafloor is deposited in elongated bars parallel to the shoreline,
- reworking of sediment discharge from rivers by a wave, current, and wind action into beaches, overwash flats, and dunes,
- engulfment of mainland beach ridges (ridges developed from the erosion of coastal plain sediments around 5000 years ago) due to sea level rise and resulting in the breaching of the ridges and flooding of the coastal lowlands, forming shallow lagoons,
- elongation of barrier spits from the erosion of headlands due to the action of longshore currents, with the spits growing in the direction of the littoral drift. [citation needed]
Fjord-type
Fjords were formed where Pleistocene glaciers deepened and widened existing river valleys so that they become U-shaped in cross-sections. At their mouths there are typically rocks, bars or sills of glacial deposits, which have the effects of modifying the estuarine circulation.
Fjord-type estuaries are formed in deeply eroded valleys formed by glaciers. These U-shaped estuaries typically have steep sides, rock bottoms, and underwater sills contoured by glacial movement. The estuary is shallowest at its mouth, where terminal glacial moraines or rock bars form sills that restrict water flow. In the upper reaches of the estuary, the depth can exceed 300 m (1,000 ft). The width-to-depth ratio is generally small. In estuaries with very shallow sills, tidal oscillations only affect the water down to the depth of the sill, and the waters deeper than that may remain stagnant for a very long time, so there is only an occasional exchange of the deep water of the estuary with the ocean. If the sill depth is deep, water circulation is less restricted, and there is a slow but steady exchange of water between the estuary and the ocean. Fjord-type estuaries can be found along the coasts of Alaska, the Puget Sound region of western Washington state, British Columbia, eastern Canada, Greenland, Iceland, New Zealand, and Norway.
Tectonically produced
These estuaries are formed by subsidence or land cut off from the ocean by land movement associated with
Classification based on water circulation
Salt wedge
In this type of estuary, river output greatly exceeds marine input and tidal effects have minor importance. Freshwater floats on top of the seawater in a layer that gradually thins as it moves seaward. The denser seawater moves landward along the bottom of the estuary, forming a wedge-shaped layer that is thinner as it approaches land. As a velocity difference develops between the two layers, shear forces generate internal waves at the interface, mixing the seawater upward with the freshwater. An examples of a salt wedge estuary is Mississippi River[6] and Mandovi estuary in Goa during monsoon period.
Partially mixed
As tidal forcing increases, river output becomes less than the marine input. Here, current induced turbulence causes mixing of the whole water column such that salinity varies more longitudinally rather than vertically, leading to a moderately stratified condition. Examples include the Chesapeake Bay and Narragansett Bay.[6]
Well-mixed
Tidal mixing forces exceed river output, resulting in a well-mixed water column and the disappearance of the vertical salinity gradient. The freshwater-seawater boundary is eliminated due to the intense turbulent mixing and eddy effects. The lower reaches of Delaware Bay and the Raritan River in New Jersey are examples of vertically homogeneous estuaries.[6]
Inverse
Inverse estuaries occur in dry climates where evaporation greatly exceeds the inflow of freshwater. A salinity maximum zone is formed, and both riverine and oceanic water flow close to the surface towards this zone.[7] This water is pushed downward and spreads along the bottom in both the seaward and landward direction.[3] Examples of an inverse estuary are Spencer Gulf, South Australia,[8] Saloum River and Casamance River, Senegal.[9]
Intermittent
Estuary type varies dramatically depending on freshwater input, and is capable of changing from a wholly marine
Physiochemical variation
The most important variable characteristics of estuary water are the concentration of dissolved oxygen,
Implications of eutrophication on estuaries
Effects of eutrophication on biogeochemical cycles
Nitrogen is often the lead cause of eutrophication in estuaries in temperate zones.[13] During a eutrophication event, biogeochemical feedback decreases the amount of available silica.[14] These feedbacks also increase the supply of nitrogen and phosphorus, creating conditions where harmful algal blooms can persist. Given the now off-balance nitrogen cycle, estuaries can be driven to phosphorus limitation instead of nitrogen limitation. Estuaries can be severely impacted by an unbalanced phosphorus cycle, as phosphorus interacts with nitrogen and silica availability.
With an abundance of nutrients in the ecosystem, plants and algae overgrow and eventually decompose, which produce a significant amount of carbon dioxide.[15] While releasing CO2 into the water and atmosphere, these organisms are also intaking all or nearly all of the available oxygen creating a hypoxic environment and unbalanced oxygen cycle.[16] The excess carbon in the form of CO2 can lead to low pH levels and ocean acidification, which is more harmful for vulnerable coastal regions like estuaries.
Effects of eutrophication on estuarine plants
Eutrophication has been seen to negatively impact many plant communities in estuarine ecosystems.[17] Salt marshes are a type of ecosystem in some estuaries that have been negatively impacted by eutrophication.[17] Cordgrass vegetation dominates the salt marsh landscape.[18] Excess nutrients allow the plants to grow at greater rates in above ground biomass, however less energy is allocated to the roots since nutrients is abundant.[17][19] This leads to a lower biomass in the vegetation below ground which destabilizes the banks of the marsh causing increased rates of erosion.[17] A similar phenomenon occurs in mangrove swamps, which are another potential ecosystem in estuaries.[19][20] An increase in nitrogen causes an increase in shoot growth and a decrease in root growth.[19] Weaker root systems cause a mangrove tree to be less resilient in seasons of drought, which can lead to the death of the mangrove.[19] This shift in above ground and below ground biomass caused by eutrophication could hindered plant success in these ecosystems.[17][19]
Effects of eutrophication on estuarine animals
Across all biomes, eutrophication often results in plant death but the impacts do not end there. Plant death alters the entire food web structure which can result in the death of animals within the afflicted biome. Estuaries are hotspots for biodiversity, containing a majority of commercial fish catch, making the impacts of eutrophication that much greater within estuaries.[21] Some specific estuarine animals feel the effects of eutrophication more strongly than others. One example is the whitefish species from the European Alps.[22] Eutrophication reduced the oxygen levels in their habitats so greatly that whitefish eggs could not survive, causing local extinctions.[22] However, some animals, such as carnivorous fish, tend to do well in nutrient-enriched environments and can benefit from eutrophication.[23] This can be seen in populations of bass or pikes.[23]
Effects of eutrophication on human activities
Eutrophication can affect many marine habitats which can lead to economic consequences. The commercial fishing industry relies upon estuaries for approximately 68 percent of their catch by value because of the great biodiversity of this ecosystem.[24] During an algal bloom, fishermen have noticed a significant increase in the quantity of fish.[25] A sudden increase in primary productivity causes spikes in fish populations which leads to more oxygen being utilized.[25] It is the continued deoxygenation of the water that then causes a decline in fish populations. These effects can begin in estuaries and have a wide effect on the surrounding water bodies. In turn, this can decrease fishing industry sales in one area and across the country.[26] Production in 2016 from recreational and commercial fishing contributes billions of dollars to the United States' gross domestic product (GDP).[24] A decrease in production within this industry can affect any of the 1.7 million people the fishing industry employs yearly across the United States.
Implications for marine life
Estuaries are incredibly dynamic systems, where temperature, salinity, turbidity, depth and flow all change daily in response to the tides. This dynamism makes estuaries highly productive habitats, but also make it difficult for many species to survive year-round. As a result, estuaries large and small experience strong seasonal variation in their fish communities.[27] In winter, the fish community is dominated by hardy marine residents, and in summer a variety of marine and anadromous fishes move into and out of estuaries, capitalizing on their high productivity.[28] Estuaries provide a critical habitat to a variety of species that rely on estuaries for life-cycle completion. Pacific Herring (Clupea pallasii) are known to lay their eggs in estuaries and bays, surfperch give birth in estuaries, juvenile flatfish and rockfish migrate to estuaries to rear, and anadromous salmonids and lampreys use estuaries as migration corridors.[29] Also, migratory bird populations, such as the black-tailed godwit,[30] rely on estuaries.
Two of the main challenges of estuarine life are the variability in
A primary source of food for many organisms on estuaries, including bacteria, is detritus from the settlement of the sedimentation.
Human impact
Of the thirty-two largest cities in the world in the early 1990s, twenty-two were located on estuaries.[31]
As ecosystems, estuaries are under threat from human activities such as
Such toxins can accumulate in the tissues of many species of aquatic life in a process called
For example, Chinese and Russian industrial pollution, such as phenols and heavy metals, has devastated fish stocks in the
Estuaries tend to be naturally
Examples
Africa
- Orange River Estuary
- Lake St LuciaEstuary
- Pungwe River Estuary
- Gambia River Estuary
- Gabon Estuary
Asia
- Gulf of Ob Estuary
- Yenisei GulfEstuary
- Amur RiverEstuary
- Puerto Princesa Underground River
- Adyar River Estuary
- Hàn River Estuary
- Kraburi River Estuary[37]
- Waeru River Estuary of Chanthaburi Province[38]
- Dawei River Estuary[39]
- Naf River Estuary[40]
- Meghna River Estuary[41]
- Yangtze Riverestuary
- Hangzhou Bay
Europe
- The Gironde
- Golden Horn
- The Humber
- Severn Estuary
- Shannon Estuary
- Thames Estuary
- The Wash
- Unterelbe
- Western Scheldt
- Tagus Estuary
- Oder Estuary
- Dnieper-Bug Estuary
- Firth of Forth
- Exe Estuary
- Dee Estuary
- Firth of Clyde
- Southampton Water
North America
- Albemarle Sound including Outer Banks of North Carolina
- Chesapeake Bay including Hampton Roads
- Columbia River Estuary
- Delaware Bay
- Drake's Estero
- East River
- Estuary of Saint Lawrence
- Fraser River
- Galveston Bay
- Great Bay
- Indian River Lagoon
- Laguna Madre
- Lake Borgne
- Lake Merritt
- Long Island Sound
- Mississippi River Delta
- Mobile Bay
- Narragansett Bay
- Newport Back Bay
- New York-New Jersey Harbor
- Coos Bay
- Puget Sound
- Pamlico Sound including the Outer Banks of North Carolina
- San Francisco Bay
- Sarasota Bay
- Tampa Bay
Oceania
- Avon Heathcote Estuary (Christchurch, New Zealand)
- Gippsland Lakes
- Port Jackson (Sydney Harbour)
- Spencer Gulf[8]
South America
- Amazon River[42]
- Iguape-Cananéia-Paranaguá estuary lagoon complex
- Lagoa dos Patos and Lagoon Mirim
- Rio de la Plata
See also
- Beaches in estuaries and bays – Type of beaches
- Coastal and Estuarine Research Federation – U.S. nonprofit organization
- Estuarine acidification – Reducing pH values in coastal marine ecosystems
- Estuarine fish– Fish that inhabit the sea between the shoreline and the edge of the continental shelf
- Firth – Scottish word used for various coastal inlets and straits
- Liman – River estuary lagoon in Black Sea region
- List of estuaries of England – A list of estuaries in England
- List of estuaries of South Africa
- List of waterways – List of navigable rivers, canals, estuaries, lakes, and firths
- National Estuarine Research Reserve – Network of 30 protected areas in the US
- Region of freshwater influence – Coastal sea region
- River delta – Silt deposition landform at the mouth of a river
- Shell growth in estuaries
- Tidal bore – A water wave traveling upstream a river or narrow bay because of an incoming tide
- Tidal prism – Volume of water in an estuary or inlet between mean high tide and mean low tide
- Wetland – Land area that is permanently, or seasonally saturated with water
References
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- ^ ISBN 978-94-010-6382-1, retrieved 2022-04-20
- ^ a b Lellis-Dibble, K.A. (2008). "Estuarine Fish and Shellfish Species in US commercial and Recreational Fisheries: Economic Value as an Incentive to Protect and Restore Estuarine Habitat". National Oceanic and Atmospheric Administration.
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- ^ Osborn, Katherine (December 2017). Seasonal fish and invertebrate communities in three northern California estuaries (M.S. thesis). Humboldt State University.
- ^ Allen, Larry G. (1982). "Seasonal abundance, composition and productivity of the littoral fish assemblage in Upper Newport Bay, California" (PDF). Fishery Bulletin. 80 (4): 769–790.
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- ^ "Oyster Reefs: Ecological importance". US National Oceanic and Atmospheric Administration. Archived from the original on October 3, 2008. Retrieved 2008-01-16.
- ^ "สัณฐานชายฝั่ง - ระบบฐานข้อมูลทรัพยากรทางทะเลและชายฝั่ง กรมทรัพยากรทางทะเลและชายฝั่ง". km.dmcr.go.th.
- ^ "พื้นที่ชุ่มน้ำในประเทศไทย". wetland.onep.go.th. Archived from the original on 2019-02-09. Retrieved 2019-02-07.
- ^ "Dawei(Tavoy)". myanmarholiday.com. Archived from the original on 2020-07-31. Retrieved 2019-06-14.
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External links
- Animated documentary on Chesapeake Bay NOAA.
- "Habitats: Estuaries – Characteristics". www.onr.navy.mil. Archived from the original on 2009-05-17. Retrieved 2009-11-17.
- The Estuary Guide (Based on experience and R&D within the UK)