Editing Estuary

{{Short description|Partially enclosed coastal body of brackish water}}
{{Other uses}}
[[File:Rio de la Plata BA 2.JPG|thumb|right|[[Río de la Plata]] estuary]]
[[File:Mattole River Estuary 2005.jpg|thumb|[[Mattole River]] estuary]]
{{Ocean habitat topics}}

An '''estuary''' is a partially enclosed [[coast]]al body of [[brackish water]] with one or more rivers or streams flowing into it, and with a free connection to the open [[sea]].<ref name="james">{{cite book |last=Pritchard |first=D. W. |year=1967 |chapter= What is an estuary: physical viewpoint |pages=3–5 |editor-first=G. H. |editor-last=Lauf |title= Estuaries |series=A.A.A.S. Publ. |volume=83 |location=Washington, DC |hdl=1969.3/24383 }}</ref> 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]], [[wave]]s, 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.<ref name="McLusky">{{cite book |last1=McLusky |first1=D. S. |last2=Elliott |first2=M. |year=2004 |title=The Estuarine Ecosystem: Ecology, Threats and Management |location=New York |publisher=Oxford University Press |isbn=978-0-19-852508-0 }}</ref>

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.<ref name="Wolanski">{{cite book |last=Wolanski |first=E. |year=2007 |title=Estuarine Ecohydrology |location=Amsterdam |publisher=Elsevier |isbn=978-0-444-53066-0 }}</ref> Estuaries are typically classified according to their [[geomorphology|geomorphological]] features or to water-circulation patterns. They can have many different names, such as [[bay]]s, [[harbor]]s, [[lagoon]]s, [[inlet]]s, or [[Sound (geography)|sound]]s, although some of these water bodies do not strictly meet the above definition of an estuary and could be fully saline.

Many estuaries suffer [[Habitat destruction|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 biphenyl]]s, [[radionuclide]]s and [[hydrocarbon]]s from sewage inputs; and diking or damming for [[flood control]] or water diversion.<ref name="Wolanski"/><ref>{{cite journal |doi=10.1016/j.ecoleng.2017.02.027 |title=Can navigation locks be used to help migratory fishes with poor swimming performance pass tidal barrages? A test with lampreys |journal=Ecological Engineering |volume=102 |pages=291–302 |year=2017 |last1=Silva |first1=Sergio |last2=Lowry |first2=Maran |last3=Macaya-Solis |first3=Consuelo |last4=Byatt |first4=Barry |last5=Lucas |first5=Martyn C. |doi-access=free |bibcode=2017EcEng.102..291S }}</ref>

==Definition==

[[File:New York STS058-081-038.jpg|thumb|[[Geography of New York–New Jersey Harbor Estuary|New York–New Jersey Harbor Estuary]]]]
[[File:Exe estuary from balloon.jpg|thumb|right|[[River Exe]] estuary]]
[[File:Estuary mouth.jpg|thumb|right|Estuary mouth located in [[Darwin, Northern Territory|Darwin]], [[Northern Territory]], Australia]]
[[File:Thekkumbhagam Estuary, Paravur.jpg|right|thumb|A crowded estuary mouth in [[Paravur, Kollam|Paravur]] near the city of [[Kollam]], India]]
[[File:Yachats River estuary mouth.jpg|thumb|right|Estuary mouth of the [[Yachats River]] in [[Yachats, Oregon]]]]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".<ref name="james"/> However, this definition excludes a number of coastal water bodies such as coastal lagoons and [[Brackish water|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".<ref name="Wolanski" /> This broad definition also includes [[fjord]]s, [[lagoon]]s, [[river mouth]]s, and [[tidal creek]]s. An estuary is a dynamic [[ecosystem]] having a connection to the open sea through which the [[sea water]] enters with the rhythm of the [[tide]]s. The effects of tides on estuaries can show [[Nonlinear tides|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.<ref name="McLusky" />

==Classification based on geomorphology==

===Drowned river valleys===
{{main|Ria}}
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 Dollart Region|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}}. [[Ria#Locations|Examples of this type of estuary]] in the U.S. are the [[Hudson River]], [[Chesapeake Bay]], and [[Delaware Bay]] along the [[Mid-Atlantic states|Mid-Atlantic]] coast, and [[Galveston Bay]] and [[Tampa Bay]] along the [[Gulf Coast]].<ref>{{cite journal |last1=Kunneke |first1=J. T. |first2=T. F. |last2=Palik |year=1984 |url=http://www.nwrc.usgs.gov/wdb/pub/others/85_15.pdf |title=Tampa Bay environmental atlas |journal=U.S. Fish Wildl. Serv. Biol. Rep. |volume=85 |issue=15 |pages=3 |access-date=January 12, 2010 }}</ref>

===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 island]]s and barrier [[Spit (landform)|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 current]]s, 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 [[Aquatic sill|sills]] of [[Moraine|glacial deposits]], which have the effects of modifying the estuarine circulation.

[[Fjord]]-type estuaries are formed in deeply eroded valleys formed by [[glacier]]s. 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 [[moraine]]s 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)|Washington state]], [[British Columbia]], eastern Canada, [[Greenland]], [[Iceland]], New Zealand, [[Fjords and channels of Chile|Chile]], and Norway.

===Tectonically produced===
These estuaries are formed by subsidence or land cut off from the ocean by land movement associated with [[Fault (geology)|faulting]], [[volcano]]es, and [[landslide]]s. [[Inundation]] from eustatic sea-level rise during the [[Holocene]] [[Epoch (geology)|Epoch]] has also contributed to the formation of these estuaries. There are only a small number of [[Tectonics|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 River|Sacramento]] and [[San Joaquin River|San Joaquin rivers]].<ref name="Kennish">{{cite book |last=Kennish |first=M. J. |year=1986 |title=Ecology of Estuaries. Volume I: Physical and Chemical Aspects |location=Boca Raton, FL |publisher=CRC Press |isbn=978-0-8493-5892-0 }}</ref>

==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]]<ref name="Kennish"/> and the [[Mandovi River|Mandovi estuary]] in [[Goa]] during the 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]].<ref name="Kennish"/>

===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 [[Turbulence|turbulent mixing]] and [[Eddy (fluid dynamics)|eddy effects]]. The lower reaches of [[Delaware Bay]] and the [[Raritan River]] in [[New Jersey]] are examples of vertically homogeneous estuaries.<ref name="Kennish"/>

===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.<ref>{{cite journal |last=Wolanski |first=E. |year=1986 |title=An evaporation-driven salinity maximum zone in Australian tropical estuaries |journal=Estuarine, Coastal and Shelf Science |volume=22 |issue=4 |pages=415–424 |doi=10.1016/0272-7714(86)90065-X |bibcode = 1986ECSS...22..415W }}</ref> This water is pushed downward and spreads along the bottom in both the seaward and landward direction.<ref name="Wolanski"/> Examples of an inverse estuary are [[Spencer Gulf]], South Australia,<ref name="Gostin">[[Victor Gostin|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}}</ref> [[Saloum River]] and [[Casamance River]], Senegal.<ref>{{Cite journal |date=2020 |title=Inverse Estuaries in West Africa: Evidence of the Rainfall Recovery? |journal=Water|doi=10.3390/w12030647|doi-access= free|last1= Descroix|first1= Luc|last2= Sané|first2= Yancouba|last3= Thior|first3= Mamadou|last4= Manga|first4= Sylvie-Paméla|last5= Ba|first5= Boubacar Demba|last6= Mingou|first6= Joseph|last7= Mendy|first7= Victor|last8= Coly|first8= Saloum|last9= Dièye|first9= Arame|last10= Badiane|first10= Alexandre|last11= Senghor|first11= Marie-Jeanne|last12= Diedhiou|first12= Ange-Bouramanding|last13= Sow|first13= Djiby|last14= Bouaita|first14= Yasmin|last15= Soumaré|first15= Safietou|last16= Diop|first16= Awa|last17= Faty|first17= Bakary|last18= Sow|first18= Bamol Ali|last19= Machu|first19= Eric|last20= Montoroi|first20= Jean-Pierre|last21= Andrieu|first21= Julien|last22= Vandervaere|first22= Jean-Pierre|volume= 12|issue= 3|page= 647|bibcode=2020Water..12..647D }}</ref>

===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.<ref>{{cite web |last=Tomczak |first=M. |year=2000 |url=http://www.es.flinders.edu.au/~mattom/IntroOc/notes/lecture12.html |title=Oceanography Notes Ch. 12: Estuaries |access-date=30 November 2006 |archive-url=https://web.archive.org/web/20061207053006/http://www.es.flinders.edu.au/~mattom/IntroOc/notes/lecture12.html |archive-date=7 December 2006 |url-status=dead }}</ref><ref name="Day">{{cite book |last=Day |first=J. H. |year=1981 |title=Estuarine Ecology |location=Rotterdam |publisher=A. A. Balkema |isbn=978-90-6191-205-7 }}</ref>

==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 [[hypoxia (environmental)|hypoxic]] or [[Dead zone (ecology)|anoxic]] zones can develop.<ref>{{cite book |last=Kaiser |year=2005 |title=Marine Ecology. Processes, Systems and Impacts |location=New York |publisher=Oxford University Press |isbn=978-0199249756 |display-authors=etal}}</ref>

== 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.<ref>{{Cite journal |last1=Howarth |first1=Robert W. |last2=Marino |first2=Roxanne |date=2006 |title=Nitrogen as the limiting nutrient for eutrophication in coastal marine ecosystems: Evolving views over three decades |journal=Limnology and Oceanography |language=en |volume=51 |issue=1part2 |pages=364–376 |doi=10.4319/lo.2006.51.1_part_2.0364 |bibcode=2006LimOc..51..364H |s2cid=18144068 |issn=0024-3590|doi-access=free }}</ref> During a eutrophication event, biogeochemical feedback decreases the amount of available [[Silica cycle|silica]].<ref>{{Cite journal |last1=Howarth |first1=Robert |last2=Chan |first2=Francis |last3=Conley |first3=Daniel J |last4=Garnier |first4=Josette |last5=Doney |first5=Scott C |last6=Marino |first6=Roxanne |last7=Billen |first7=Gilles |date=2011 |title=Coupled biogeochemical cycles: eutrophication and hypoxia in temperate estuaries and coastal marine ecosystems |journal=Frontiers in Ecology and the Environment |language=en |volume=9 |issue=1 |pages=18–26 |doi=10.1890/100008 |hdl=1813/60819 |issn=1540-9295|doi-access=free |bibcode=2011FrEE....9...18H |hdl-access=free }}</ref> These feedbacks also increase the supply of [[Nitrogen cycle|nitrogen]] and phosphorus, creating conditions where harmful algal blooms can persist. Given the now off-balance [[nitrogen cycle]], estuaries can be driven to [[Phosphorus cycle|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.<ref>{{Cite journal |last1=Morales-Williams |first1=Ana M. |last2=Wanamaker |first2=Alan D. |last3=Williams |first3=Clayton J. |last4=Downing |first4=John A. |date=2021 |title=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}}</ref> While releasing {{CO2}} into the water and atmosphere, these organisms are also intaking all or nearly all of the available oxygen creating a [[Dead zone (ecology)|hypoxic]] environment and unbalanced [[oxygen cycle]].<ref>{{Cite book |last1=Selman |first1=Mindy |url=https://www.wri.org/research/eutrophication-and-hypoxia-coastal-areas |title=Eutrophication and Hypoxia in Coastal Areas |last2=Sugg |first2=Zachary |last3=Greenhalgh |first3=Suzie |date=2008 |publisher=World Resources Institute |isbn=978-1-56973-681-4 |language=en}}</ref> 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 [[ecosystem]]s.<ref name=":5">{{Cite journal |last1=Deegan |first1=Linda A. |last2=Johnson |first2=David Samuel |last3=Warren |first3=R. Scott |last4=Peterson |first4=Bruce J. |last5=Fleeger |first5=John W. |last6=Fagherazzi |first6=Sergio |last7=Wollheim |first7=Wilfred M. |date=2012 |title=Coastal eutrophication as a driver of salt marsh loss |url=http://dx.doi.org/10.1038/nature11533 |journal=Nature |volume=490 |issue=7420 |pages=388–392 |doi=10.1038/nature11533 |pmid=23075989 |bibcode=2012Natur.490..388D |s2cid=4414196 |issn=0028-0836}}</ref> [[Salt marsh]]es are a type of ecosystem in some estuaries that have been negatively impacted by eutrophication.<ref name=":5" /> [[Spartina|Cordgrass]] vegetation dominates the salt marsh landscape.<ref>{{Cite journal |last1=Donnelly |first1=Jeffrey P. |last2=Bertness |first2=Mark D. |date=2001 |title=Rapid shoreward encroachment of salt marsh cordgrass in response to accelerated sea-level rise |journal=Proceedings of the National Academy of Sciences |volume=98 |issue=25 |pages=14218–14223 |doi=10.1073/pnas.251209298 |pmid=11724926 |pmc=64662 |bibcode=2001PNAS...9814218D |issn=0027-8424|doi-access=free }}</ref> 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.<ref name=":5" /><ref name=":0">{{Cite journal |last1=Lovelock |first1=Catherine E. |last2=Ball |first2=Marilyn C. |last3=Martin |first3=Katherine C. |last4=C. Feller |first4=Ilka |date=2009 |title=Nutrient Enrichment Increases Mortality of Mangroves |journal=PLOS ONE |volume=4 |issue=5 |pages=e5600 |doi=10.1371/journal.pone.0005600 |pmid=19440554 |pmc=2679148 |bibcode=2009PLoSO...4.5600L |issn=1932-6203|doi-access=free }}</ref> This leads to a lower [[Biomass (ecology)|biomass]] in the vegetation below ground which destabilizes the banks of the marsh causing increased rates of [[erosion]].<ref name=":5" /> A similar phenomenon occurs in [[Mangrove forest|mangrove swamps]], which are another potential ecosystem in estuaries.<ref name=":0" /><ref>{{Cite journal |last1=Guest |first1=Michaela A. |last2=Connolly |first2=Rod M. |date=2005 |title=Fine-scale movement and assimilation of carbon in saltmarsh and mangrove habitat by resident animals |url=http://dx.doi.org/10.1007/s10452-005-0442-9 |journal=Aquatic Ecology |volume=38 |issue=4 |pages=599–609 |doi=10.1007/s10452-005-0442-9 |bibcode=2005AqEco..38..599G |s2cid=20771999 |issn=1386-2588}}</ref> An increase in nitrogen causes an increase in shoot growth and a decrease in root growth.<ref name=":0" /> Weaker root systems cause a mangrove tree to be less resilient in seasons of drought, which can lead to the death of the mangrove.<ref name=":0" /> This shift in above ground and below ground biomass caused by eutrophication could hindered plant success in these ecosystems.<ref name=":5" /><ref name=":0" />

=== 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.<ref>{{Cite journal |last1=Waltham |first1=Nathan J. |last2=McCann |first2=Jack |last3=Power |first3=Trent |last4=Moore |first4=Matt |last5=Buelow |first5=Christina |date=2020 |title=Patterns of fish use in urban estuaries: Engineering maintenance schedules to protect broader seascape habitat |journal=[[Estuarine, Coastal and Shelf Science]] |volume=238 |pages=106729 |doi=10.1016/j.ecss.2020.106729 |bibcode=2020ECSS..23806729W |s2cid=216460098 |issn=0272-7714|doi-access=free }}</ref> Some specific estuarine animals feel the effects of eutrophication more strongly than others. One example is the whitefish species from the [[Alps|European Alps]].<ref name=":1">[https://ec.europa.eu/environment/integration/research/newsalert/pdf/290na1_en.pdf 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.]</ref> Eutrophication reduced the oxygen levels in their habitats so greatly that whitefish eggs could not survive, causing local extinctions.<ref name=":1" /> However, some animals, such as carnivorous fish, tend to do well in nutrient-enriched environments and can benefit from eutrophication.<ref name=":2">{{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 }}</ref> This can be seen in populations of bass or pikes.<ref name=":2" />

=== 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.<ref name=":4">{{Cite journal |last=Lellis-Dibble |first=K.A. |date=2008 |title=Estuarine Fish and Shellfish Species in US commercial and Recreational Fisheries: Economic Value as an Incentive to Protect and Restore Estuarine Habitat |journal=National Oceanic and Atmospheric Administration}}</ref> During an [[algal bloom]], fishermen have noticed a significant increase in the quantity of fish.<ref name=":3">{{Cite journal |last1=Gao |first1=Yang |last2=Lee |first2=Jeong-Yeol |date=2012-12-30 |title=Compensatory Responses of Nile Tilapia Oreochromis niloticus under Different Feed-Deprivation Regimes |journal=Fisheries and Aquatic Sciences |volume=15 |issue=4 |pages=305–311 |doi=10.5657/fas.2012.0305 |issn=2234-1749|doi-access=free }}</ref> A sudden increase in primary productivity causes spikes in fish populations which leads to more oxygen being utilized.<ref name=":3" /> 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.<ref>{{Cite journal |last1=Fay |first1=Gavin |last2=DePiper |first2=Geret |last3=Steinback |first3=Scott |last4=Gamble |first4=Robert J. |last5=Link |first5=Jason S. |date=2019 |title=Economic and Ecosystem Effects of Fishing on the Northeast US Shelf |journal=Frontiers in Marine Science |volume=6 |page=133 |doi=10.3389/fmars.2019.00133 |issn=2296-7745|doi-access=free |bibcode=2019FrMaS...6..133F }}</ref> Production in 2016 from recreational and commercial fishing contributes billions of dollars to the United States' gross domestic product (GDP).<ref name=":4" /> 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.<ref>{{cite thesis|last=Osborn|first=Katherine|title=Seasonal fish and invertebrate communities in three northern California estuaries|date=December 2017|degree=M.S.|publisher=Humboldt State University|url=https://digitalcommons.humboldt.edu/etd/101/}}</ref> 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.<ref>{{Cite journal|last=Allen|first=Larry G.|date=1982|title=Seasonal abundance, composition and productivity of the littoral fish assemblage in Upper Newport Bay, California|url=https://spo.nmfs.noaa.gov/sites/default/files/pdf-content/1982/804/allen.pdf|journal=Fishery Bulletin|volume=80|issue=4|pages=769–790}}</ref> 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 [[Fish migration|anadromous]] [[Salmonidae|salmonids]] and [[lamprey]]s use estuaries as migration corridors.<ref>{{cite journal |doi=10.3354/meps247281 |jstor=24866466 |title=Evidence of connectivity between juvenile and adult habitats for mobile marine fauna: An important component of nurseries |journal=Marine Ecology Progress Series |volume=247 |pages=281–295 |year=2003 |last1=Gillanders |first1=BM |last2=Able |first2=KW |last3=Brown |first3=JA |last4=Eggleston |first4=DB |last5=Sheridan |first5=PF |bibcode=2003MEPS..247..281G |doi-access=free |hdl=2440/1877 |hdl-access=free }}</ref> Also, [[Bird migration|migratory]] bird populations, such as the [[black-tailed godwit]],<ref>{{cite journal |doi=10.1038/35086568 |pmid=11473317 |title=The buffer effect and large-scale population regulation in migratory birds |journal=Nature |volume=412 |issue=6845 |pages=436–438 |year=2001 |last1=Gill |first1=Jennifer A. |last2=Norris |first2=Ken |last3=Potts |first3=Peter M. |last4=Gunnarsson |first4=Tómas Grétar |last5=Atkinson |first5=Philip W. |last6=Sutherland |first6=William J. |bibcode=2001Natur.412..436G |s2cid=4308197 }}</ref> rely on estuaries.

Two of the main challenges of estuarine life are the variability in [[salinity]] and [[sedimentation]]. Many species of [[fish]] and [[invertebrate]]s have various methods to control or conform to the shifts in salt concentrations and are termed [[osmoconformer]]s and [[osmoregulator]]s. Many animals also [[burrowing|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 waters|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 [[tide]]s. 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.<ref name="RossDA">{{cite book |last=Ross |first=D. A. |year=1995 |title=Introduction to Oceanography |location=New York |publisher=Harper Collins College Publishers |isbn=978-0-673-46938-0 }}</ref>

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.<ref>{{cite journal |doi=10.1016/S0169-5347(99)01732-2 |title=Estuarine vulnerability and ecological impacts |journal=Trends in Ecology & Evolution |volume=14 |issue=12 |pages=499 |year=1999 |last1=Branch |first1=George }}</ref> 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 [[Marine debris|plastics]], [[pesticide]]s, [[Polychlorinated dibenzofurans|furans]], [[Polychlorinated dibenzodioxins|dioxins]], [[phenol]]s and [[Heavy metal (chemistry)|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.<ref name="Río de la Plata">{{cite journal |doi=10.1016/j.ecss.2016.12.018 |title=Total and extractable elemental composition of the intertidal estuarine biofilm of the Río de la Plata: Disentangling natural and anthropogenic influences |journal=Estuarine, Coastal and Shelf Science |volume=187 |pages=53–61 |year=2017 |last1=García-Alonso |first1=J. |last2=Lercari |first2=D. |last3=Araujo |first3=B.F. |last4=Almeida |first4=M.G. |last5=Rezende |first5=C.E. |bibcode=2017ECSS..187...53G }}</ref>

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.<ref>[http://www.npolar.no/ansipra/english/Indexpages/Ethnic_groups.html#19 "Indigenous Peoples of the Russian North, Siberia and Far East: Nivkh"] {{Webarchive|url=https://web.archive.org/web/20090807054552/http://www.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 Arctic</ref>

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 (environmental)|hypoxia]] and the creation of [[dead zone (ecology)|dead zones]].<ref>{{cite book |first=Sebastian A.|last=Gerlach |title=Marine Pollution: Diagnosis and Therapy |url=https://archive.org/details/marinepollutiond0000gerl|url-access=registration|publisher=Springer |location=Berlin |year=1981 |isbn=978-0387109404 }}</ref> 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,<ref>{{cite web|url=http://habitat.noaa.gov/restorationtechniques/public/habitat.cfm?HabitatID=2&HabitatTopicID=11 |title=Oyster Reefs: Ecological importance |publisher=US National Oceanic and Atmospheric Administration |access-date=2008-01-16 |url-status=dead |archive-url=https://web.archive.org/web/20081003114619/http://habitat.noaa.gov/restorationtechniques/public/habitat.cfm?HabitatID=2&HabitatTopicID=11 |archive-date=October 3, 2008 }}</ref> and sediment, nutrients, and algae can cause problems in local waters.

Some major rivers that run through deserts historically had vast, expansive estuaries that have been reduced to a fraction of their former size, because of dams and diversions. One example is the [[Colorado River Delta]] in Mexico, historically covered with marshlands and forests, but now essentially a salt flat.

==Examples==
{{div col|colwidth=22em}}

===Africa===
* [[Congo River]] Estuary
* [[Estuário do Espírito Santo]]
* [[Gambia River]] Estuary
* [[Gabon Estuary]]
* [[Lake St Lucia]] Estuary
* [[Orange River]] Estuary
* [[Pungwe River]] Estuary

===Asia===
* [[Amur River]] Estuary 
* [[Adyar River]] Estuary
* [[Dawei River]] Estuary<ref>{{Cite web|url=https://myanmarholiday.com/index.php/daweitavoy|title=Dawei(Tavoy)|website=myanmarholiday.com|access-date=2019-06-14|archive-date=2020-07-31|archive-url=https://web.archive.org/web/20200731020705/https://myanmarholiday.com/index.php/daweitavoy|url-status=dead}}</ref>
* [[Gulf of Ob]] Estuary
* [[Hangzhou Bay]]
* [[Hàn River (Vietnam)|Hàn River]] Estuary
* [[Kraburi River]] Estuary<ref>{{Cite web|url=https://km.dmcr.go.th/th/c_56/s_77/d_2766|title=สัณฐานชายฝั่ง - ระบบฐานข้อมูลทรัพยากรทางทะเลและชายฝั่ง กรมทรัพยากรทางทะเลและชายฝั่ง|website=km.dmcr.go.th}}</ref>
* [[Meghna River]] Estuary<ref>{{cite journal |doi=10.1006/ecss.2001.0929 |title=Residual Flow in the Meghna Estuary on the Coastline of Bangladesh |journal=Estuarine, Coastal and Shelf Science |volume=55 |issue=4 |pages=587–597 |year=2002 |last1=Jakobsen |first1=F. |last2=Azam |first2=M.H. |last3=Mahboob-Ul-Kabir |first3=M. |bibcode=2002ECSS...55..587J }}</ref>
* [[Naf River]] Estuary<ref>{{cite journal |doi=10.1007/s00343-019-8063-7 |title=Spatial and seasonal distribution of Intertidal Macrobenthos with their biomass and functional feeding guilds in the Naf River estuary, Bangladesh |journal=Journal of Oceanology and Limnology |volume=37 |issue=3 |pages=1010–1023 |year=2018 |last1=Noman |first1=Md. Abu |last2=Mamunur |first2=Rashid |last3=Islam |first3=M. Shahanul |last4=Hossain |first4=M. Belal |bibcode=2019JOL....37.1010N |s2cid=92734488 }}</ref>
* Narmada river estuary 
* [[Puerto Princesa Subterranean River National Park|Puerto Princesa Underground River]]
* Waeru River Estuary of [[Chanthaburi Province]]<ref>{{Cite web|url=http://wetland.onep.go.th/2559-1-Waeru.html|title=พื้นที่ชุ่มน้ำในประเทศไทย|website=wetland.onep.go.th|access-date=2019-02-07|archive-date=2019-02-09|archive-url=https://web.archive.org/web/20190209124327/http://wetland.onep.go.th/2559-1-Waeru.html|url-status=dead}}</ref>
* [[Yangtze River]] estuary
* [[Yenisei Gulf]] Estuary

===Europe===
* [[Dee Estuary]]
* [[Dnieper-Bug Estuary]]
* [[Exe Estuary]]
* [[Firth of Clyde]]
* [[Firth of Forth]]
* [[Gironde estuary]]
* [[Golden Horn]]
* [[Humber]]
* [[Oder]] Estuary
* [[Severn Estuary]]
* [[Shannon Estuary]]
* [[Solway Firth]]
* [[Southampton Water]]
* [[Tagus Estuary]]
* [[Thames Estuary]]
* [[The Wash]]
* [[Unterelbe]]
* [[Western Scheldt]]

===North America===
* [[Albemarle Sound]] including [[Outer Banks]] of [[North Carolina]]
* [[Chesapeake Bay]] including [[Hampton Roads]]
* [[Columbia River Estuary]]
* [[Coos Bay]]
* [[Delaware Bay]]
* [[Drakes Estero|Drake's Estero]]
* [[East River]]
* [[Estuary of Saint Lawrence]]
* [[Fraser River]]
* [[Galveston Bay]]
* [[Great Bay (New Hampshire)|Great Bay]]
* [[Indian River Lagoon]]
* [[Laguna de Términos]]
* [[Laguna Madre]]
* [[Lake Borgne]]
* [[Lake Merritt]]
* [[Long Island Sound]]
* [[Miramichi Bay]]
* [[Mississippi River Delta]]
** [[Lake Pontchartrain]]
* [[Mobile Bay]]
* [[Narragansett Bay]]
* [[Newport Back Bay]]
* [[Geography of New York-New Jersey Harbor Estuary|New York-New Jersey Harbor]]
* [[Pamlico Sound]] including the [[Outer Banks]] of [[North Carolina]]
* [[Puget Sound]]
** [[Billy Frank Jr. Nisqually National Wildlife Refuge]] on [[Puget Sound]]
* [[San Francisco Bay]]
* [[Sarasota Bay]]
* [[Tampa Bay]]

===Oceania===
* [[Avon Heathcote Estuary]] (Christchurch, New Zealand)
* [[Gippsland Lakes]]
* [[Port Jackson]] (Sydney Harbour)
* [[Spencer Gulf]]<ref name="Gostin" />

===South America===
* [[Amazon River]]<ref>{{Cite web|url=http://etaisweb.weebly.com/the-amazon-river-estuary.html|title=The Amazon River Estuary|website=etai's web}}</ref>
* [[Iguape-Cananéia-Paranaguá estuary lagoon complex]]
* [[Lagoa dos Patos]] and [[Lagoon Mirim]]
* [[Mearim River]]
**São Marcos Bay
**São José Bay
* [[Rio de la Plata]]{{div col end}}

==See also==
{{Portal|Rivers|Oceans|Water|Environment|Wetlands}}
{{div col}}
* {{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}}
* [https://web.archive.org/web/20101023203928/http://ecopath.org/LifeInTheChesapeakeBay/ Animated documentary on Chesapeake Bay] [[NOAA]].
* {{cite web
 |url         = http://www.onr.navy.mil/Focus/ocean/habitats/estuaries1.htm
 |title       = Habitats: Estuaries – Characteristics
 |publisher   = www.onr.navy.mil
 |access-date  = 2009-11-17
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20090517130442/http://www.onr.navy.mil/focus/ocean/habitats/estuaries1.htm
 |archive-date = 2009-05-17
}}
* [http://www.estuary-guide.net The Estuary Guide (Based on experience and R&D within the UK)]

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

[[Category:Estuaries| ]]
[[Category:Geodesy]]
[[Category:Coastal geography]]
[[Category:Environmental impact of fishing]]
[[Category:Coastal and oceanic landforms]]
[[Category:Bodies of water]]
[[Category:Aquatic ecology]]