View source for Eutrophication

{{Short description|Phenomenon where nutrients accumulate in water bodies}}
{{about|a process in aquatic ecosystems|one of the main causes|Nutrient pollution|one of the effects|Harmful algal bloom}}
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[[File:River algae Sichuan.jpg|thumb|313x313px|Eutrophication can cause harmful algal blooms like this one in a river near [[Chengdu]], China.]]
{{plankton sidebar|bloom}}
'''Eutrophication''' is a general term describing a process in which [[nutrient]]s accumulate in a body of water, resulting in an increased growth of organisms that may deplete the [[oxygen]] in the water; ie. the process of too many plants growing on the surface of a river, lake, etc., often because chemicals that are used to help crops grow have been carried there by rain.<ref name="usgs.gov">{{Cite web |title=Nutrients and Eutrophication {{!}} U.S. Geological Survey | date=March 2, 2019 |url=https://www.usgs.gov/mission-areas/water-resources/science/nutrients-and-eutrophication#:~:text=Eutrophication%20is%20a%20natural%20process,and%20clogging%20water-intake%20pipes. |access-date=February 9, 2024 |publisher=USGS}}</ref><ref>{{Cite journal |title=What Is the Nitrogen Cycle and Why Is It Key to Life? |year=2019 |language=en |doi=10.3389/frym.2019.00041 |doi-access=free |last1=Aczel |first1=Miriam R. |journal=Frontiers for Young Minds |volume=7 |article-number=41 |hdl=10044/1/71039 |hdl-access=free }}</ref> Eutrophication may occur naturally or as a result of human actions. Manmade, or cultural, eutrophication occurs when [[sewage]], [[Industrial wastewater treatment|industrial wastewater]], [[fertilizer runoff]], and other nutrient sources are released into the environment.<ref>{{Cite web |title=Cultural eutrophication {{!}} ecology {{!}} Britannica |url=https://www.britannica.com/science/cultural-eutrophication |access-date=February 9, 2024 |website=Britannica |language=en}}</ref> Such [[nutrient pollution]] usually causes [[algal bloom]]s and bacterial growth, resulting in the depletion of [[dissolved oxygen]] in water and causing substantial [[environmental degradation]].<ref>{{Cite journal |doi=10.1073/pnas.0806112105|pmid=18685114|title=Phosphorus control is critical to mitigating eutrophication|journal=Proceedings of the National Academy of Sciences|volume=105|issue=32|pages=11039–11040|year=2008|last1=Carpenter|first1=S. R.|bibcode=2008PNAS..10511039C|doi-access=free|pmc=2516213}}</ref> Many policies have been introduced to combat eutrophication, including the United Nations Development Program (UNDP)'s sustainability development goals.<ref name=":17" />

Approaches for prevention and reversal of eutrophication include minimizing [[point source pollution]] from sewage and agriculture as well as other [[Nonpoint source pollution|nonpoint pollution]] sources.<ref name="usgs.gov"/> Additionally, the introduction of bacteria and algae-inhibiting organisms such as [[shellfish]] and [[Seaweed farming|seaweed]] can also help reduce nitrogen pollution, which in turn controls the growth of [[cyanobacteria]], the main source of [[Harmful algal bloom|harmful algae blooms]].<ref>{{Cite web |title=Eutrophication and Oyster Aquaculture in the Potomac River Estuary |url=https://coastalscience.noaa.gov/project/eutrophication-oyster-aquaculture-potomac-river/ |access-date=February 9, 2024 |website=NCCOS Coastal Science Website |language=en-US}}</ref>

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== History and terminology==
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The term "eutrophication" comes from the [[Greek language|Greek]] ''eutrophos'', meaning "well-nourished",.<ref>{{citation |title=eutrophia |date=2016 |url=https://www.thefreedictionary.com/eutrophia |website=American Heritage Dictionary of the English Language |edition=Fifth |publisher=Houghton Mifflin Harcourt Publishing Company |access-date=March 10, 2018 |archive-date=March 11, 2018 |archive-url=https://web.archive.org/web/20180311021548/https://www.thefreedictionary.com/eutrophia |url-status=live }}</ref> because the waterway has had an excessive amount of nutrients for the growth of algae and other organisms that it eventually strangulates the oxygen out of the water.  

This overgrowth ultimately makes the water brackish as the organisms that once throve in it, now die in what is basically a mass extinction of all life within that area leaving behind it "dead zones"  i.e. non-oxygenated water.  As these pockets grow, they destroy more and more life until eventually they reach an area that enough oxygen to deter their advance.  The problem is that aquatic life does not know where eutrophic water starts and can swim into and immediately die.

Water bodies with very low nutrient levels are termed [[oligotrophic]] and those with moderate nutrient levels are termed [[Trophic state index#Mesotrophic|mesotrophic]]. Advanced eutrophication may also be referred to as [[Dystrophic lake|dystrophic]] and hypertrophic conditions.<ref>{{Cite book|last=Wetzel|first=Robert|title=Limnology|publisher=W.B. Saunders|year=1975|isbn=0-7216-9240-0|location=Philadelphia-London-Toronto|page=743}}</ref> Thus, eutrophication has been defined as "degradation of water quality owing to enrichment by nutrients which results in excessive plant (principally algae) growth and decay."<ref>{{cite web |last1=Smil |first1=Vaclav |title=Nitrogen Cycle and World Food Production |url=https://vaclavsmil.com/wp-content/uploads/docs/smil-article-worldagriculture.pdf |access-date=March 5, 2024 |archive-date=September 13, 2024 |archive-url=https://web.archive.org/web/20240913113039/https://www.vaclavsmil.com/wp-content/uploads/docs/smil-article-worldagriculture.pdf }}</ref> 

Eutrophication was recognized as a [[water pollution]] problem in European and North American lakes and reservoirs in the mid-20th century.<ref name="Rohde 1969">Rodhe, W. (1969) "Crystallization of Eutrophication Concepts in North Europe". In: ''Eutrophication, Causes, Consequences, Correctives''. National Academy of Sciences, Washington D.C., {{ISBN|9780309017008}} <!--Not 10 or 13 digits long-->, pp. 50–64.</ref> Breakthrough research carried out at the [[Experimental Lakes Area]] (ELA) in Ontario, Canada, in the 1970s provided the evidence that freshwater bodies are phosphorus-limited. ELA uses the whole [[ecosystem approach]] and long-term, whole-lake investigations of freshwater focusing on cultural eutrophication.<ref>{{Cite journal |last=Schindler |first=David |date=1974 |title=Eutrophication and Recovery in Experimental Lakes: Implications for Lake Management |journal=Science |volume=184 (4139) |issue=4139 |pages=897–899 |bibcode=1974Sci...184..897S |doi=10.1126/science.184.4139.897 |pmid=17782381 |s2cid=25620329}}</ref> 

==Causes==
[[Image:Sodium tripolyphosphate.svg|thumb|[[Sodium triphosphate]], once a component of many detergents, was a major contributor to eutrophication.|230x230px]]
[[File:NRCSTN83003 - Tennessee (6251)(NRCS Photo Gallery).jpg|thumb|212x212px|An example in [[Tennessee]] of how soil from fertilized fields can turn into runoff after a storm, creating a flux of nutrients that flow into local bodies of water such as lakes and creeks]]
Eutrophication is caused by excessive concentrations of nutrients, most commonly [[phosphate|phosphates]] and [[nitrate|nitrates]],<ref name=":6">Schindler, David and Vallentyne, John R. (2004) ''Over fertilization of the World's Freshwaters and Estuaries'', University of Alberta Press, p. 1, {{ISBN|0-88864-484-1}}</ref>  although this varies with location. Prior to their being phasing out in the 1970's, phosphate-containing detergents contributed to eutrophication. Since then, sewage and agriculture have emerged as the dominant phosphate sources.<ref name=":11">Werner, Wilfried (2002) "Fertilizers, 6. Environmental Aspects". ''Ullmann's Encyclopedia of Industrial Biology'', Wiley-VCH, Weinheim. {{doi|10.1002/14356007.n10_n05}}</ref> The main sources of nitrogen pollution are from agricultural runoff containing fertilizers and animal wastes, from sewage, and from atmospheric deposition of nitrogen originating from combustion or animal waste.<ref>{{Cite journal|author1-link=David Fowler (physicist)|last1=Fowler|first1=David|last2=Coyle|first2=Mhairi|last3=Skiba|first3=Ute|last4=Sutton|first4=Mark A.|last5=Cape|first5=J. Neil|last6=Reis|first6=Stefan|last7=Sheppard|first7=Lucy J.|last8=Jenkins|first8=Alan|last9=Grizzetti|first9=Bruna|last10=Galloway|first10=James N.|last11=Vitousek|first11=Peter|date=2013|title=The global nitrogen cycle in the twenty-first century|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|volume=368|issue=1621|article-number=20130164|doi=10.1098/rstb.2013.0164|pmc=3682748|pmid=23713126}}</ref>

The limitation of productivity in any aquatic system varies with the rate of supply (from external sources) and removal (flushing out) of nutrients from the body of water.<ref>{{Cite journal |last1=Moore |first1=C. M. |last2=Mills |first2=M. M. |last3=Arrigo |first3=K. R. |last4=Berman-Frank |first4=I. |last5=Bopp |first5=L. |last6=Boyd |first6=P. W. |last7=Galbraith |first7=E. D. |last8=Geider |first8=R. J. |last9=Guieu |first9=C. |last10=Jaccard |first10=S. L. |last11=Jickells |first11=T. D. |last12=La Roche |first12=J. |last13=Lenton |first13=T. M. |last14=Mahowald |first14=N. M. |last15=Marañón |first15=E. |date=September 2013 |title=Processes and patterns of oceanic nutrient limitation |url=https://www.nature.com/articles/ngeo1765 |journal=Nature Geoscience |language=en |volume=6 |issue=9 |pages=701–710 |doi=10.1038/ngeo1765 |bibcode=2013NatGe...6..701M |s2cid=249514 |issn=1752-0908|url-access=subscription }}</ref> This means that some nutrients are more prevalent in certain areas than others and different ecosystems and environments have different limiting factors. 

Phosphorus is the limiting factor for plant growth in most freshwater ecosystems,<ref>{{Cite journal |last1=Elser |first1=James J. |last2=Bracken |first2=Matthew E.S. |last3=Cleland |first3=Elsa E. |last4=Gruner |first4=Daniel S. |last5=Harpole |first5=W. Stanley |last6=Hillebrand |first6=Helmut |last7=Ngai |first7=Jacqueline T. |last8=Seabloom |first8=Eric W. |last9=Shurin |first9=Jonathan B. |last10=Smith |first10=Jennifer E. |date=July 2007 |title=Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1461-0248.2007.01113.x |journal=Ecology Letters |language=en |volume=10 |issue=12 |pages=1135–1142 |doi=10.1111/j.1461-0248.2007.01113.x |pmid=17922835 |bibcode=2007EcolL..10.1135E |hdl=1903/7447 |s2cid=12083235 |issn=1461-023X|hdl-access=free |url-access=subscription }}</ref> and because phosphate adheres tightly to soil particles and sinks in areas such as wetlands and lakes,<ref>{{Cite web |title=Phosphorus Basics: Understanding Phosphorus Forms and Their Cycling in the Soil |work=Alabama Cooperative Extension System |url=https://www.aces.edu/blog/topics/crop-production/understanding-phosphorus-forms-and-their-cycling-in-the-soil/ |access-date=February 10, 2024 |language=en-US}}</ref> due to its prevalence nowadays more and more phosphorus is accumulating inside freshwater bodies.<ref>{{Cite web |last=US EPA |first=OW |date=November 27, 2013 |title=Indicators: Phosphorus |url=https://www.epa.gov/national-aquatic-resource-surveys/indicators-phosphorus |access-date=February 10, 2024 |publisher=EPA |language=en}}</ref><ref name=":18">{{Cite journal |last=Schindler |first=David W. |date=2012 |title=The dilemma of controlling cultural eutrophication of lakes |journal=Proceedings of the Royal Society B: Biological Sciences |volume=279 |issue=1746 |pages=4322–4333 |doi=10.1098/rspb.2012.1032 |pmc=3479793 |pmid=22915669}}</ref> 

In [[marine ecosystem]]s, nitrogen is the primary limiting nutrient; [[nitrous oxide]] (created by the combustion of [[fossil fuel]]s) and its deposition in the water from the atmosphere has led to an increase in nitrogen levels,<ref>{{Cite web |last=Reay |first=Dave |date=November 9, 2002 |title=Nitrous oxide Sources - Oceans |url=https://www.ghgonline.org/nitrousoceans.htm#:~:text=As%20with%20methane%2C%20man's%20impact,in%20estuaries%20and%20coastal%20waters. |access-date=February 11, 2024 |website=ghgonline |archive-date=December 7, 2023 |archive-url=https://web.archive.org/web/20231207225440/http://ghgonline.org/nitrousoceans.htm#:~:text=As%20with%20methane%2C%20man's%20impact,in%20estuaries%20and%20coastal%20waters. }}</ref> and also the heightened levels of eutrophication in the ocean.<ref>{{Cite journal |last1=Bristow |first1=L. |last2=Mohr |first2=W. |date=2017 |title=Nutrients that limit growth in the ocean |journal=Current Biology |volume=27 |issue=11 |pages=R431–R510 |doi=10.1016/j.cub.2017.03.030 |pmid=28586682 |bibcode=2017CBio...27.R474B |s2cid=21052483 |hdl-access=free |hdl=21.11116/0000-0001-C1AA-5}}</ref>

===Cultural eutrophication===
Cultural or [[Human impact on the environment|anthropogenic]] eutrophication is the process that causes eutrophication because of human activity.<ref name="Smith">{{cite journal |last1=Smith |first1=Val H. |last2=Schindler |first2=David W. |date=2009 |title=Eutrophication science: Where do we go from here? |journal=Trends in Ecology & Evolution |volume=24 |issue=4 |pages=201–207 |doi=10.1016/j.tree.2008.11.009 |pmid=19246117|bibcode=2009TEcoE..24..201S }}</ref><ref name=":7">[https://www.britannica.com/EBchecked/topic/146210/cultural-eutrophication Cultural eutrophication] {{Webarchive|url=https://web.archive.org/web/20150504105403/http://www.britannica.com/EBchecked/topic/146210/cultural-eutrophication |date=May 4, 2015 }} (2010) ''Encyclopedia Britannica''. Retrieved April 26, 2010, from Encyclopedia Britannica Online:</ref> The problem became more apparent following the introduction of chemical fertilizers in agriculture (green revolution of the mid-1900s).<ref>{{Cite journal|last=Smil|first=Vaclav|date=November 2000|title=Phosphorus in the Environment: Natural Flows and Human Interferences|journal=[[Annual Review of Energy and the Environment]]|volume=25|issue=1|pages=53–88|doi=10.1146/annurev.energy.25.1.53|doi-access=free|issn=1056-3466}}</ref> 

Phosphorus and nitrogen are the two main nutrients that cause cultural eutrophication as they enrich the water, allowing for some aquatic plants, especially algae to grow rapidly and bloom in high densities. Algal blooms can shade out benthic plants thereby altering the overall plant community.<ref name=":2">{{Cite journal|last=Moss|first=Brian|date=1983|title=The Norfolk Broadland: Experiments in the Restoration of a Complex Wetland|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-185X.1983.tb00399.x|journal=Biological Reviews|language=en|volume=58|issue=4|pages=521–561|doi=10.1111/j.1469-185X.1983.tb00399.x|s2cid=83803387|issn=1469-185X|access-date=February 8, 2022|archive-date=February 8, 2022|archive-url=https://web.archive.org/web/20220208192439/https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-185X.1983.tb00399.x|url-status=live|url-access=subscription}}</ref> When [[algae]] die off, their degradation by bacteria removes oxygen, potentially, generating [[Anoxic waters|anoxic]] conditions. 

This anoxic environment kills off aerobic organisms (e.g. fish and invertebrates) in the water body. This also affects terrestrial animals, restricting their access to affected water (e.g. as drinking sources). Selection for algal and aquatic plant species that can thrive in nutrient-rich conditions can cause structural and functional disruption to entire aquatic ecosystems and their food webs, resulting in loss of habitat and species biodiversity.<ref name=":1" />

There are several sources of excessive nutrients from human activity including run-off from fertilized fields, lawns, and golf courses, untreated sewage and wastewater and internal combustion of fuels creating nitrogen pollution.<ref name=":5">Schindler, David W., Vallentyne, John R. (2008). ''The Algal Bowl: Overfertilization of the World's Freshwaters and Estuaries'', University of Alberta Press, {{ISBN|0-88864-484-1}}.</ref>  Cultural eutrophication can occur in fresh water and salt water bodies, shallow waters being the most susceptible. In shore lines and shallow lakes, sediments are frequently resuspended by wind and waves which can result in nutrient release from sediments into the overlying water, enhancing eutrophication.<ref>{{Cite journal|last1=Qin|first1=Boqiang|last2=Yang|first2=Liuyan|last3=Chen|first3=Feizhou|last4=Zhu|first4=Guangwei|last5=Zhang|first5=Lu|last6=Chen|first6=Yiyu|date=October 1, 2006|title=Mechanism and control of lake eutrophication|journal=Chinese Science Bulletin|language=en|volume=51|issue=19|pages=2401–2412|doi=10.1007/s11434-006-2096-y|bibcode=2006ChSBu..51.2401Q|s2cid=198137333|issn=1861-9541}}</ref> The deterioration of water quality caused by cultural eutrophication can therefore negatively impact human uses including potable supply for consumption, industrial uses and recreation.<ref>{{Citation|last1=Khan|first1=M. Nasir|title=Eutrophication: Challenges and Solutions|date=2014|work=Eutrophication: Causes, Consequences and Control: Volume 2|pages=1–15|editor-last=Ansari|editor-first=Abid A.|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-007-7814-6_1|isbn=978-94-007-7814-6|last2=Mohammad|first2=Firoz|editor2-last=Gill|editor2-first=Sarvajeet Singh}}</ref>
[[File:Mono Lake sat zoomed.jpg|thumb|260x260px|The eutrophication of [[Mono Lake]], which is a [[cyanobacteria]]-rich [[soda lake]]]]

=== Natural eutrophication ===
Eutrophication can be a natural process and occurs naturally through the gradual accumulation of sediment and nutrients.  Naturally, eutrophication is usually caused by the natural accumulation of nutrients from dissolved phosphate minerals and dead plant matter in water.<ref name="Sawyer">{{cite journal|title=Basic Concepts of Eutrophication|author=Clair N. Sawyer|journal=Journal (Water Pollution Control Federation)|date=May 1966|volume=38|number=5|pages=737–744|publisher=Wiley|jstor=25035549}}</ref><ref>{{Cite web |last=Addy |first=Kelly |date=1996 |title=Phosphorus and Lake Aging |url=https://web.uri.edu/watershedwatch/files/Phosphorus.pdf |url-status=live |archive-url=https://web.archive.org/web/20210728121848/https://web.uri.edu/watershedwatch/files/Phosphorus.pdf |archive-date=July 28, 2021 |access-date=June 16, 2021 |website=Natural Resources Facts - University of Rhode Island}}</ref>

Natural eutrophication has been well-characterized in lakes. [[paleolimnology|Paleolimnologists]] now recognise that climate change, geology, and other external influences are also critical in regulating the natural productivity of lakes. 

A few artificial lakes also demonstrate the reverse process ([[meiotrophication]]<ref>{{Cite book|last=Wetzel|first=Robert G.|title=Limnology: lake and river ecosystems|date=2001|publisher=Academic Press|isbn=0-12-744760-1|edition=3rd|location=San Diego|oclc=46393244}}</ref>), becoming less nutrient rich with time as nutrient poor inputs slowly elute the nutrient richer water mass of the lake.<ref name=":8">Walker, I. R. (2006) "Chironomid overview", pp. 360–366 in S.A. EIias (ed.) ''Encyclopedia of Quaternary Science'', Vol. 1, Elsevier,</ref><ref name=":9">{{Cite journal|last1=Whiteside|first1=M. C.|year=1983|title=The mythical concept of eutrophication|journal=Hydrobiologia|volume=103|issue=1 |pages=107–150|doi=10.1007/BF00028437|bibcode=1983HyBio.103..107W |s2cid=19039247}}</ref> This process may be seen in artificial lakes and reservoirs which tend to be highly eutrophic on first filling but may become more oligotrophic with time. The main difference between natural and anthropogenic eutrophication is that the natural process is very slow, occurring on geological time scales.<ref name=":10">Callisto, Marcos; Molozzi, Joseline and Barbosa, José Lucena Etham (2014) "Eutrophication of Lakes" in A. A. Ansari, S. S. Gill (eds.), ''Eutrophication: Causes, Consequences and Control'', Springer Science+Business Media Dordrecht. {{doi|10.1007/978-94-007-7814-6_5}}. {{ISBN|978-94-007-7814-6}}.</ref>

== Effects ==
{{Further|Harmful algal bloom#Harmful effects}}
[[File:Caspian Sea from orbit.jpg|thumb|306x306px|Eutrophication is apparent as increased [[turbidity]] in the northern part of the [[Caspian Sea]], imaged from orbit.]]

=== Ecological effects ===
Eutrophication can have the following ecological effects: increased biomass of [[phytoplankton]], changes in [[macrophyte]] [[species composition]] and [[biomass]], [[Oxygen saturation|dissolved oxygen]] depletion, increased incidences of [[fish kill]]s, loss of desirable fish species.<ref>{{Cite web |title=Nutrients and Eutrophication {{!}} U.S. Geological Survey | date=March 2, 2019 |url=https://www.usgs.gov/mission-areas/water-resources/science/nutrients-and-eutrophication#:~:text=Eutrophication%20is%20a%20natural%20process,and%20clogging%20water-intake%20pipes |access-date=2024-09-29 |publisher=USGS}}</ref>

==== Decreased biodiversity ====
When an ecosystem experiences an increase in nutrients, [[primary producer]]s reap the benefits first. In aquatic ecosystems, species such as [[algae]] experience a population increase (called an [[algal bloom]]). 

Algal blooms limit the sunlight available to bottom-dwelling organisms and cause wide swings in the amount of dissolved oxygen in the water. Oxygen is required by all aerobically [[Respiration (physiology)|respiring]] plants and animals and it is replenished in daylight by [[photosynthesis|photosynthesizing]] plants and algae. 

Under eutrophic conditions, dissolved oxygen greatly increases during the day, but is greatly reduced after dark by the respiring algae and by microorganisms that feed on the increasing mass of dead algae. When dissolved oxygen levels decline to [[hypoxia (environmental)|hypoxic]] levels, fish and other [[Marine life|marine animals]] suffocate. As a result, creatures such as fish, shrimp, and especially immobile bottom dwellers die off.<ref name="Horrigan 2002">{{Cite journal|last1=Horrigan|first1=L.|last2=Lawrence|first2=R. S.|last3=Walker|first3=P.|year=2002|title=How sustainable agriculture can address the environmental and human health harms of industrial agriculture|journal=Environmental Health Perspectives|volume=110|issue=5|pages=445–456|doi=10.1289/ehp.02110445|pmc=1240832|pmid=12003747|bibcode=2002EnvHP.110..445H }}</ref> In extreme cases, [[Anaerobic organism|anaerobic]] conditions ensue, promoting growth of bacteria. Zones where this occurs are known as [[Dead zone (ecology)|dead zones]].

==== New species invasion ====
Eutrophication may cause competitive release by making abundant a normally [[Limiting factor|limiting nutrient]]. This process causes shifts in the [[species composition]] of ecosystems. For instance, an increase in nitrogen might allow new, [[invasive species|competitive species]] to invade and out-compete original inhabitant species. This has been shown to occur in [[New England]] [[salt marsh]]es.<ref name="Bertness 2002">{{cite journal|last1=Bertness|first1=M. D.|last2=Ewanchuk|first2=P. J.|last3=Silliman|first3=B. R.|year=2002|title=Anthropogenic modification of New England salt marsh landscapes|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=99|issue=3|pages=1395–1398|bibcode=2002PNAS...99.1395B|doi=10.1073/pnas.022447299|jstor=3057772|pmc=122201|pmid=11818525|doi-access=free}}</ref> In Europe and Asia, the [[common carp]] frequently lives in naturally eutrophic or hypereutrophic areas, and is adapted to living in such conditions. The eutrophication of areas outside its natural range partially explain the fish's success in colonizing these areas after being introduced.

==== Toxicity ====
Some [[harmful algal bloom]]s resulting from eutrophication, are [[toxic]] to plants and animals.<ref name=Smith/><ref name="Anderson 1994">{{cite journal|author=Anderson D. M.|year=1994|title=Red tides|url=http://www.whoi.edu/cms/files/Anderson_1994_SciAm-redtides_31132.pdf|journal=Scientific American|volume=271|issue=2|pages=62–68|bibcode=1994SciAm.271b..62A|doi=10.1038/scientificamerican0894-62|pmid=8066432|access-date=March 31, 2013|archive-date=May 11, 2013|archive-url=https://web.archive.org/web/20130511185552/http://www.whoi.edu/cms/files/Anderson_1994_SciAm-redtides_31132.pdf|url-status=live}}</ref> Freshwater algal blooms can pose a threat to livestock. When the algae die or are eaten, [[neurotoxin|neuro]]- and [[hepatotoxins]] are released which can kill animals and may pose a threat to humans.<ref name="Lawton 1991">{{cite journal|last=Lawton|first=L.A.|author2=G.A. Codd|year=1991|title=Cyanobacterial (blue-green algae) toxins and their significance in UK and European waters|journal=Journal of Soil and Water Conservation|volume=40|issue=4|pages=87–97|doi=10.1111/j.1747-6593.1991.tb00643.x|bibcode=1991WaEnJ...5..460L }}</ref><ref name="Martin 1994">{{cite journal|last=Martin|first=A.|author2=G.D. Cooke|year=1994|title=Health risks in eutrophic water supplies|journal=Lake Line|volume=14|pages=24–26}}</ref> An example of algal toxins working their way into humans is the case of [[shellfish]] poisoning.<ref name="Shumway 1990">{{Cite journal|last1=Shumway|first1=S. E.|year=1990|title=A Review of the Effects of Algal Blooms on Shellfish and Aquaculture|journal=Journal of the World Aquaculture Society|volume=21|issue=2|pages=65–104|doi=10.1111/j.1749-7345.1990.tb00529.x|bibcode=1990JWAS...21...65S }}</ref> Biotoxins created during algal blooms are taken up by shellfish ([[mussel]]s, [[oyster]]s), leading to these human foods acquiring the toxicity and poisoning humans. Examples include paralytic, neurotoxic, and diarrhoetic [[shellfish poisoning]]. Other marine animals can be [[Vector (epidemiology)|vectors]] for such toxins, as in the case of [[ciguatera]], where it is typically a predator fish that accumulates the toxin and then poisons humans.

There are five types of toxins associated with harmful algal blooms (HABs). They include [[domoic acid]], [[ciguatoxin]], [[okadaic acid]], [[brevetoxin]]s, and [[saxitoxin]]s. All of these toxins, with the exception of ciguatoxin, can cause different types of shellfish poisoning. Domoic acid<ref>{{Cite journal |last1=Lie |first1=Alle A. Y. |last2=Zimmer-Faust |first2=Amity G. |last3=Diner |first3=Rachel E. |last4=Kunselman |first4=Emily |last5=Daniel |first5=Zachary |last6=Van Artsdalen |first6=Kathryn |last7=Salas Garcia |first7=Mariana C. |last8=Gilbert |first8=Jack A. |last9=Shultz |first9=Dana |last10=Chokry |first10=Jeff |last11=Langlois |first11=Kylie |last12=Smith |first12=Jayme |date=May 2024 |title=Understanding the risks of co-exposures in a changing world: a case study of dual monitoring of the biotoxin domoic acid and ''Vibrio'' spp. in Pacific oyster |url=https://link.springer.com/10.1007/s10661-024-12614-1 |journal=Environmental Monitoring and Assessment |language=en |volume=196 |issue=5 |page=447 |doi=10.1007/s10661-024-12614-1 |pmid=38607511 |bibcode=2024EMnAs.196..447L |issn=0167-6369|url-access=subscription }}</ref> is associated with amnesic shellfish poisoning; okadaic acid<ref>{{Cite journal |last1=Zhang |first1=Yuting |last2=Song |first2=Shanshan |last3=Zhang |first3=Bin |last4=Zhang |first4=Yang |last5=Tian |first5=Miao |last6=Wu |first6=Ziyi |last7=Chen |first7=Huorong |last8=Ding |first8=Guangmao |last9=Liu |first9=Renyan |last10=Mu |first10=Jingli |date=February 2023 |title=Comparison of short-term toxicity of 14 common phycotoxins (alone and in combination) to the survival of brine shrimp ''Artemia salina''|url=https://link.springer.com/10.1007/s13131-022-2120-3 |journal=Acta Oceanologica Sinica |language=en |volume=42 |issue=2 |pages=134–141 |doi=10.1007/s13131-022-2120-3 |issn=0253-505X|url-access=subscription }}</ref> is associated with diarrhetic shellfish poisoning; brevetoxins<ref>{{Cite journal |last1=Perrault |first1=Justin R. |last2=Stacy |first2=Nicole I. |last3=Lehner |first3=Andreas F. |last4=Mott |first4=Cody R. |last5=Hirsch |first5=Sarah |last6=Gorham |first6=Jonathan C. |last7=Buchweitz |first7=John P. |last8=Bresette |first8=Michael J. |last9=Walsh |first9=Catherine J. |date=December 2017 |title=Potential effects of brevetoxins and toxic elements on various health variables in Kemp's ridley (''Lepidochelys kempii'') and green (''Chelonia mydas'') sea turtles after a red tide bloom event |url=https://linkinghub.elsevier.com/retrieve/pii/S004896971731567X |journal=Science of the Total Environment |language=en |volume=605-606 |pages=967–979 |doi=10.1016/j.scitotenv.2017.06.149|pmid=28693110 |bibcode=2017ScTEn.605..967P |url-access=subscription }}</ref> are associated with neurotoxic shellfish poisoning; and saxitoxins<ref>{{Cite book |last1=Lalli |first1=Carol M. |title=Biological oceanography: an introduction |last2=Parsons |first2=Timothy Richard |date=1997 |publisher=Butterworth Heinemann |isbn=978-0-7506-3384-0 |edition=2nd |series=Open University oceanography series |location=Oxford [England]}}</ref> are associated with paralytic shellfish poisoning. Different species of algae are associated with the different toxins.<ref>{{Cite book |last1=Lalli |first1=Carol M. |title=Biological oceanography: an introduction |last2=Parsons |first2=Timothy Richard |date=1997 |publisher=Butterworth Heinemann |isbn=978-0-7506-3384-0 |edition=2nd |series=Open University oceanography series |location=Oxford [England]}}</ref> For example, ''Alexandrium'', ''Pyrodinium'', and ''Gymnodinium'' species generate saxitoxins.<ref>{{Cite book |last1=Lalli |first1=Carol M. |title=Biological oceanography: an introduction |last2=Parsons |first2=Timothy Richard |date=1997 |publisher=Butterworth Heinemann |isbn=978-0-7506-3384-0 |edition=2nd |series=Open University oceanography series |location=Oxford [England]}}</ref> Saxitoxin is known to be 50 times more lethal than strychnine and 10,000 times more lethal than cyanide.<ref>{{Cite book |last1=Lalli |first1=Carol M. |title=Biological oceanography: an introduction |last2=Parsons |first2=Timothy Richard |date=1997 |publisher=Butterworth Heinemann |isbn=978-0-7506-3384-0 |edition=2nd |series=Open University oceanography series |location=Oxford [England]}}</ref>
[[File:Lake Pyramid algal bloom (Copernicus).jpg|thumb|Lake Pyramid Algal Bloom]]

=== Economic effects ===
Eutrophication and harmful algal blooms can have economic impacts due to increasing [[water treatment]] costs, commercial fishing and shellfish losses, recreational fishing losses (reductions in harvestable fish and [[shellfish]]), and reduced tourism income (decreases in perceived aesthetic value of the water body and decreases in safety due to high bacteria levels from decomposition).<ref>{{Cite web |last=US EPA |first=OW |date=2013 |title=The Effects: Economy |url=https://www.epa.gov/nutrientpollution/effects-economy |access-date=February 15, 2022 |publisher=EPA |language=en |archive-date=September 28, 2022 |archive-url=https://web.archive.org/web/20220928180552/https://www.epa.gov/nutrientpollution/effects-economy |url-status=live }}</ref><ref>{{Cite web |last=Finch |first=Allison |date=June 12, 2023 |title=Thousands of dead fish washed up along a 6-mile stretch of Texas coast |url=https://www.accuweather.com/en/weather-news/thousands-of-dead-fish-washed-up-along-a-6-mile-stretch-of-texas-coast/1540159 |website=AccuWeather}}</ref> Water treatment costs can be increased due to decreases in water transparency (increased [[turbidity]]). There can also be issues with color and smell during drinking water treatment. However, controlled eutrophication can potentially be used to increase production in fisheries, which in turn increases community income.<ref>{{Cite journal |title=Eutrophication in a tropical estuary: Is it good or bad? | date=2021|doi=10.1088/1755-1315/744/1/012010 | journal=IOP Conference Series: Earth and Environmental Science | volume=744 | issue=1 | article-number=012010 | bibcode=2021E&ES..744a2010D | vauthors = Damar A, Ervinia A, Kurniawan F, Rudianto BY | doi-access=free }}</ref> Notably, there is a delicate line where eutrophication can become damaging very quickly, and as such is not recommended currently due to high eutrophication rates.

=== Health impacts ===
Human health effects of eutrophication derive from two main issues excess nitrate in drinking water and exposure to toxic algae.<ref>{{Cite web |last=Xiao |date=2017-06-02 |title=Water Eutrophication and its Effect • EnvGuide |url=https://us.envguide.com/eutrop/#:~:text=Drinking,%20accidentally%20swallowing%20or%20swimming,cause%20serious%20health%20problems%20including:&text=Rashes&text=Stomach%20or%20liver%20illness&text=Respiratory%20problems |access-date=2024-10-28 |website=EnvGuide |language=en-US}}</ref> Nitrates in drinking water can cause [[blue baby syndrome]] in infants and can react with chemicals used to treat water to create [[Disinfection by-product|disinfection by-products]] in drinking water.<ref>{{cite web |author=<!--Not stated--> |date=March 1, 2021 |title=The Effects: Human Health |url=https://www.epa.gov/nutrientpollution/effects-human-and-animal-health |url-status=live |archive-url=https://web.archive.org/web/20200219144746/https://www.epa.gov/nutrientpollution/effects-human-health |archive-date=February 19, 2020 |access-date=February 21, 2022 |website=Nutrient Pollution |publisher=EPA}}</ref> Getting direct contact with toxic algae through swimming or drinking can cause rashes, stomach or liver illness, and respiratory or neurological problems  .<ref>{{Cite web |last=US EPA |first=OW |date=2013 |title=The Effects: Human Health |url=https://www.epa.gov/nutrientpollution/effects-human-and-animal-health |url-status=live |archive-url=https://web.archive.org/web/20200219144746/https://www.epa.gov/nutrientpollution/effects-human-health |archive-date=February 19, 2020 |access-date=February 15, 2022 |publisher=EPA |language=en}}</ref>
== Causes and effects for different types of water bodies ==
[[File:Algal bloom in the Lake Valencia - Venezuela.jpg|thumb|253x253px|An algal bloom in Lake Valencia, the largest freshwater lake in Venezuela. Since 1976 the lake has been affected by eutrophication caused by wastewater.]]

=== Freshwater systems ===

One response to added amounts of nutrients in [[aquatic ecosystem]]s is the rapid growth of microscopic algae, creating an [[algal bloom]]. In [[freshwater ecosystem]]s, the formation of floating algal blooms are commonly nitrogen-fixing [[cyanobacteria]] (blue-green algae). This outcome is favored when soluble nitrogen becomes limiting and phosphorus inputs remain significant.<ref name=":12">{{cite journal|last1=Schindler|first1=David W.|last2=Hecky|first2=R.E.|last3=Findlay|first3=D.L.|last4=Stainton|first4=M.P.|last5=Parker|first5=B.R.|last6=Paterson|first6=M.J.|last7=Beaty|first7=K.G.|last8=Lyng|first8=M.|last9=Kasian|first9=S. E. M.|title=Eutrophication of lakes cannot be controlled by reducing nitrogen input: Results of a 37-year whole-ecosystem experiment|journal=Proceedings of the National Academy of Sciences of the United States of America|date=August 2008|volume=105|issue=32|pages=11254–11258|doi=10.1073/pnas.0805108105|pmid=18667696|pmc=2491484|doi-access=free}}</ref> [[Nutrient pollution]] is a major cause of algal blooms and excess growth of other aquatic plants leading to overcrowding competition for sunlight, space, and oxygen. Increased competition for the added nutrients can cause potential disruption to entire ecosystems and food webs, as well as a loss of habitat, and biodiversity of species.<ref name=":1">{{cite journal|last1=Rabalais|first1=NN|title=Nitrogen in aquatic ecosystems|journal=Ambio: A Journal of the Human Environment|date=Mar 2002|volume=31|issue=2|pages=102–112|doi=10.1579/0044-7447-31.2.102|pmid=12077998|bibcode=2002Ambio..31..102R |s2cid=19172194}}</ref>

When overproduced [[macrophyte]]s and algae die in eutrophic water, their decompose further consumes dissolved oxygen. The depleted oxygen levels in turn may lead to [[fish kill]]s and a range of other effects reducing biodiversity. Nutrients may become concentrated in an anoxic zone, often in deeper waters cut off by stratification of the water column and may only be made available again during autumn turn-over in temperate areas or in conditions of turbulent flow. The dead algae and organic load carried by the water inflows into a lake settle to the bottom and undergo [[anaerobic digestion]] releasing [[greenhouse gases]] such as methane and CO<sub>2</sub>. Some of the methane gas may be oxidised by anaerobic [[Methanotroph|methane oxidation bacteria]] such as ''[[Methylococcus capsulatus]]'', which in turn may provide a food source for [[zooplankton]].<ref>{{cite web |url= https://www.igb-berlin.de/en/news/climate-gases-water-bodies |title= Climate gases from water bodies |access-date= September 22, 2018 |archive-date= February 2, 2019 |archive-url= https://web.archive.org/web/20190202042542/https://www.igb-berlin.de/en/news/climate-gases-water-bodies |url-status= live }}</ref>  Thus a self-sustaining biological process can take place to generate [[Primary production|primary food source]] for the [[phytoplankton]] and zooplankton depending on the availability of adequate dissolved oxygen in the water body.<ref>{{cite web |url= https://www.ntva.no/wp-content/uploads/2014/01/04-huslid.pdf |title= Nature's Value Chain... |access-date= September 22, 2018 |archive-url= https://web.archive.org/web/20161221084521/https://www.ntva.no/wp-content/uploads/2014/01/04-huslid.pdf |archive-date= December 21, 2016 }}</ref>

Enhanced growth of aquatic vegetation, phytoplankton and algal blooms disrupts normal functioning of the ecosystem, causing a variety of problems such as a lack of [[oxygen]] which is needed for fish and [[shellfish]] to survive. The growth of dense algae in surface waters can shade the deeper water and reduce the viability of benthic shelter plants with resultant impacts on the wider ecosystem.<ref name=":2" /><ref>{{Cite journal|last1=Jeppesen|first1=Erik|last2=Søndergaard|first2=Martin|last3=Jensen|first3=Jens Peder|last4=Havens|first4=Karl E.|last5=Anneville|first5=Orlane|last6=Carvalho|first6=Laurence|last7=Coveney|first7=Michael F.|last8=Deneke|first8=Rainer|last9=Dokulil|first9=Martin T.|last10=Foy|first10=Bob|last11=Gerdeaux|first11=Daniel|date=2005|title=Lake responses to reduced nutrient loading – an analysis of contemporary long-term data from 35 case studies|journal=Freshwater Biology|language=en|volume=50|issue=10|pages=1747–1771|doi=10.1111/j.1365-2427.2005.01415.x|issn=1365-2427|doi-access=free|bibcode=2005FrBio..50.1747J }}</ref> Eutrophication also decreases the value of rivers, lakes and aesthetic enjoyment. Health problems can occur where [[eutrophic]] conditions interfere with drinking [[water treatment]].<ref name="Bartram 1999">Bartram, J., Wayne W. Carmichael, Ingrid Chorus, Gary Jones, and [[Olav M. Skulberg]] (1999). "Chapter 1. Introduction", in: ''Toxic Cyanobacteria in Water: A guide to their public health consequences, monitoring and management''. [[World Health Organization]]. URL: [https://www.who.int/water_sanitation_health/resourcesquality/toxicyanbact/en/ WHO document] {{webarchive|url=https://web.archive.org/web/20070124215138/http://www.who.int/water_sanitation_health/resourcesquality/toxicyanbact/en/ |date=January 24, 2007 }}</ref>

[[Phosphorus]] is often regarded as the main culprit in cases of eutrophication in lakes subjected to "point source" pollution from sewage pipes.  The concentration of algae and the [[Trophic state index|trophic state]] of lakes correspond well to phosphorus levels in water. Studies conducted in the Experimental Lakes Area in Ontario have shown a relationship between the addition of phosphorus and the rate of eutrophication. Later stages of eutrophication lead to blooms of nitrogen-fixing cyanobacteria limited solely by the phosphorus concentration.<ref>{{cite journal |last1=Higgins |first1=Scott N. |last2=Paterson |first2=Michael J. |last3=Hecky |first3=Robert E. |last4=Schindler |first4=David W. |last5=Venkiteswaran |first5=Jason J. |last6=Findlay |first6=David L. |title=Biological Nitrogen Fixation Prevents the Response of a Eutrophic Lake to Reduced Loading of Nitrogen: Evidence from a 46-Year Whole-Lake Experiment |journal=Ecosystems |date=November 27, 2017 |volume=21 |issue=6 |pages=1088–1100 |doi=10.1007/s10021-017-0204-2 |s2cid=26030685 }}</ref>  Phosphorus-base eutrophication in fresh water lakes has been addressed in several cases.

===Coastal waters===
{{Further|Harmful algal bloom#Causes or contributing factors of coastal HABs}}
{{Further|Estuary#Implications of eutrophication on estuaries}}
<gallery mode=packed heights=200px>
IMAGE-Map of measured Gulf hypoxia zone, July 25-31, 2021-LUMCON-NOAA.png|Map of measured Gulf hypoxia zone, July 25–31, 2021, LUMCON-NOAA
UNESCO global ocean deoxygenation map.png|Oxygen minimum zones (OMZs) (blue) and areas with coastal hypoxia (red) in the world's ocean<ref name=":15" />
</gallery>
Eutrophication is a common phenomenon in [[coastal waters]], where nitrogenous sources are the main culprit.<ref name=Smith/> In coastal waters, nitrogen is commonly the key limiting nutrient of [[Seawater|marine waters]] (unlike the freshwater systems where phosphorus is often the limiting nutrient). Therefore, [[nitrogen]] levels are more important than phosphorus levels for understanding and controlling eutrophication problems in salt water.<ref name=":0">{{Cite journal|last1=Paerl|first1=Hans W.|last2=Valdes|first2=Lexia M.|last3=Joyner|first3=Alan R.|last4=Piehler|first4=Michael F.|last5=Lebo|first5=Martin E.|date=2004|title=Solving problems resulting from solutions: Evolution of a dual nutrient management strategy for the eutrophying Neuse River Estuary, North Carolina|journal=Environmental Science and Technology|volume=38|issue=11|pages=3068–3073|doi=10.1021/es0352350|pmid=15224737|bibcode=2004EnST...38.3068P}}</ref> [[Estuary|Estuaries]], as the interface between freshwater and saltwater, can be both phosphorus and nitrogen limited and commonly exhibit symptoms of eutrophication. Eutrophication in estuaries often results in bottom water hypoxia or anoxia, leading to fish kills and habitat degradation.<ref name="huang">{{cite journal|last1=Huang|first1=Jing|last2=Xu|first2=Chang-chun|last3=Ridoutt|first3=Bradley|last4=Wang|first4=Xue-chun|last5=Ren|first5=Pin-an|date=August 2017|title=Nitrogen and phosphorus losses and eutrophication potential associated with fertilizer application to cropland in China|journal=Journal of Cleaner Production|volume=159|pages=171–179|doi=10.1016/j.jclepro.2017.05.008|bibcode=2017JCPro.159..171H }}</ref> Upwelling in coastal systems also promotes increased productivity by conveying deep, nutrient-rich waters to the surface, where the nutrients can be assimilated by [[algae]].

Examples of anthropogenic sources of nitrogen-rich pollution to coastal waters include sea cage [[fish farming]] and discharges of [[ammonia]] from the production of [[Coke (fuel)|coke]] from coal.<ref>{{Cite web|date=2019|title=Recovery of Ammonia during Production of Coke from Coking Coal|url=https://www.ispatguru.com/recovery-of-ammonia-during-production-of-coke-from-coking-coal/|url-status=live|access-date=June 17, 2021|website=Ispat Guru|archive-date=June 24, 2021|archive-url=https://web.archive.org/web/20210624200906/https://www.ispatguru.com/recovery-of-ammonia-during-production-of-coke-from-coking-coal/}}</ref> In addition to runoff from land, wastes from fish farming and industrial ammonia discharges, atmospheric [[Nitrogen fixation|fixed nitrogen]] can be an important nutrient source in the open ocean. This could account for around one third of the ocean's external (non-recycled) nitrogen supply, and up to 3% of the annual new marine biological production.<ref>{{cite journal | last1 = Duce | first1 = R A | display-authors = etal   | year = 2008 | title = Impacts of Atmospheric Anthropogenic Nitrogen on the Open Ocean | journal = Science | volume = 320 | issue = 5878| pages = 893–89 | doi=10.1126/science.1150369 | pmid=18487184| bibcode = 2008Sci...320..893D| s2cid = 11204131 | hdl = 21.11116/0000-0001-CD7A-0 | hdl-access = free }}</ref>

Coastal waters embrace a wide range of [[marine habitats]] from enclosed [[Estuary|estuaries]] to the [[Pelagic zone|open waters]] of the continental shelf. Phytoplankton productivity in coastal waters depends on both nutrient and light supply, with the latter an important limiting factor in waters near to shore where sediment resuspension often limits light penetration.

Nutrients are supplied to coastal waters from land via river and groundwater and also via the atmosphere. There is also an important source from the open ocean, via mixing of relatively nutrient rich deep ocean waters.<ref name=":3">{{Cite journal|last=Jickells|first=T. D.|date=1998|title=Nutrient Biogeochemistry of the Coastal Zone|url=https://www.science.org/doi/10.1126/science.281.5374.217|journal=Science|language=en|volume=281|issue=5374|pages=217–222|doi=10.1126/science.281.5374.217|issn=0036-8075|pmid=9660744|url-access=subscription}}</ref> Nutrient inputs from the ocean are little changed by human activity, although [[climate change]] may alter the water flows across the shelf break. By contrast, inputs from land to coastal zones of the nutrients nitrogen and phosphorus have been increased by human activity globally. The extent of increases varies greatly from place to place depending on human activities in the catchments.<ref>{{Cite journal|last1=Seitzinger|first1=S. P.|last2=Mayorga|first2=E.|last3=Bouwman|first3=A. F.|last4=Kroeze|first4=C.|last5=Beusen|first5=A. H. W.|last6=Billen|first6=G.|last7=Van Drecht|first7=G.|last8=Dumont|first8=E.|last9=Fekete|first9=B. M.|last10=Garnier|first10=J.|last11=Harrison|first11=J. A.|date=2010|title=Global river nutrient export: A scenario analysis of past and future trends: GLOBAL RIVER EXPORT SCENARIOS|url=http://doi.wiley.com/10.1029/2009GB003587|journal=Global Biogeochemical Cycles|language=en|volume=24|issue=4|pages=n/a|doi=10.1029/2009GB003587|s2cid=55095122|url-access=subscription}}</ref><ref>{{Cite journal|last1=Jickells|first1=T. D.|last2=Buitenhuis|first2=E.|last3=Altieri|first3=K.|last4=Baker|first4=A. R.|last5=Capone|first5=D.|last6=Duce|first6=R. A.|last7=Dentener|first7=F.|last8=Fennel|first8=K.|last9=Kanakidou|first9=M.|last10=LaRoche|first10=J.|last11=Lee|first11=K.|date=2017|title=A reevaluation of the magnitude and impacts of anthropogenic atmospheric nitrogen inputs on the ocean: Atmospheric nitrogen inputs|url=http://doi.wiley.com/10.1002/2016GB005586|journal=Global Biogeochemical Cycles|volume=31 |issue=2 |pages=289–305 |language=en|doi=10.1002/2016GB005586|bibcode=2017GBioC..31..289J |hdl=1874/348077|s2cid=5158406|hdl-access=free}}</ref> A third key nutrient, dissolved [[silicon]], is derived primarily from sediment [[weathering]] to rivers and from offshore and is therefore much less affected by human activity.

==== Effects of coastal eutrophication ====
These increasing nitrogen and phosphorus nutrient inputs exert eutrophication pressures on coastal zones. These pressures vary geographically depending on the catchment activities and associated nutrient load. The geographical setting of the coastal zone is another important factor as it controls dilution of the nutrient load and oxygen exchange with the atmosphere. The effects of these eutrophication pressures can be seen in several different ways:

# There is evidence from [[Earth observation satellite|satellite monitoring]] that the amounts of [[chlorophyll]] as a measure of overall [[phytoplankton]] activity are increasing in many coastal areas worldwide due to increased nutrient inputs.<ref>{{Cite journal|last1=Maúre|first1=Elígio de Raús|last2=Terauchi|first2=Genki|last3=Ishizaka|first3=Joji|last4=Clinton|first4=Nicholas|last5=DeWitt|first5=Michael|date=2021|title=Globally consistent assessment of coastal eutrophication|journal=Nature Communications|language=en|volume=12|issue=1|page=6142|doi=10.1038/s41467-021-26391-9|issn=2041-1723|pmc=8536747|pmid=34686688}}</ref>
# The phytoplankton [[species composition]] may change due to increased nutrient loadings and changes in the proportions of key nutrients. In particular the increases in nitrogen and phosphorus inputs, along with much smaller changes in silicon inputs, create changes in the ratio of nitrogen and phosphorus to silicon. These changing nutrient ratios drive changes in phytoplankton species composition, particularly disadvantaging silica rich phytoplankton species like diatoms compared to other species.<ref name=":3" /> This process leads to the development of nuisance algal blooms in areas such as the North Sea<ref>{{Cite web|editor-first1=Emily|editor-last1=Corcoran|editor-first2=Jo|editor-last2=Foden|editor-first3=Jennifer|editor-last3=Godwin|editor-first4=Barbara|editor-last4=Middleton|editor-first5=Colin|editor-last5=Moffat|editor-first6=John|editor-last6=Mouat|editor-first7=Bernardas|editor-last7=Padegimas|editor-first8=Carolyn|editor-last8=Symon|year=2017|title=Intermediate Assessment 2017|url=https://oap.ospar.org/en/ospar-assessments/intermediate-assessment-2017/|access-date=February 9, 2022|publisher=OSPAR|language=en|archive-date=February 9, 2022|archive-url=https://web.archive.org/web/20220209160519/https://oap.ospar.org/en/ospar-assessments/intermediate-assessment-2017/|url-status=live}}</ref> (see also [[OSPAR Convention]]) and the [[Black Sea]].<ref name=":13">{{Cite journal|last1=Mee|first1=Laurence|last2=Friedrich|first2=Jana|last3=Gomoiu|first3=Marian|date=2005|title=Restoring the Black Sea in Times of Uncertainty|journal=Oceanography|volume=18|issue=2|pages=100–111|doi=10.5670/oceanog.2005.45|issn=1042-8275|doi-access=free|bibcode=2005Ocgpy..18b.100M }}</ref> In some cases nutrient enrichment can lead to [[harmful algal bloom]]s (HABs). Such blooms can occur naturally, but there is good evidence that these are increasing as a result of nutrient enrichment, although the causal linkage between nutrient enrichment and HABs is not straightforward.<ref name=":16">{{Cite journal|last1=Glibert|first1=Patricia|last2=Burford|first2=Michele|date=2017|title=Globally Changing Nutrient Loads and Harmful Algal Blooms: Recent Advances, New Paradigms, and Continuing Challenges|url=https://tos.org/oceanography/article/globally-changing-nutrient-loads-and-harmful-algal-blooms-recent-advances-n|journal=Oceanography|volume=30|issue=1|pages=58–69|doi=10.5670/oceanog.2017.110|access-date=February 9, 2022|archive-date=January 21, 2022|archive-url=https://web.archive.org/web/20220121234844/https://tos.org/oceanography/article/globally-changing-nutrient-loads-and-harmful-algal-blooms-recent-advances-n|url-status=live|doi-access=free|bibcode=2017Ocgpy..30a..58G |hdl=10072/377577|hdl-access=free}}</ref>
# [[Hypoxia (environmental)|Oxygen depletion]] has existed in some coastal seas such as the [[Baltic Sea hypoxia|Baltic for thousands of years]]. In such areas the density structure of the water column severely restricts water column mixing and associated oxygenation of deep water. However, increases in the inputs of bacterially degradable organic matter to such isolated deep waters can exacerbate such [[Ocean deoxygenation|oxygen depletion in oceans]]. These areas of lower dissolved oxygen have increased globally in recent decades. They are usually connected with nutrient enrichment and resulting algal blooms.<ref name=":15">{{Cite journal|last1=Breitburg|first1=Denise|last2=Levin|first2=Lisa A.|last3=Oschlies|first3=Andreas|last4=Grégoire|first4=Marilaure|last5=Chavez|first5=Francisco P.|last6=Conley|first6=Daniel J.|last7=Garçon|first7=Véronique|last8=Gilbert|first8=Denis|last9=Gutiérrez|first9=Dimitri|last10=Isensee|first10=Kirsten|last11=Jacinto|first11=Gil S.|date=2018|title=Declining oxygen in the global ocean and coastal waters|journal=Science|language=EN|volume=359|issue=6371|article-number=eaam7240 |bibcode=2018Sci...359M7240B|doi=10.1126/science.aam7240|pmid=29301986|s2cid=206657115|doi-access=free}}</ref> Climate change will generally tend to increase water column stratification and so exacerbate this oxygen depletion problem.<ref>{{Cite journal|last1=Li|first1=Guancheng|last2=Cheng|first2=Lijing|last3=Zhu|first3=Jiang|last4=Trenberth|first4=Kevin E.|last5=Mann|first5=Michael E.|last6=Abraham|first6=John P.|date=2020|title=Increasing ocean stratification over the past half-century|url=https://www.nature.com/articles/s41558-020-00918-2|journal=Nature Climate Change|language=en|volume=10|issue=12|pages=1116–1123|doi=10.1038/s41558-020-00918-2|bibcode=2020NatCC..10.1116L|s2cid=221985871|issn=1758-678X|access-date=February 18, 2022|archive-date=February 18, 2022|archive-url=https://web.archive.org/web/20220218194651/https://www.nature.com/articles/s41558-020-00918-2|url-status=live|url-access=subscription}}</ref> An example of such coastal oxygen depletion is in the [[Gulf of Mexico]] where an area of seasonal anoxia more than 5000 square miles in area has developed since the 1950s. The increased primary production driving this anoxia is fueled by nutrients supplied by the [[Mississippi River|Mississippi river]].<ref>{{Cite journal|last1=Rabalais|first1=Nancy N.|last2=Turner|first2=R. Eugene|date=2019|title=Gulf of Mexico Hypoxia: Past, Present, and Future|journal=Limnology and Oceanography Bulletin|language=en|volume=28|issue=4|pages=117–124|doi=10.1002/lob.10351|issn=1539-6088|s2cid=209578424|doi-access=free|bibcode=2019LimOB..28..117R }}</ref> A similar process has been documented in the Black Sea.<ref name=":13" />
# [[Hypolimnion|Hypolimnetic]] oxygen depletion can lead to summer "kills". During summer [[Stratification (water)|stratification]], inputs or organic matter and [[sedimentation]] of [[primary producer]]s can increase rates of [[Respiration (physiology)|respiration]] in the [[hypolimnion]]. If oxygen depletion becomes extreme, aerobic organisms (such as fish) may die, resulting in what is known as a "summer kill".<ref name="Limnology">Wetzel, R. G. (2001). ''Limnology: Lake and river ecosystems''. San Diego: Academic Press.</ref>

== Extent of the problem ==
Surveys showed that 54% of lakes in [[Asia]] are eutrophic; in [[Europe]], 53%; in [[North America]], 48%; in [[South America]], 41%; and in [[Africa]], 28%.<ref name="ILEC">ILEC/Lake Biwa Research Institute [Eds]. 1988–1993 Survey of the State of the World's Lakes. Volumes I-IV. International Lake Environment Committee, Otsu and United Nations Environment Programme, Nairobi.</ref> In South Africa, a study by the CSIR using [[remote sensing]] has shown more than 60% of the reservoirs surveyed were eutrophic.<ref>{{cite journal|last1=Matthews|first1=Mark|last2=Bernard|first2=Stewart|year=2015|title=Eutrophication and cyanobacteria in South Africa's standing water bodies: A view from space|journal=South African Journal of Science|volume=111|issue=5/6|pages=1–8 |doi=10.17159/sajs.2015/20140193|doi-access=free}}</ref>

The [[World Resources Institute]] has identified 375 [[Hypoxia (environmental)|hypoxic]] coastal zones in the world, concentrated in coastal areas in Western Europe, the Eastern and Southern coasts of the US, and [[East Asia]], particularly [[Japan]].<ref>
Selman, Mindy (2007) ''Eutrophication: An Overview of Status, Trends, Policies, and Strategies.'' World Resources Institute.</ref>

==Prevention==
{{Further|Harmful algal bloom}}

As a society, there are certain steps we can take to ensure the minimization of eutrophication, thereby reducing its harmful effects on humans and other living organisms in order to sustain a healthy norm of living, some of which are as follows:

=== Minimizing pollution from sewage ===
{{Further|Nutrient pollution#Mitigation of nutrient pollutant discharges}}
There are multiple different ways to fix cultural eutrophication with raw sewage being a [[point source]] of pollution. For example, [[Sewage treatment|sewage treatment plants]] can be upgraded for biological nutrient removal so that they discharge much less nitrogen and phosphorus to the receiving water body. However, even with good [[secondary treatment]], most final effluents from sewage treatment works contain substantial concentrations of nitrogen as nitrate, nitrite or ammonia. Removal of these nutrients is an expensive and often difficult process.

Laws regulating the discharge and treatment of sewage have led to dramatic nutrient reductions to surrounding ecosystems.<ref name="Smith 1999">{{Cite journal |last1=Smith |first1=V. H. |last2=Tilman |first2=G. D. |last3=Nekola |first3=J. C. |year=1999 |title=Eutrophication: Impacts of excess nutrient inputs on freshwater, marine, and terrestrial ecosystems |journal=Environmental Pollution |volume=100 |issue=1–3 |pages=179–196 |doi=10.1016/S0269-7491(99)00091-3 |pmid=15093117 |s2cid=969039}}</ref> As a major contributor to the nonpoint source nutrient loading of water bodies is untreated domestic sewage, it is necessary to provide treatment facilities to highly urbanized areas, particularly those in [[Developing country|developing countries]], in which treatment of domestic waste water is a scarcity. The technology to safely and efficiently [[Reclaimed water|reuse wastewater]], both from domestic and industrial sources, should be a primary concern for policy regarding eutrophication.

=== Minimizing nutrient pollution by agriculture ===
There are many ways to help fix cultural eutrophication caused by agriculture. Some recommendations issued by the U.S. Department of Agriculture include:<ref name=":4">{{Cite web|date=March 12, 2013|title=The Sources and Solutions: Agriculture|url=https://www.epa.gov/nutrientpollution/sources-and-solutions-agriculture|url-status=live|archive-url=https://web.archive.org/web/20210622084443/https://www.epa.gov/nutrientpollution/sources-and-solutions-agriculture|archive-date=June 22, 2021|publisher=United States EPA}}</ref>
# [[Nutrient management]] techniques - Anyone using fertilizers should apply fertilizer in the correct amount, at the right time of year, with the right method and placement. Organically fertilized fields can "significantly reduce harmful nitrate leaching" compared to conventionally fertilized fields.<ref name="PNAS 2006-3-21">{{Cite journal | doi = 10.1073/pnas.0600359103|bibcode = 2006PNAS..103.4522K | title = Reduced nitrate leaching and enhanced denitrifier activity and efficiency in organically fertilized soils | year = 2006 | last1 = Kramer | first1 = S. B. | journal = Proceedings of the National Academy of Sciences | volume = 103 | issue = 12 | pages = 4522–4527 | pmid=16537377 | pmc=1450204|doi-access = free }}</ref> Eutrophication impacts are in some cases higher from organic production than they are from conventional production.<ref>Williams, A.G., Audsley, E. and Sandars, D.L. (2006) [http://randd.defra.gov.uk/Default.aspx?Module=More&Location=None&ProjectID=11442 Determining the environmental burdens and resource use in the production of agricultural and horticultural commodities] {{Webarchive|url=https://web.archive.org/web/20180925104257/http://randd.defra.gov.uk/Default.aspx?Module=More&Location=None&ProjectID=11442 |date=September 25, 2018 }}. Main Report. Defra Research Project IS0205. Bedford: Cranfield University and Defra.</ref>  In Japan the amount of nitrogen produced by livestock is adequate to serve the fertilizer needs for the agriculture industry.<ref name="Kumazawa 2002">{{Cite journal | last1 = Kumazawa | first1 = K. | journal = Nutrient Cycling in Agroecosystems | volume = 63 | issue = 2/3 | pages = 129–137 | doi = 10.1023/A:1021198721003 |title=Nitrogen fertilization and nitrate pollution in groundwater in Japan: Present status and measures for sustainable agriculture| year = 2002 | bibcode = 2002NCyAg..63..129K | s2cid = 22847510 }}</ref>
# Year-round ground cover - a [[cover crop]] will prevent periods of bare ground thus eliminating erosion and runoff of nutrients even after the growing season has passed.
# Planting field buffers - Planting trees, shrubs and grasses along the edges of fields can help catch the runoff and absorb some nutrients before the water makes it to a nearby water body.<ref name="Carpenter, S.R. 1998">{{cite journal|last1=Carpenter|first1=S. R.|last2=Caraco|first2=N. F.|last3=Correll|first3=D. L.|last4=Howarth|first4=R. W.|last5=Sharpley|first5=A. N.|last6=Smith|first6=V. H.|date=August 1998|title=Nonpoint Pollution of Surface Waters with Phosphorus and Nitrogen|journal=Ecological Applications|volume=8|issue=3|page=559|doi=10.2307/2641247|jstor=2641247|hdl-access=free|hdl=1813/60811}}</ref> [[riparian|Riparian buffer zones]] are interfaces between a flowing body of water and land, and have been created near waterways in an attempt to filter pollutants; [[sediment]] and nutrients are deposited here instead of in water. Creating buffer zones near farms and roads is another possible way to prevent nutrients from traveling too far. 
# [[Tillage#Conservation_tillage|Conservation tillage]] - By reducing frequency and intensity of tilling, the land will enhance the chance of nutrients absorbing into the ground.
[[File:Orange like Autumn.jpg|thumb|Eutrophication in a canal]]

===Policy===
The [[United Nations]] framework for [[Sustainable Development Goals]] recognizes the damaging effects of eutrophication for marine environments. It has established a timeline for creating an Index of Coastal Eutrophication and Floating Plastic Debris Density (ICEP) within [[Sustainable Development Goal 14]] (life below water).<ref name=":14">{{Cite web|title=14.1.1 Index of Coastal Eutrophication (ICEP) and Floating Plastic debris Density|url=https://uneplive.unep.org/indicator/index/14_1_1|access-date=October 14, 2020|website=UN Environment|archive-date=August 13, 2020|archive-url=https://web.archive.org/web/20200813115053/https://uneplive.unep.org/indicator/index/14_1_1|url-status=live}}</ref> SDG 14 specifically has a target to: "by 2025, prevent and significantly reduce marine pollution of all kinds, in particular from land-based activities, including marine debris and nutrient pollution".<ref name=":17">{{Cite web|title=Goal 14 targets|url=https://www.undp.org/content/undp/en/home/sustainable-development-goals/goal-14-life-below-water/targets.html|archive-url=https://web.archive.org/web/20200930060036/https://www.undp.org/content/undp/en/home/sustainable-development-goals/goal-14-life-below-water/targets.html|archive-date=September 30, 2020|access-date=September 24, 2020|publisher=UNDP|language=en}}</ref>

Policy and regulations are a set of tools to minimize causes of eutrophication.<ref name="ReferenceA">[http://www.unep.or.jp/ietc/Publications/TechPublications/TechPub-11/ "Planning and Management of Lakes and Reservoirs: An Integrated Approach to Eutrophication."] {{Webarchive|url=https://web.archive.org/web/20121111202652/http://www.unep.or.jp/ietc/Publications/techpublications/TechPub-11/ |date=November 11, 2012 }} United Nations Environment Programme, Newsletter and Technical Publications. International Environmental Technology Centre. Ch.3.4 (2000).</ref> Nonpoint sources of pollution are the primary contributors to eutrophication, and their effects can be minimized through common agricultural practices. Reducing the amount of pollutants that reach a watershed can be achieved through the protection of its forest cover, reducing the amount of erosion leeching into a watershed. Also, through the efficient, controlled use of land using sustainable agricultural practices to minimize [[land degradation]], the amount of soil runoff and nitrogen-based fertilizers reaching a watershed can be reduced.<ref>{{cite journal|author1=Oglesby, R. T.  |author2=Edmondson, W. T. |title=Control of Eutrophication|journal= Journal (Water Pollution Control Federation)|volume= 38|issue= 9 |year=1966|pages= 1452–1460|jstor=25035632}}</ref> Waste disposal technology constitutes another factor in eutrophication prevention.

The current policies proposed are mainly command-and-control policies, which are based on commonly used regulation standards.<ref name=":19">{{Cite journal |last1=Hammarlund |first1=Cecilia |last2=Andersson |first2=Anna |last3=Nordström |first3=Jonas |date=2024-04-02 |title=Nutrient policies and the performance of aquaculture in developed countries – a literature review |url=https://www.tandfonline.com/doi/full/10.1080/13657305.2024.2314511 |journal=Aquaculture Economics & Management |language=en |volume=28 |issue=2 |pages=208–237 |doi=10.1080/13657305.2024.2314511 |bibcode=2024AqEM...28..208H |issn=1365-7305}}</ref> Although these policies are easier to implement, they are not as cost-effective.<ref name=":19" /> This typically involves implementing limitations on input resources, emissions, or technologies, which are all common command-and-control policies that have been implemented by multiple countries.<ref name=":19" />

Because a body of water can have an effect on a range of people reaching far beyond that of the watershed, cooperation between different organizations is necessary to prevent the intrusion of contaminants that can lead to eutrophication. Agencies ranging from state governments to those of water resource management and non-governmental organizations, going as low as the local population, are responsible for preventing eutrophication of water bodies. In the United States, the most well known inter-state effort to prevent eutrophication is the [[Chesapeake Bay]].<ref>[https://web.archive.org/web/20150111125613/http://dnr.state.md.us/bay/monitoring/limit/index.html Nutrient Limitation]. Department of Natural Resources, Maryland, U.S.</ref>

== Reversal and remediation ==
Reducing nutrient inputs is a crucial precondition for restoration. Still, there are two caveats: Firstly, it can take a long time, mainly because of the storage of nutrients in [[sediment]]s. Secondly, restoration may need more than a simple reversal of inputs since there are sometimes several stable but very different ecological states.<ref>{{Cite journal|last1=May|first1=L|last2=Olszewska|first2=J|last3=Gunn|first3=I D M|last4=Meis|first4=S|last5=Spears|first5=B M|date=2020|title=Eutrophication and restoration in temperate lakes|journal=IOP Conference Series: Earth and Environmental Science|volume=535|issue=1|article-number=012001|doi=10.1088/1755-1315/535/1/012001|bibcode=2020E&ES..535a2001M|s2cid=225481650|issn=1755-1307|doi-access=free}}</ref> Recovery of eutrophicated lakes is slow, often requiring several decades.<ref name=":18" />

In [[environmental remediation]], nutrient removal technologies include [[biofiltration]], which uses living material to capture and biologically degrade pollutants. Examples include green belts, [[riparian]] areas, natural and constructed wetlands, and treatment ponds.

===Algae bloom forecasting===
The National Oceanic Atmospheric Admiration in the United States has created a forecasting tool for regions such as the Great Lakes, the Gulf of Maine, and The Gulf of Mexico.<ref>{{cite web |title=Lake Erie Harmful Algal Bloom Forecast |url=https://coastalscience.noaa.gov/science-areas/habs/hab-forecasts/lake-erie/ |website=NCCOS |publisher=NOAA |access-date=12 February 2024}}</ref> Shorter term predictions can help to show the intensity, location, and trajectory of blooms in order to warn more directly affected communities. Longer term tests in specific regions and bodies help to predict larger scale factors like scale of future blooms and factors that could lead to more adverse effects.<ref>{{Cite web |title=HAB Forecasts |url=https://coastalscience.noaa.gov/science-areas/habs/hab-forecasts/ |access-date=2024-11-04 |website=NCCOS Coastal Science Website |language=en-US}}</ref>

===Nutrient bioextraction===
Nutrient bioextraction is bioremediation involving cultured plants and animals. Nutrient bioextraction or bioharvesting is the practice of farming and harvesting [[shellfish]] and [[seaweed]] to remove nitrogen and other nutrients from natural water bodies.<ref>{{cite web|title=Nutrient Bioextraction Overview|url=http://longislandsoundstudy.net/issues-actions/water-quality/nutrient-bioextraction-overview/|access-date=March 22, 2018|publisher=Long Island Sound Study partnership|location=Stamford, Conn.|archive-date=October 6, 2017|archive-url=https://web.archive.org/web/20171006062352/http://longislandsoundstudy.net/issues-actions/water-quality/nutrient-bioextraction-overview/|url-status=live}}</ref>

==== Shellfish in estuaries ====
[[File:Mussels at Strawberry Rocks PC013145.JPG|thumb|Mussels are an example of organisms that act as nutrient bioextractors. They consume the nitrogen in water, depleting algae of their nutrients.|231x231px]]
{{See also|Nutrient pollution}}It has been suggested that nitrogen removal by oyster reefs could generate net benefits for sources facing nitrogen emission restrictions, similar to other nutrient trading scenarios. Specifically, if oysters maintain nitrogen levels in estuaries below thresholds, then oysters effectively stave off an enforcement response, and compliance costs parties responsible for nitrogen emission would otherwise incur.<ref>{{cite web|last=Kroeger|first=Timm|year=2012|title=Dollars and Sense: Economic Benefits and Impacts from two Oyster Reef Restoration Projects in the Northern Gulf of Mexico|url=http://www.nature.org/ourinitiatives/regions/northamerica/oyster-restoration-study-kroeger.pdf|archive-url=https://web.archive.org/web/20160304002313/http://www.nature.org/ourinitiatives/regions/northamerica/oyster-restoration-study-kroeger.pdf|archive-date=March 4, 2016|access-date=May 29, 2013|publisher=The Nature Conservancy}}</ref> Several studies have shown that oysters and mussels can dramatically impact nitrogen levels in estuaries.<ref>{{cite book| vauthors = Newell RI, Fisher TR, Holyoke RR, Cornwell JC |title=The Comparative Roles of Suspension Feeders in Ecosystems | volume = 47 |publisher=Springer|year=2005| veditors = Dame R, Olenin S |edition=NATO Science Series IV: Earth and Environmental Sciences|location=Netherlands|pages=93–120|contribution=Influence of eastern oysters on nitrogen and phosphorus regeneration in Chesapeake Bay, USA}}</ref><ref>{{cite book| vauthors = Grabowski JH, Petersen CM |title=Restoring oyster reefs to recover ecosystem services|publisher=Elsevier-Academic Press|year=2007| veditors = Cuddington K, Byers JE, Wilson WG, Hastings A |edition=Ecosystem Engineers: Concepts, Theory and Applications|location=Amsterdam|pages=281–298}}</ref><ref>{{cite web|year=2010|title=International Workshop on Bioextractive Technologies for Nutrient Remediation Summary Report|url=http://www.nefsc.noaa.gov/nefsc/publications/|publisher=US Dept Commerce, Northeast Fish Sci Cent Ref Doc. 10-19; 12 p. Available from: National Marine Fisheries Service, 166 Water Street, Woods Hole, MA 02543-1026|vauthors=Rose JM, Tedesco M, Wikfors GH, Yarish C|access-date=February 15, 2022|archive-date=October 29, 2019|archive-url=https://web.archive.org/web/20191029030853/https://www.nefsc.noaa.gov/nefsc/publications/|url-status=live}}</ref> Filter feeding activity is considered beneficial to water quality<ref>Burkholder, JoAnn M. and Sandra E. Shumway. (2011) "Bivalve shellfish aquaculture and eutrophication", in ''Shellfish Aquaculture and the Environment''. Ed. Sandra E. Shumway. John Wiley & Sons, {{ISBN|0-8138-1413-8}}.</ref> by controlling phytoplankton density and sequestering nutrients, which can be removed from the system through shellfish harvest, buried in the sediments, or lost through [[denitrification]].<ref>{{Cite journal|last1=Kaspar|first1=H. F.|last2=Gillespie|first2=P. A.|last3=Boyer|first3=I. C.|last4=MacKenzie|first4=A. L.|year=1985|title=Effects of mussel aquaculture on the nitrogen cycle and benthic communities in Kenepuru Sound, Marlborough Sounds, New Zealand|journal=Marine Biology|volume=85|issue=2|pages=127–136|doi=10.1007/BF00397431|bibcode=1985MarBi..85..127K |s2cid=83551118}}</ref><ref>{{Cite journal|last1=Newell|first1=R. I. E.|last2=Cornwell|first2=J. C.|last3=Owens|first3=M. S.|year=2002|title=Influence of simulated bivalve biodeposition and microphytobenthos on sediment nitrogen dynamics: A laboratory study|journal=Limnology and Oceanography|volume=47|issue=5|pages=1367–1379|bibcode=2002LimOc..47.1367N|doi=10.4319/lo.2002.47.5.1367|doi-access=free}}</ref> Foundational work toward the idea of improving marine water quality through shellfish cultivation was conducted by Odd Lindahl et al., using [[mussels]] in Sweden.<ref>{{Cite journal|last1=Lindahl|first1=O.|last2=Hart|first2=R.|last3=Hernroth|first3=B.|last4=Kollberg|first4=S.|last5=Loo|first5=L. O.|last6=Olrog|first6=L.|last7=Rehnstam-Holm|first7=A. S.|last8=Svensson|first8=J.|last9=Svensson|first9=S.|last10=Syversen|first10=U.|year=2005|title=Improving marine water quality by mussel farming: A profitable solution for Swedish society|url=http://www.aquacircle.org/images/pdfdokumenter/efterret07/ambi3402_131-138.pdf|journal=Ambio|volume=34|issue=2|pages=131–138|citeseerx=10.1.1.589.3995|doi=10.1579/0044-7447-34.2.131|pmid=15865310|bibcode=2005Ambio..34..131L |s2cid=25371433|access-date=November 1, 2017|archive-date=September 22, 2017|archive-url=https://web.archive.org/web/20170922011833/http://www.aquacircle.org/images/pdfdokumenter/efterret07/ambi3402_131-138.pdf|url-status=live}}</ref> In the United States, shellfish restoration projects have been conducted on the East, West and Gulf coasts.<ref>Brumbaugh, R.D. et al. (2006). [http://www.conservationgateway.org/Files/Pages/practitioner%E2%80%99s-guide-desi.aspx A Practitioners Guide to the Design and Monitoring of Shellfish Restoration Projects: An Ecosystem Services Approach] {{Webarchive|url=https://web.archive.org/web/20130701024305/http://www.conservationgateway.org/Files/Pages/practitioner%E2%80%99s-guide-desi.aspx |date=July 1, 2013 }}. The Nature Conservancy, Arlington, Va.</ref>

====Seaweed farming====
Studies have demonstrated seaweed's potential to improve nitrogen levels.<ref>{{Cite journal |last1=Kim |first1=Jang K. |last2=Kraemer |first2=George P. |last3=Yarish |first3=Charles |date=2014 |title=Field scale evaluation of seaweed aquaculture as a nutrient bioextraction strategy in Long Island Sound and the Bronx River Estuary |journal=Aquaculture |volume=433 |pages=148–156 |bibcode=2014Aquac.433..148K |doi=10.1016/j.aquaculture.2014.05.034}}</ref><ref>{{cite web |last1=Kroeger |first1=Timm |date=May 2012 |title=Dollars and Sense: Economic Benefits and Impacts from two Oyster Reef Restoration Projects in the Northern Gulf of Mexico |url=https://www.conservationgateway.org/Files/Pages/dollars-and-sense-economi.aspx |url-status=live |archive-url=https://web.archive.org/web/20200803151420/http://www.conservationgateway.org/Files/Pages/dollars-and-sense-economi.aspx |archive-date=August 3, 2020 |access-date=July 29, 2020 |publisher=The Nature Conservancy}}</ref>  [[Seaweed farming|Seaweed aquaculture]] offers an opportunity to mitigate, and adapt to climate change.<ref>{{cite journal|last1=Duarte|first1=Carlos M.|last2=Wu|first2=Jiaping|last3=Xiao|first3=Xi|last4=Bruhn|first4=Annette|last5=Krause-Jensen|first5=Dorte|date=April 12, 2017|title=Can Seaweed Farming Play a Role in Climate Change Mitigation and Adaptation?|journal=Frontiers in Marine Science|volume=4|page=100 |doi=10.3389/fmars.2017.00100|doi-access=free|bibcode=2017FrMaS...4..100D |hdl=10754/623247|hdl-access=free}}</ref> Seaweed, such as kelp, also absorbs phosphorus and nitrogen<ref>{{Cite web|url=https://e360.yale.edu/features/new_breed_of_ocean_farmer_aims_to_revive_global_seas|title=Can We Save the Oceans By Farming Them?|website=Yale E360|access-date=March 8, 2019|archive-date=October 19, 2019|archive-url=https://web.archive.org/web/20191019150923/https://e360.yale.edu/features/new_breed_of_ocean_farmer_aims_to_revive_global_seas|url-status=live}}</ref> and is thus helpful to remove excessive nutrients from polluted parts of the sea.<ref>{{Cite journal|last1=Xiao|first1=X.|last2=Agusti|first2=S.|last3=Lin|first3=F.|last4=Li|first4=K.|last5=Pan|first5=Y.|last6=Yu|first6=Y.|last7=Zheng|first7=Y.|last8=Wu|first8=J.|last9=Duarte|first9=C. M.|year=2017|title=Nutrient removal from Chinese coastal waters by large-scale seaweed aquaculture|journal=Scientific Reports|volume=7|article-number=46613|bibcode=2017NatSR...746613X|doi=10.1038/srep46613|pmc=5399451|pmid=28429792}}</ref> Some cultivated seaweeds have very high productivity and could absorb large quantities of N, P, {{CO2}}, producing large amounts of {{chem2|O2}} having an excellent effect on decreasing eutrophication.<ref>{{Citation|last=Duarte|first=Carlos M.|title=Coastal eutrophication research: A new awareness |date=2009|work=Eutrophication in Coastal Ecosystems|pages=263–269|publisher=Springer Netherlands|doi=10.1007/978-90-481-3385-7_22|isbn=978-90-481-3384-0}}</ref> It is believed that seaweed cultivation in large scale should be a good solution to the eutrophication problem in [[coastal waters]].

===Geo-engineering===
[[File:Application of a phosphorus sorbent to a lake - The Netherlands.jpg|thumb|Application of a phosphorus sorbent to a lake - The Netherlands]]
Another technique for combatting [[Hypoxia (environmental)|hypoxia]]/eutrophication in localized situations is direct injection of compressed air, a technique used in the restoration of the [[Salford Docks]] area of the [[Manchester Ship Canal]] in England.<ref>{{cite web|url= http://www.mangeogsoc.org.uk/egm/5_1.pdf|access-date=December 11, 2007|date=August 21, 2003|title= Exploring Greater Manchester&nbsp;– a fieldwork guide: The fluvioglacial gravel ridges of Salford and flooding on the River Irwell|author= Hindle, P.|publisher=Manchester Geographical Society}} p. 13</ref> For smaller-scale waters such as aquaculture ponds, pump aeration is standard.<ref>{{cite web | url=https://thefishsite.com/articles/pond-aeration | title=Pond Aeration | date=April 10, 2006 }}</ref>

===Chemical removal of phosphorus===
{{Further|Chemical phosphorus removal}}
Removing [[phosphorus cycle|phosphorus]] can remediate eutrophication.<ref>{{Cite journal|doi=10.1021/es5036267|pmid=25137490|title=Geo-Engineering in Lakes: A Crisis of Confidence?|journal=Environmental Science & Technology|volume=48|issue=17|pages=9977–9979|year=2014|last1=Spears|first1=Bryan M.|last2=Maberly|first2=Stephen C.|last3=Pan|first3=Gang|last4=MacKay|first4=Ellie|last5=Bruere|first5=Andy|last6=Corker|first6=Nicholas|last7=Douglas|first7=Grant|last8=Egemose|first8=Sara|last9=Hamilton|first9=David|last10=Hatton-Ellis|first10=Tristan|last11=Huser|first11=Brian|last12=Li|first12=Wei|last13=Meis|first13=Sebastian|last14=Moss|first14=Brian|last15=Lürling|first15=Miquel|last16=Phillips|first16=Geoff|last17=Yasseri|first17=Said|last18=Reitzel|first18=Kasper|bibcode=2014EnST...48.9977S|url=http://ir.rcees.ac.cn/handle/311016/9551|access-date=September 8, 2020|archive-date=October 21, 2021|archive-url=https://web.archive.org/web/20211021121418/http://ir.rcees.ac.cn/handle/311016/9551|url-status=live|url-access=subscription}}</ref><ref>{{Cite journal |doi=10.5268/IW-4.4.769|title=Geoengineering in lakes: Welcome attraction or fatal distraction?|journal=Inland Waters|volume=4|issue=4|pages=349–356|year=2014|last1=MacKay|first1=Eleanor|last2=Maberly|first2=Stephen|last3=Pan|first3=Gang|last4=Reitzel|first4=Kasper|last5=Bruere|first5=Andy|last6=Corker|first6=Nicholas|last7=Douglas|first7=Grant|last8=Egemose|first8=Sara|last9=Hamilton|first9=David|last10=Hatton-Ellis|first10=Tristan|last11=Huser|first11=Brian|last12=Li|first12=Wei|last13=Meis|first13=Sebastian|last14=Moss|first14=Brian|last15=Lürling|first15=Miquel|last16=Phillips|first16=Geoff|last17=Yasseri|first17=Said|last18=Spears|first18=Bryan|bibcode=2014InWat...4..349M |hdl=10072/337267|s2cid=55610343|hdl-access=free}}</ref> Of the several phosphate sorbents, [[alum]] ([[aluminium sulfate]]) is of practical interest.<ref>{{cite web|url=http://www.dnr.state.wi.us/org/water/fhp/papers/alum_brochure.pdf |title=Wisconsin Department of Natural Resources |access-date=August 3, 2010 |archive-url=https://web.archive.org/web/20091128030217/http://www.dnr.state.wi.us/org/water/fhp/papers/alum_brochure.pdf |archive-date=November 28, 2009 }}</ref>)   Many materials have been investigated.<ref>{{Cite journal|doi=10.1007/s10452-016-9575-2|title=Guiding principles for the development and application of solid-phase phosphorus adsorbents for freshwater ecosystems|journal=Aquatic Ecology|volume=50|issue=3|pages=385–405|year=2016|last1=Douglas|first1=G. B.|last2=Hamilton|first2=D. P.|last3=Robb|first3=M. S.|last4=Pan|first4=G.|last5=Spears|first5=B. M.|last6=Lurling|first6=M.|bibcode=2016AqEco..50..385D |hdl=10072/406333 |s2cid=18154662|url=http://irep.ntu.ac.uk/id/eprint/27767/1/PubSub5337_Pan.pdf|access-date=December 15, 2019|archive-date=September 19, 2020|archive-url=https://web.archive.org/web/20200919184710/http://irep.ntu.ac.uk/id/eprint/27767/1/PubSub5337_Pan.pdf|url-status=live}}</ref><ref>{{Cite journal|doi=10.1016/J.WATRES.2016.03.035|pmid=27039034|title=Editorial – A critical perspective on geo-engineering for eutrophication management in lakes|journal=Water Research|volume=97|pages=1–10|year=2016|last1=Lürling|first1=Miquel|last2=MacKay|first2=Eleanor|last3=Reitzel|first3=Kasper|last4=Spears|first4=Bryan M.|bibcode=2016WatRe..97....1L |url=http://nora.nerc.ac.uk/id/eprint/513724/1/N513724PP.pdf|access-date=December 15, 2019|archive-date=July 31, 2020|archive-url=https://web.archive.org/web/20200731023813/http://nora.nerc.ac.uk/id/eprint/513724/1/N513724PP.pdf|url-status=live}}</ref>  The phosphate sorbent is commonly applied in the surface of the water body and it sinks to the bottom of the lake reducing phosphate, such sorbents have been applied worldwide to manage eutrophication and algal bloom (for example under the commercial name [[Phoslock]]).<ref>{{Cite journal |doi=10.1016/j.watres.2015.06.051|pmid=26250754|title=Longevity and effectiveness of aluminum addition to reduce sediment phosphorus release and restore lake water quality|journal=Water Research|volume=97|pages=122–132|year=2016|last1=Huser|first1=Brian J.|last2=Egemose|first2=Sara|last3=Harper|first3=Harvey|last4=Hupfer|first4=Michael|last5=Jensen|first5=Henning|last6=Pilgrim|first6=Keith M.|last7=Reitzel|first7=Kasper|last8=Rydin|first8=Emil|last9=Futter|first9=Martyn|bibcode=2016WatRe..97..122H |doi-access=free}}</ref><ref>{{Cite journal |doi=10.1016/j.watres.2013.08.019|pmid=24041525|title=Controlling eutrophication by combined bloom precipitation and sediment phosphorus inactivation|journal=Water Research|volume=47|issue=17|pages=6527–6537|year=2013|last1=Lürling|first1=Miquel|last2=Oosterhout|first2=Frank van|bibcode=2013WatRe..47.6527L }}</ref><ref>{{Cite journal |doi=10.1080/10402381.2016.1265618|title=Attempted management of cyanobacteria by Phoslock (Lanthanum-modified clay) in Canadian lakes: Water quality results and predictions|journal=Lake and Reservoir Management|volume=33|issue=2|pages=163–170|year=2017|last1=Nürnberg|first1=Gertrud K.|bibcode=2017LRMan..33..163N |s2cid=89762486}}</ref><ref>{{Cite journal |doi=10.1080/10402381.2016.1263693|title=Nine years of phosphorus management with lanthanum modified bentonite (Phoslock) in a eutrophic, shallow swimming lake in Germany|journal=Lake and Reservoir Management|volume=33|issue=2|pages=119–129|year=2017|last1=Epe|first1=Tim Sebastian|last2=Finsterle|first2=Karin|last3=Yasseri|first3=Said|bibcode=2017LRMan..33..119E |s2cid=90314146}}</ref><ref name=":110">{{Cite journal |last1=Kennedy |first1=Robert H. |last2=Cook |first2=G. Dennis |title=Control of Lake Phosphorus with Aluminum Sulfate: Dose Determination and Application Techniques |date=June 1982 |journal=Journal of the American Water Resources Association |volume=18 |issue=3 |pages=389–395 |doi=10.1111/j.1752-1688.1982.tb00005.x |bibcode=1982JAWRA..18..389K |issn=1093-474X}}</ref> In a large-scale study, 114 lakes were monitored for the effectiveness of alum at phosphorus reduction. Across all lakes, alum effectively reduced the phosphorus for 11 years. While there was variety in longevity (21 years in deep lakes and 5.7 years in shallow lakes), the results express the effectiveness of alum at controlling phosphorus within lakes.<ref>{{Cite book |last1=Huser |first1=Brian J. |last2=Egemose |first2=Sara |last3=Harper |first3=Harvey |last4=Hupfer |first4=Michael |last5=Jensen |first5=Henning |last6=Pilgrim |first6=Keith M. |last7=Reitzel |first7=Kasper |last8=Rydin |first8=Emil |last9=Futter |first9=Martyn |date=2016 |title=Longevity and effectiveness of aluminum addition to reduce sediment phosphorus release and restore lake water quality |location=[[Fjärdingen]] |publisher=Uppsala universitet, Limnologi Uppsala universitet |oclc=1233676585 }}</ref> Alum treatment is less effective in deep lakes, as well as lakes with substantial external phosphorus loading.<ref name=":22">Cooke, G. D., Welch, E. B., Martin, A. B., Fulmer, D. G., Hyde, J. B., & Schrieve, G. D. (1993). Effectiveness of Al, Ca, and Fe salts for control of internal phosphorus loading in shallow and deep lakes. ''Hydrobiologia'', ''253''(1), 323-335.</ref>

Finnish phosphorus removal measures started in the mid-1970s and have targeted rivers and lakes polluted by industrial and municipal discharges. These efforts have had a 90% removal efficiency.<ref name="Raike 2003">{{Cite journal|last1=Räike|first1=A.|last2=Pietiläinen|first2=O. -P.|last3=Rekolainen|first3=S.|last4=Kauppila|first4=P.|last5=Pitkänen|first5=H.|last6=Niemi|first6=J.|last7=Raateland|first7=A.|last8=Vuorenmaa|first8=J.|year=2003|title=Trends of phosphorus, nitrogen and chlorophyll a concentrations in Finnish rivers and lakes in 1975–2000|journal=Science of the Total Environment|volume=310|issue=1–3|pages=47–59|bibcode=2003ScTEn.310...47R|doi=10.1016/S0048-9697(02)00622-8|pmid=12812730}}</ref> Still, some targeted point sources did not show a decrease in runoff despite reduction efforts.

==See also==
* {{annotated link|Biogeochemical cycle}}
* {{annotated link|Ecological Quality Ratio}}
* {{annotated link|Effluent}}
* {{annotated link|Nitrogen cycle}}
* {{annotated link|Trophic state index}}
* {{annotated link|Upland and lowland (freshwater ecology)}}
* {{annotated link|Water Framework Directive}}

==External links==
{{wiktionary}}
* [https://initrogen.org/ International Nitrogen Initiative]

==References==
{{reflist}}
{{aquatic ecosystem topics}}
{{marine pollution}}
{{pollution}}
{{Authority control}}
[[Category:Eutrophication| ]]
[[Category:Nutrient pollution]]
[[Category:Water pollution]]
[[Category:Environmental chemistry]]
[[Category:Environmental issues with water]]
[[Category:Aquatic ecology]]