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{{Short description|Partially enclosed coastal body of brackish water}}{{Other uses}}File:Rio de la Plata BA 2.JPG|thumb|right|Río de la PlataRío de la Plata{{ocean habitat topics|image=(File:Mattole River Estuary 2005.jpg|250px)|caption= Mattole River estuary}}An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea.BOOK, Pritchard, D. W., 1967, What is an estuary: physical viewpoint, 3–5, G. H., Lauf, Estuaries, A.A.A.S. Publ., 83, Washington, DC, 1969.3/24383, Estuaries form a transition zone between river environments and maritime environments and are an example of an ecotone. Estuaries are subject both to marine influences such as tides, waves, and the influx of saline water, and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.BOOK, McLusky, D. S., Elliott, M., 2004, The Estuarine Ecosystem: Ecology, Threats and Management, New York, Oxford University Press, 978-0-19-852508-0, 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.BOOK, Wolanski, E., 2007, Estuarine Ecohydrology, Amsterdam, Elsevier, 978-0-444-53066-0, 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.JOURNAL, 10.1016/j.ecoleng.2017.02.027, Can navigation locks be used to help migratory fishes with poor swimming performance pass tidal barrages? A test with lampreys, Ecological Engineering, 102, 291–302, 2017, Silva, Sergio, Lowry, Maran, Macaya-Solis, Consuelo, Byatt, Barry, Lucas, Martyn C., free, 2017EcEng.102..291S,

Definition

File:New York STS058-081-038.jpg|thumb|New York–New Jersey Harbor Estuary ]]File:Exe estuary from balloon.jpg|thumb|right|River ExeRiver ExeFile:Estuary mouth.jpg|thumb|right|Estuary mouth located in Darwin, Northern TerritoryNorthern TerritoryFile:Thekkumbhagam Estuary, Paravur.jpg|right|thumb|A crowded estuary mouth in Paravur near the city of KollamKollam(File:Estuary-mouth.jpg|thumb|right|Estuary mouth)File:Yachats River estuary mouth.jpg|thumb|right|Estuary mouth of the Yachats River in Yachats, OregonYachats, Oregon File:Mouths of amazon geocover 1990.png|thumb|Amazon RiverAmazon RiverThe 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”. 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 tidal limit or the salt intrusion limit and receiving freshwater runoff; however the freshwater inflow may not be perennial, the connection to the sea may be closed for part of the year and tidal influence may be negligible”. This broad definition also includes fjords, lagoons, river mouths, and tidal creeks. An estuary is a dynamic ecosystem having a connection to the open sea through which the sea water enters with the rhythm of the tides. The effects of tides on estuaries can show nonlinear effects on the movement of water which can have important impacts on the ecosystem and waterflow. The seawater entering the estuary is diluted by the fresh water flowing from rivers and streams. The pattern of dilution varies between different estuaries and depends on the volume of freshwater, the tidal range, and the extent of evaporation of the water in the estuary.

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 {{convert|30|m|ft|-2|abbr=on}}. Examples of this type of estuary in the U.S. are the Hudson River, Chesapeake Bay, and Delaware Bay along the Mid-Atlantic coast, and Galveston Bay and Tampa Bay along the Gulf Coast.JOURNAL, Kunneke, J. T., T. F., Palik, 1984,www.nwrc.usgs.gov/wdb/pub/others/85_15.pdf, Tampa Bay environmental atlas, U.S. Fish Wildl. Serv. Biol. Rep., 85, 15, 3, January 12, 2010,

Lagoon-type or bar-built

{{see also|Mouth bar}}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 {{convert|4|m|ft|abbr=on}}. 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|date=December 2020}}

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 {{convert|300|m|ft|-2|abbr=on}}. 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 faulting, volcanoes, and landslides. Inundation from eustatic sea-level rise during the Holocene Epoch has also contributed to the formation of these estuaries. There are only a small number of tectonically produced estuaries; one example is the San Francisco Bay, which was formed by the crustal movements of the San Andreas fault system causing the inundation of the lower reaches of the Sacramento and San Joaquin rivers.BOOK, Kennish, M. J., 1986, Ecology of Estuaries. Volume I: Physical and Chemical Aspects, Boca Raton, FL, CRC Press, 978-0-8493-5892-0,

Classification based on water circulation

{{See also|Estuarine water circulation}}

Salt wedge

{{Redirect-distinguish|Salt wedge|Salt wedging (geology)}}{{See also|Saltwater intrusion}}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 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.

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.

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.JOURNAL, Wolanski, E., 1986, An evaporation-driven salinity maximum zone in Australian tropical estuaries, Estuarine, Coastal and Shelf Science, 22, 4, 415–424, 10.1016/0272-7714(86)90065-X, 1986ECSS...22..415W, This water is pushed downward and spreads along the bottom in both the seaward and landward direction. Examples of an inverse estuary are Spencer Gulf, South Australia,Gostin, V. & Hall, S.M. (2014): Spencer Gulf: Geological setting and evolution. In: Natural History of Spencer Gulf. Royal Society of South Australia Inc. p. 21. {{ISBN|9780959662764}} Saloum River and Casamance River, Senegal.JOURNAL, 2020, Inverse Estuaries in West Africa: Evidence of the Rainfall Recovery?, Water, 10.3390/w12030647, free, Descroix, Luc, Sané, Yancouba, Thior, Mamadou, Manga, Sylvie-Paméla, Ba, Boubacar Demba, Mingou, Joseph, Mendy, Victor, Coly, Saloum, Dièye, Arame, Badiane, Alexandre, Senghor, Marie-Jeanne, Diedhiou, Ange-Bouramanding, Sow, Djiby, Bouaita, Yasmin, Soumaré, Safietou, Diop, Awa, Faty, Bakary, Sow, Bamol Ali, Machu, Eric, Montoroi, Jean-Pierre, Andrieu, Julien, Vandervaere, Jean-Pierre, 12, 3, 647,

Intermittent

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.WEB, Tomczak, M., 2000,www.es.flinders.edu.au/~mattom/IntroOc/notes/lecture12.html, Oceanography Notes Ch. 12: Estuaries, 30 November 2006,www.es.flinders.edu.au/~mattom/IntroOc/notes/lecture12.html," title="web.archive.org/web/20061207053006www.es.flinders.edu.au/~mattom/IntroOc/notes/lecture12.html,">web.archive.org/web/20061207053006www.es.flinders.edu.au/~mattom/IntroOc/notes/lecture12.html, 7 December 2006, dead, BOOK, Day, J. H., 1981, Estuarine Ecology, Rotterdam, A. A. Balkema, 978-90-6191-205-7,

Physiochemical variation

The most important variable characteristics of estuary water are the concentration of dissolved oxygen, salinity and sediment load. There is extreme spatial variability in salinity, with a range of near-zero at the tidal limit of tributary rivers to 3.4% at the estuary mouth. At any one point, the salinity will vary considerably over time and seasons, making it a harsh environment for organisms. Sediment often settles in intertidal mudflats which are extremely difficult to colonize. No points of attachment exist for algae, so vegetation based habitat is not established.{{clarify|date=June 2012}} Sediment can also clog feeding and respiratory structures of species, and special adaptations exist within mudflat species to cope with this problem. Lastly, dissolved oxygen variation can cause problems for life forms. Nutrient-rich sediment from human-made sources can promote primary production life cycles, perhaps leading to eventual decay removing the dissolved oxygen from the water; thus hypoxic or anoxic zones can develop.BOOK, Kaiser, 2005, Marine Ecology. Processes, Systems and Impacts, New York, Oxford University Press, 978-0199249756, etal,

Implications of eutrophication on estuaries

{{Further|Eutrophication#Coastal waters}}

Effects of eutrophication on biogeochemical cycles

(File:Table_of_the_Processes_in_the_Nitrogen_Cycle.jpg|thumb|Processes that nitrogen undergo in estuarine systems)Nitrogen is often the lead cause of eutrophication in estuaries in temperate zones.JOURNAL, Howarth, Robert W., Marino, Roxanne, 2006, Nitrogen as the limiting nutrient for eutrophication in coastal marine ecosystems: Evolving views over three decades, Limnology and Oceanography, en, 51, 1part2, 364–376, 10.4319/lo.2006.51.1_part_2.0364, 2006LimOc..51..364H, 18144068, 0024-3590, free, During a eutrophication event, biogeochemical feedback decreases the amount of available silica.JOURNAL, Howarth, Robert, Chan, Francis, Conley, Daniel J, Garnier, Josette, Doney, Scott C, Marino, Roxanne, Billen, Gilles, 2011, Coupled biogeochemical cycles: eutrophication and hypoxia in temperate estuaries and coastal marine ecosystems, Frontiers in Ecology and the Environment, en, 9, 1, 18–26, 10.1890/100008, 1813/60819, 1540-9295, free, 2011FrEE....9...18H, free, 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.JOURNAL, Morales-Williams, Ana M., Wanamaker, Alan D., Williams, Clayton J., Downing, John A., 2021, Eutrophication Drives Extreme Seasonal {{CO2, Flux in Lake Ecosystems |url=https://link.springer.com/10.1007/s10021-020-00527-2 |journal=Ecosystems |language=en |volume=24 |issue=2 |pages=434–450 |doi=10.1007/s10021-020-00527-2 |bibcode=2021Ecosy..24..434M |s2cid=220856626 |issn=1432-9840}} 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.BOOK, Selman, Mindy,www.wri.org/research/eutrophication-and-hypoxia-coastal-areas, Eutrophication and Hypoxia in Coastal Areas, Sugg, Zachary, Greenhalgh, Suzie, 2008, World Resources Institute, 978-1-56973-681-4, en, 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

(File:Wood_storks_wading_in_a_marsh.jpg|thumb|A salt marsh with wood storks wading)Eutrophication has been seen to negatively impact many plant communities in estuarine ecosystems.JOURNAL, Deegan, Linda A., Johnson, David Samuel, Warren, R. Scott, Peterson, Bruce J., Fleeger, John W., Fagherazzi, Sergio, Wollheim, Wilfred M., 2012, Coastal eutrophication as a driver of salt marsh loss,dx.doi.org/10.1038/nature11533, Nature, 490, 7420, 388–392, 10.1038/nature11533, 23075989, 2012Natur.490..388D, 4414196, 0028-0836, Salt marshes are a type of ecosystem in some estuaries that have been negatively impacted by eutrophication. Cordgrass vegetation dominates the salt marsh landscape.JOURNAL, Donnelly, Jeffrey P., Bertness, Mark D., 2001, Rapid shoreward encroachment of salt marsh cordgrass in response to accelerated sea-level rise, Proceedings of the National Academy of Sciences, 98, 25, 14218–14223, 10.1073/pnas.251209298, 11724926, 64662, 2001PNAS...9814218D, 0027-8424, free, 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.JOURNAL, Lovelock, Catherine E., Ball, Marilyn C., Martin, Katherine C., C. Feller, Ilka, 2009, Nutrient Enrichment Increases Mortality of Mangroves, PLOS ONE, 4, 5, e5600, 10.1371/journal.pone.0005600, 19440554, 2679148, 2009PLoSO...4.5600L, 1932-6203, free, This leads to a lower biomass in the vegetation below ground which destabilizes the banks of the marsh causing increased rates of erosion. A similar phenomenon occurs in mangrove swamps, which are another potential ecosystem in estuaries.JOURNAL, Guest, Michaela A., Connolly, Rod M., 2005, Fine-scale movement and assimilation of carbon in saltmarsh and mangrove habitat by resident animals,dx.doi.org/10.1007/s10452-005-0442-9, Aquatic Ecology, 38, 4, 599–609, 10.1007/s10452-005-0442-9, 2005AqEco..38..599G, 20771999, 1386-2588, An increase in nitrogen causes an increase in shoot growth and a decrease in root growth. Weaker root systems cause a mangrove tree to be less resilient in seasons of drought, which can lead to the death of the mangrove. This shift in above ground and below ground biomass caused by eutrophication could hindered plant success in these ecosystems.

Effects of eutrophication on estuarine animals

(File:Whitefish, from the Fish from American Waters series (N8) for Allen & Ginter Cigarettes Brands MET DP830737.jpg|thumb|Example of a whitefish)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.JOURNAL, Waltham, Nathan J., McCann, Jack, Power, Trent, Moore, Matt, Buelow, Christina, 2020, Patterns of fish use in urban estuaries: Engineering maintenance schedules to protect broader seascape habitat, Estuarine, Coastal and Shelf Science, 238, 106729, 10.1016/j.ecss.2020.106729, 2020ECSS..23806729W, 216460098, 0272-7714, free, Some specific estuarine animals feel the effects of eutrophication more strongly than others. One example is the whitefish species from the European Alps.Vonlanthen, P., Bittner, D., Hudson A.G., et al. (2012). Eutrophication causes speciation reversal in whitefish adaptive radiations. Nature. 482, 337-362. DOI: 10.1038/nature0824. Eutrophication reduced the oxygen levels in their habitats so greatly that whitefish eggs could not survive, causing local extinctions. However, some animals, such as carnivorous fish, tend to do well in nutrient-enriched environments and can benefit from eutrophication.{{Citation |last1=Jeppesen |first1=Erik |date=1997 |url=http://dx.doi.org/10.1007/978-94-011-5648-6_17 |work=Shallow Lakes ‘95 |pages=151–164 |place=Dordrecht |publisher=Springer Netherlands |isbn=978-94-010-6382-1 |access-date=2022-04-20 |last2=Peder Jensen |first2=Jens |last3=Søndergaard |first3=Martin |last4=Lauridsen |first4=Torben |last5=Junge Pedersen |first5=Leif |last6=Jensen |first6=Lars|title=Top-down control in freshwater lakes: The role of nutrient state, submerged macrophytes and water depth |doi=10.1007/978-94-011-5648-6_17 }} This can be seen in populations of bass or pikes.

Effects of eutrophication on human activities

(File:Fishing_boat_at_Wrangell_Harbor.jpg|thumb|Commercial fishing boat)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.JOURNAL, 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, During an algal bloom, fishermen have noticed a significant increase in the quantity of fish.JOURNAL, Gao, Yang, Lee, Jeong-Yeol, 2012-12-30, Compensatory Responses of Nile Tilapia Oreochromis niloticus under Different Feed-Deprivation Regimes, Fisheries and Aquatic Sciences, 15, 4, 305–311, 10.5657/fas.2012.0305, 2234-1749, free, A sudden increase in primary productivity causes spikes in fish populations which leads to more oxygen being utilized. 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.JOURNAL, Fay, Gavin, DePiper, Geret, Steinback, Scott, Gamble, Robert J., Link, Jason S., 2019, Economic and Ecosystem Effects of Fishing on the Northeast US Shelf, Frontiers in Marine Science, 6, 10.3389/fmars.2019.00133, 2296-7745, free, Production in 2016 from recreational and commercial fishing contributes billions of dollars to the United States’ gross domestic product (GDP). 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.THESIS, Osborn, Katherine, Seasonal fish and invertebrate communities in three northern California estuaries, December 2017, M.S., Humboldt State University,digitalcommons.humboldt.edu/etd/101/, 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.JOURNAL, Allen, Larry G., 1982, Seasonal abundance, composition and productivity of the littoral fish assemblage in Upper Newport Bay, California,spo.nmfs.noaa.gov/sites/default/files/pdf-content/1982/804/allen.pdf, Fishery Bulletin, 80, 4, 769–790, 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.JOURNAL, 10.3354/meps247281, 24866466, Evidence of connectivity between juvenile and adult habitats for mobile marine fauna: An important component of nurseries, Marine Ecology Progress Series, 247, 281–295, 2003, Gillanders, BM, Able, KW, Brown, JA, Eggleston, DB, Sheridan, PF, 2003MEPS..247..281G, free, 2440/1877, free, Also, migratory bird populations, such as the black-tailed godwit,JOURNAL, 10.1038/35086568, 11473317, The buffer effect and large-scale population regulation in migratory birds, Nature, 412, 6845, 436–438, 2001, Gill, Jennifer A., Norris, Ken, Potts, Peter M., Gunnarsson, Tómas Grétar, Atkinson, Philip W., Sutherland, William J., 2001Natur.412..436G, 4308197, rely on 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 a more stable sedimental environment. However, large numbers of bacteria are found within the sediment which has 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 flow.Phytoplankton 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 dependent upon the turbidity of the water. The main phytoplankton present are diatoms and dinoflagellates which are abundant in the sediment.A primary source of food for many organisms on estuaries, including bacteria, is detritus from the settlement of the sedimentation.

Human impact

{{Further|Ecosystem service#Estuarine and coastal ecosystem services|Marine ecosystem#Threats}}Of the thirty-two largest cities in the world in the early 1990s, twenty-two were located on estuaries.BOOK, Ross, D. A., 1995, Introduction to Oceanography, New York, Harper Collins College Publishers, 978-0-673-46938-0, 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.JOURNAL, 10.1016/S0169-5347(99)01732-2, Estuarine vulnerability and ecological impacts, Trends in Ecology & Evolution, 14, 12, 499, 1999, Branch, George, 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. The elemental composition of biofilm reflect areas of the estuary impacted by human activities, and over time may shift the basic composition of the ecosystem, and the reversible or irreversible changes in the abiotic and biotic parts of the systems from the bottom up.JOURNAL, 10.1016/j.ecss.2016.12.018, Total and extractable elemental composition of the intertidal estuarine biofilm of the Río de la Plata: Disentangling natural and anthropogenic influences, Estuarine, Coastal and Shelf Science, 187, 53–61, 2017, García-Alonso, J., Lercari, D., Araujo, B.F., Almeida, M.G., Rezende, C.E., 2017ECSS..187...53G, For example, Chinese and Russian industrial pollution, such as phenols and heavy metals, has devastated fish stocks in the Amur River and damaged its estuary soil.“Indigenous Peoples of the Russian North, Siberia and Far East: Nivkh” {{Webarchive|url=https://web.archive.org/web/20090807054552www.npolar.no/ansipra/english/Indexpages/Ethnic_groups.html#19 |date=2009-08-07 }} by Arctic Network for the Support of the Indigenous Peoples of the Russian ArcticEstuaries 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.BOOK, Sebastian A., Gerlach, Marine Pollution: Diagnosis and Therapy,archive.org/details/marinepollutiond0000gerl, registration, Springer, Berlin, 1981, 978-0387109404, This can result in reductions in water quality, fish, and other animal populations.Overfishing also occurs. Chesapeake Bay once had a flourishing oyster population that has been almost wiped out by overfishing. Oysters filter these pollutants, and either eat them or shape them into small packets that are deposited on the bottom where they are harmless. 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,WEB,habitat.noaa.gov/restorationtechniques/public/habitat.cfm?HabitatID=2&HabitatTopicID=11, Oyster Reefs: Ecological importance, US National Oceanic and Atmospheric Administration, 2008-01-16, dead,habitat.noaa.gov/restorationtechniques/public/habitat.cfm?HabitatID=2&HabitatTopicID=11," title="web.archive.org/web/20081003114619habitat.noaa.gov/restorationtechniques/public/habitat.cfm?HabitatID=2&HabitatTopicID=11,">web.archive.org/web/20081003114619habitat.noaa.gov/restorationtechniques/public/habitat.cfm?HabitatID=2&HabitatTopicID=11, October 3, 2008, and sediment, nutrients, and algae can cause problems in local waters.

Examples

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Africa

• Orange River Estuary
• Lake St Lucia Estuary
• Pungwe River Estuary
• Gambia River Estuary
• Gabon Estuary

Asia

• Gulf of Ob Estuary
• Yenisei Gulf Estuary
• Amur River Estuary
• Puerto Princesa Underground River
• Adyar River Estuary
• Hàn River Estuary
• Kraburi River EstuaryWEB,km.dmcr.go.th/th/c_56/s_77/d_2766, สัณฐานชายฝั่ง - ระบบฐานข้อมูลทรัพยากรทางทะเลและชายฝั่ง กรมทรัพยากรทางทะเลและชายฝั่ง, km.dmcr.go.th,
• Waeru River Estuary of Chanthaburi ProvinceWEB,wetland.onep.go.th/2559-1-Waeru.html, พื้นที่ชุ่มน้ำในประเทศไทย, wetland.onep.go.th, 2019-02-07, 2019-02-09,wetland.onep.go.th/2559-1-Waeru.html," title="web.archive.org/web/20190209124327wetland.onep.go.th/2559-1-Waeru.html,">web.archive.org/web/20190209124327wetland.onep.go.th/2559-1-Waeru.html, dead,
• Dawei River EstuaryWEB,myanmarholiday.com/index.php/daweitavoy, Dawei(Tavoy), myanmarholiday.com, 2019-06-14, 2020-07-31,web.archive.org/web/20200731020705/https://myanmarholiday.com/index.php/daweitavoy, dead,
• Naf River EstuaryJOURNAL, 10.1007/s00343-019-8063-7, Spatial and seasonal distribution of Intertidal Macrobenthos with their biomass and functional feeding guilds in the Naf River estuary, Bangladesh, Journal of Oceanology and Limnology, 37, 3, 1010–1023, 2018, Noman, Md. Abu, Mamunur, Rashid, Islam, M. Shahanul, Hossain, M. Belal, 2019JOL....37.1010N, 92734488,
• Meghna River EstuaryJOURNAL, 10.1006/ecss.2001.0929, Residual Flow in the Meghna Estuary on the Coastline of Bangladesh, Estuarine, Coastal and Shelf Science, 55, 4, 587–597, 2002, Jakobsen, F., Azam, M.H., Mahboob-Ul-Kabir, M., 2002ECSS...55..587J,
• Yangtze River estuary
• 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
  • Lake Pontchartrain
• 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

South America

• Amazon RiverWEB,etaisweb.weebly.com/the-amazon-river-estuary.html, The Amazon River Estuary, etai’s web,
• Iguape-Cananéia-Paranaguá estuary lagoon complex
• Lagoa dos Patos and Lagoon Mirim
• Rio de la Plata{{div col end}}

See also

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• {{annotated link|Beaches in estuaries and bays}}
• {{annotated link|Coastal and Estuarine Research Federation}}
• {{annotated link|Estuarine acidification}}
• {{annotated link|Estuarine fish}}
• {{annotated link|Firth}}
• {{annotated link|Liman (landform)|Liman}}
• {{annotated link|List of estuaries of England}}
• {{annotated link|List of estuaries of South Africa}}
• {{annotated link|List of waterways}}
• {{annotated link|National Estuarine Research Reserve}}
• {{annotated link|Region of freshwater influence}}
• {{annotated link|River delta}}
• {{annotated link|Shell growth in estuaries}}
• {{annotated link|Tidal bore}}
• {{annotated link|Tidal prism}}
• {{annotated link|Wetland}}

{{div col end}}{{clear}}

References

{{Reflist|30em}}

External links

{{Commons category|Estuaries}}{{Wiktionary|estuary}}

• ecopath.org/LifeInTheChesapeakeBay/" title="web.archive.org/web/20101023203928ecopath.org/LifeInTheChesapeakeBay/">Animated documentary on Chesapeake Bay NOAA.
• WEB

,

• The Estuary Guide (Based on experience and R&D within the UK)

{{aquatic ecosystem topics|expanded=marine}}{{coastal geography}}{{river morphology}}{{Wetlands}}{{Authority control}}