Difference between revisions of "Possible consequences of eutrophication"

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==Introduction==
 
==Introduction==
<P ALIGN="justify">
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Enhanced plant production and improved fish yields are sometimes described as positive impacts of [[Eutrophication|eutrophication]], especially in countries where fish and other aquatic organisms are a significant source of food<ref>Nixon, S.W. and Buckley, B. A. 2002. “A strikingly rich zone” – Nutrient enrichment and secondary production in coastal marine ecosystems. Estuaries 25: 782–796</ref>. However, detrimental ecological impacts can in turn have other negative consequences and impacts which are described below. The entire aquatic [[ecosystem]] may change with eutrophication. The diagram below gives an overview on the eutrophication process and its causes and consequences.  
Enhanced plant production and improved fish yields are sometimes described as positive impacts of [[Eutrophication|eutrophication]], especially in countries where fish and other aquatic organisms are a significant source of food. However detrimental ecological impacts can in turn have other negative consequences and impacts which are described below. Essentially the entire aquatic [[ecosystem]] changes with eutrophication. The diagram below gives an overview on the eutrophication process and its causes and consequences. </P>
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[[Image:eutrocon.png|705|center]]
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[[File:eutrocon.png|705|center]]
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==Ecological impacts==
 
==Ecological impacts==
==== Increased biomass of phytoplankton resulting in algal blooms ====
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===Increased biomass of phytoplankton resulting in [[algal bloom]]s===
[[Image:Plentiful_plankton.jpg|175px|thumb|right|<small>Envisat satellite image of an algal bloom captured with MERIS (Photo Credit: ESA, 2009)</small>]]
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[[File:Plentiful_plankton.jpg|250px|thumb|right|<small>Fig. 1. Envisat satellite image of an algal bloom captured with MERIS (Photo Credit: ESA, 2009)</small>]]
<P ALIGN="justify">[[Phytoplankton]] or microalgae are [[photosynthesizing]] microscopic organisms. They contain chlorophyll and require sunlight in order to live and grow. Most phytoplankton are buoyant and float in the upper part of the ocean where sunlight penetrates the water. In a balanced ecosystem they provide food for a wide range of organisms such as whales, shrimp, snails and jellyfish.
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[[Phytoplankton]] or microalgae are [[photosynthesizing]] microscopic organisms. They contain chlorophyll and require sunlight in order to live and grow. Most phytoplankton are buoyant and float in the upper part of the ocean where sunlight penetrates the water. In a balanced ecosystem they provide food for a wide range of organisms such as whales, shrimp, snails and jellyfish.
Among the more important groups are the diatoms, cyanobacteria, dinoflagellates and coccolithophores.  
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Among the more important groups are the diatoms, cyanobacteria, dinoflagellates and coccolithophores (see: [[Marine Plankton]]).  
Phytoplankton species require inorganic [[Nutrient|nutrients]] such as nitrates, phosphates, and sulfur which they convert into proteins, fats and carbohydrates. When '''too many''' of these '''nutrients''' (by natural or [[Anthropogenic|anthropogenic]] cause) are available in the water phytoplankton may grow and multiply very fast forming [[Algal_bloom | algal blooms]]. Algal blooms may occur in freshwater as well as marine environments. Only one or a small number of phytoplankton species are involved and some blooms discolor (green, yellow-brown or red) the water due to their high density of pigmented cells. Blooms in the ocean may cover a large area and are easily visible in '''satellite images'''.</P>
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Phytoplankton species require inorganic [[Nutrient|nutrients]] such as nitrates, phosphates, and sulfur which they convert into proteins, fats and carbohydrates. When '''too many''' of these '''nutrients''' (by natural or [[Anthropogenic|anthropogenic]] cause) are available in the water, phytoplankton may grow and multiply very fast forming algal blooms (see [[Algal bloom dynamics]]). Algal blooms may occur in freshwater as well as marine environments. Only one or a small number of phytoplankton species are involved and some blooms discolor (green, yellow-brown or red) the water due to their high density of pigmented cells. Blooms in the ocean may cover a large area and are easily visible in satellite images (Fig. 1) – see the article [[Determining coastal water constituents from space]].
==== Toxic or inedible phytoplankton species (harmful algal blooms)====
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===Toxic or inedible phytoplankton species ([[harmful algal blooms]])===
<P ALIGN="justify">'''[[Harmful_algal_bloom | Harmful algal blooms (HAB)]]''' are bloom events involving '''toxic or harmful phytoplankton'''. These cause harm through the production of toxins or by their accumulated biomass, which can effect co-occurring organisms and alter food web dynamics. Impacts include:</P>
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'''Harmful algal blooms (HAB)''' are bloom events involving '''toxic or harmful phytoplankton'''. These cause harm through the production of toxins or by their accumulated biomass, which can effect co-occurring organisms and alter food web dynamics. Impacts include:
 
*Human illness,
 
*Human illness,
 
*Mortality of fish, birds and mammals following consumption or indirect exposure to HAB toxins,
 
*Mortality of fish, birds and mammals following consumption or indirect exposure to HAB toxins,
 
*Substantially economic losses to coastal communities and commercial fisheries.
 
*Substantially economic losses to coastal communities and commercial fisheries.
 +
For more details on toxic algal blooms, see the article [[Harmful algal bloom]].
 +
===Increased blooms of gelatinous zooplankton===
 +
[[Phytoplankton]] are the food source for numerous other organisms, especially the zooplankton. [[Zooplankton]] are heterotrophic plankton. They are primarily transported by ambient water currents but many can swim. Through their consumption and processing of phytoplankton and other food sources they play a role in aquatic food webs as a resource for higher trophic levels including fish. Zooplankton can be divided in two important groups: crustacean (copepods and krill) and '''gelatinous zooplankton'''. Gelatinous zooplankton have relatively fragile, plastic gelatinous bodies that contain at least 95% water and which lack rigid skeletal parts. The most well-known are the jellyfish. There is some evidence that gelatinous zooplankton organisms increase in relative importance versus crustacean zooplankton in areas where the natural species diversity has been affected by pollution, over-fishing and climate change<ref> Condon, R.H., Duarte, C.M., Pitt, K.A., Robinson, K.L., Lucas, C.H., Sutherland, K.R., Mianzan, H.W., Bogeberg, M., Purcell, J.E., Decker, M.B., Uye, S.-I., Madin, L.P., Brodeur, R.D., Haddock, S.H.D., Malej, A., Parry, G.D., Eriksen, E., Quiñones, J., Acha, M., Harvey, M., Arthur, J.M. and Graham W.M. 2013. Recurrent jellyfish blooms are a consequence of global oscillations. PNAS 110: 1000-1005</ref>.
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===Decreases in water transparency (increased turbidity)===
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The growth of phytoplankton causes increased [[Turbidity|turbidity]] or decreased penetration of light into the lower depths of the water column. In lakes and rivers this can inhibit growth of submerged aquatic plants and affect species which are dependent on them (fish, shellfish).
 +
===Dissolved oxygen depletion or hypoxia===
 +
[[File:FishKill.jpg|250px|thumb|right|<small>Fig. 2. Fish kill (menhaden) due to severe hypoxia (Photo credit: Chris Deacutis, IAN Image library )</small>]]
 +
Oxygen is required for all life forms on the planet. Oxygen is produced by plants during [[photosynthesis]]. At night animals and plants, as well as aerobic micro-organisms respire and so consume oxygen which results in a decrease in dissolved oxygen levels. [[Algal bloom]]s may cause strong fluctuations in dissolved oxygen levels. When the algae population is growing at a fast rate, it may block sunlight from reaching other organisms and cause a decrease of dissolved oxygen levels. When algae die, they are decomposed by bacteria which in this process consume oxygen so that the water can become temporarily hypoxic. Oxygen depletion, or [[Hypoxia|hypoxia]], is a common consequence of eutrophication, both in fresh water and seawater. The '''direct effects''' of hypoxia include '''fish kills''', especially the death of fish that need high levels of dissolved oxygen (Fig. 2).
 +
There is some evidence that hypoxic conditions promote the growth of cyanobacteria as a consequence of enhanced phosphorus release<ref>Funkey, C.P., Conley, D.J., Reuss, N.S., Humborg, C, Jilbert, T. and Slomp, C.P. 2014. Hypoxia Sustains Cyanobacteria Blooms in the Baltic Sea. Environ. Sci. Technol. 48: 2598−2602</ref>. Many cyanobacteria species produce toxins that are lethal to birds and animals.
 +
===Dead zones===
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Zones with extreme hypoxia are called [https://en.wikipedia.org/wiki/Dead_zone_(ecology) dead zones]. Upwelling of nutrient-rich waters may produce 'natural' dead zones in some ocean regions where water mass circulation is minimal. However, most severe dead zones occur as a result of anthropogenic eutrophication of stratified water masses. Stratification is mainly caused by temperature-induced density differences between surface waters and deeper waters in the ocean; stratification in estuaries and coastal waters is mainly due to the salinity-induced density difference between seawater and river water. When an algal bloom in the surface layer decays, the deeper water layers become laden with sinking dead algae that consume oxygen while decomposing. Stratification inhibits turbulent mixing of the oxygen-rich surface layer into the underlying water layer that therefore becomes depleted from oxygen.
  
==== Increased blooms of gelatinous zooplankton====
 
<P ALIGN="justify">[[Phytoplankton]] are the food source for numerous other organisms, especially the zooplankton. [[Zooplankton]] are heterotrophic plankton. They are primarily transported by ambient water currents but many have locomotion. Through their consumption and processing of phytoplankton and other food sources they play a role in aquatic food webs as a resource for higher trophic levels including fish. Zooplankton can be divided in two important groups: crustacean (copepods and krill) and '''gelatinous zooplankton'''. Gelatinous zooplankton have relatively fragile, plastic gelatinous bodies that contain at least 95% water and which lack rigid skeletal parts. The most well-known are the jellyfish. Eutrophication is believed to cause an '''increase''' in the relative importance of '''gelatinous''' versus crustacean '''zooplankton'''. On many areas of the world where the natural species diversity has been affected by pollution, over-fishing and climate change gelatinous zooplankton organisms may be becoming the dominant species.</P>
 
  
==== Decreases in water transparency (increased turbidity)====
+
[[File:OceanHypoxia.jpg|thumb|500px|left|<small>Fig. 3. Areas with hypoxia in coastal seas (red) and in the oceans (blue). From IOC-UNESCO 2018<ref name=BG>Breitburg, D., Gregoire, M. Isensee, K. (eds.) 2018. The ocean is losing its breath: Declining oxygen in the world’s ocean and coastal waters. IOC-UNESCO, IOC Technical Series, No. 137 40pp. (IOC/2018/TS/137)</ref> Creative Commons Licence.</small>]]
<P ALIGN="justify">The growth of phytoplankton can cause increased [[Turbidity|turbidity]] or decreased penetration of light into the lower depths of the water column. In lakes and rivers this can inhibit growth of submerged aquatic plants and affect species which are dependent on them (fish, shellfish).</P>
 
  
==== Dissolved oxygen depletion or hypoxia resulting in increased incidences of fish kills and / or dead benthic animals====
 
[[Image:FishKill.jpg|160px|thumb|right|<small>A menhaden fish kill due severe hypoxia (Photo credit: Chris Deacutis, IAN Image library )</small>]]
 
<P ALIGN="justify">Oxygen is required for all life forms on the planet. Oxygen is produced by plants during ([[photosynthesis]]). At night animals and plants, as well as aerobic micro-organisms and decomposing dead organisms respire and so consume oxygen which results in a decrease in dissolved oxygen levels. Large fluctuations in dissolved oxygen levels may be the result of an algal blooms. While the algae population is growing at a fast rate, dissolved oxygen levels decrease. When these algae die, they are decomposed by bacteria which consume oxygen in this process so that the water can become temporarily hypoxic. Oxygen depletion, or [[Hypoxia|hypoxia]], is a common effect of eutrophication in water. The '''direct effects''' of hypoxia include '''fish kills''', especially the death of fish that need high levels of dissolved oxygen. Changes in fish communities may have an impact on the whole aquatic ecosystem and may deplete fish stocks. In extreme cases hypoxic conditions promote the growth of bacteria that produce toxins deadly to birds and animals. Zones where this occurs are called [[Case_studies_eutrophication#Ecological_impacts_of_eutrophication_.28Case_study:_Eutrophication_and_dead_zones.29|dead zones]].</P>
 
  
==== Species biodiversity decreases and the dominant biota changes====
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Fig. 3 shows a map of coastal and ocean regions with frequently occurring hypoxia. The largest and most persistent dead zones are situated in the Baltic Sea, the Black Sea, the Arabian Sea and the Gulf of Mexico. The size of dead zones decreases during severe storms when strong wind-driven turbulent water motion partially neutralizes stratification.
<P ALIGN="justify">Eutrophication leads to changes in the availability of light and certain nutrients to an ecosystem. This causes shifts in the species composition so that only the more tolerant species survive and new competitive species invade and out-compete original inhabitants. Examples are macroalgae and their massive biomass which inhibits the growth of other aquatic plants and algal blooms that consists of one type of phytoplankton species because other species are expelled.</P>
 
  
==== Increased biomass of macroalgae====
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Climate change is expected to worsen oxygen depletion in the ocean and coastal waters by increasing surface temperatures and stratification<ref> Li, G., Cheng, L., Zhu, J., Trenberth, K.E., Mann, M.E. and Abraham, J.P. 2020. Increasing ocean stratification over the past half-century. Nature Climate Change https://doi.org/10.1038/s41558-020-00918-2</ref>; in turn, ocean oxygen depletion can contribute to climate change by increased microbial denitrification with production of the greenhouse gas NO<sub>2</sub><ref name=BG/>.
<P ALIGN="justify">Algal blooms may also consist of '''marine seaweeds''' or '''macroalgae'''. These blooms are recognizable by large blades of algae that may wash up into the shoreline. The seaweed is harmless when it is alive, but when decomposed by anaerobic bacteria toxic gases (such as the colorless hydrogen sulfide (H<sub>2</sub>S)) can be released.</P>
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===Species biodiversity decreases and the dominant biota changes===
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[[File: GreenAlgae_D_Ramirez.jpg|thumb|250px|right|<small>Fig. 4. Macroalgae washed ashore. Photo credit D. Ramirez</small>.]]
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Eutrophication leads to changes in the availability of light and certain nutrients to an ecosystem. This causes shifts in the species composition so that only the more tolerant species survive and new competitive species invade and out-compete original inhabitants. Examples are macroalgae and their massive biomass which inhibits the growth of other aquatic plants and algal blooms that consists of one type of phytoplankton species because other species are outcompeted.
 +
===Green tides===
 +
Many observations and experiments provide evidence that the occurrence of so-called 'green tides', consisting of '''marine seaweeds''' or '''macroalgae''', is increased in eutrophicated coastal waters<ref name=SZ>Smetacek, V. and Zingone, A. 2013. Green and golden seaweed tides on the rise. Nature 504, doi:10.1038/nature12860</ref>. These blooms are recognizable by large blades of algae that may wash up onto the shoreline (Fig. 4). Green tides can asphyxiate aquacultures or disrupt traditional artisanal fisheries. Macroalgae can also be valorized as a raw material; harvesting could therefore be an effective solution<ref name=SZ/>.
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<br clear=all>
  
 
==Human health impacts==
 
==Human health impacts==
<P ALIGN="justify">Harmful algal bloom species have the capacity to produce '''toxins''' dangerous to humans. Algal [[Toxic|toxins]] are observed in marine ecosystems where they can accumulate in shellfish and more generally in seafood reaching dangerous levels for human as well as animal health. Examples include paralytic, neurotoxic and diarrhoeic shellfish poisoning. Several algal species able of producing toxins harmful to human or marine life have been identified in European coastal waters. The table gives an overview of some species that are regularly observed and represent a risk for seafood consumers.</P>
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Harmful algal bloom species have the capacity to produce '''toxins''' dangerous to humans. Algal [[Toxic|toxins]] are observed in marine ecosystems where they can accumulate in shellfish and more generally in seafood reaching dangerous levels for human as well as animal health. Examples include paralytic, neurotoxic and diarrhoeic shellfish poisoning. Several algal species able of producing toxins harmful to human or marine life have been identified in European coastal waters<ref>Eutrophication and health. European Commission 2002. Office for Official Publications of the European Communities: Luxembourg. ISBN 92-894-4413-4.28 pp.</ref>. The table gives an overview of some species that are regularly observed and represent a risk for seafood consumers<ref name=B16>Berdalet, E., Fleming, L.E., Gowen, R., Davidson, K., Hess, P., Backer, L.C., Moore, S.K., Hoagland, P. and Enevoldsen, H. 2016. Marine harmful algal blooms, human health and wellbeing: challenges and opportunities in the 21st century. J. Mar. Biol. Assoc. U.K. 96: 61–91</ref>.
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<P ALIGN="justify">Other marine mammals can be vectors for toxins, as in the case of ciguatera, where it is typically predator fish whose flesh is contaminated with the toxins originally produced by dinoflagellates and then poison humans. Symptoms include gastrointestinal and neurological effects.</P>
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 +
 
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Finfish can also be a vector for toxins, as in the case of ciguatera, where it is typically predator fish whose flesh is contaminated with the toxins originally produced by dinoflagellates<ref name=B16/>. People are poisoned when consuming contaminated fish. Symptoms include gastrointestinal and neurological effects.
  
 
==Socio-economic impacts==
 
==Socio-economic impacts==
 
Nearly all of the above described impacts have a direct or indirect socio-economic impact.
 
Nearly all of the above described impacts have a direct or indirect socio-economic impact.
==== Impact on recreation and tourism ====
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===Impact on recreation and tourism===
<P ALIGN="justify">The enrichment of nutrients to an ecosystem can result in a massive growth of macroalgae. The existence of such dense algal growth areas can inhibit or prevent access to waterways. This decreases the fitness for '''use of the water for water sports''' (swimming, boating and fishing).</P>
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The enrichment of nutrients to an ecosystem can result in a massive growth of macroalgae. The existence of such dense algal growth areas can inhibit or prevent access to waterways. The seaweed is harmless to humans but decreases the fitness for use of the water for water sports (swimming, boating and fishing).  
==== Aesthetic impacts====
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===Aesthetic impacts===
[[Image:Beach_closed.jpg|200px|thumb|right|<small>As a result of toxic algal blooms beaches can be closed (Photo credit: Elizabeth Halliday, Woods Hole Oceanographic Institution)</small>]]
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[[Image:Beach_closed.jpg|200px|thumb|right|<small>Fig. 5. Beaches can be closed as a result of toxic algal blooms (Photo credit: Elizabeth Halliday, Woods Hole Oceanographic Institution).</small>]]
 
Algal blooms are unsightly and can have unpleasant smells for example:
 
Algal blooms are unsightly and can have unpleasant smells for example:
* The appearance of a '''white yellowish foam''' on the beach in spring on the shores along the North Sea. The foam is formed by the wind that sweeps up the decaying remains of ''Phaeocystis'' algal colonies.
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* The appearance of a white yellowish foam on the beach in spring, for example on the shores along the North Sea. The foam is formed by the wind that sweeps up the decaying remains of ''Phaeocystis'' algal colonies. An extreme case is shown in [[Foam beach, Sydney]].
* When macroalgae or seaweed are decomposed by anaerobic bacteria hydrogen sulfide is (H<sub>2</sub>S) released. This gas is characterized by a very unpleasant characteristic foul odor of rotten eggs.
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* When macroalgae or seaweed are decomposed by anaerobic bacteria hydrogen sulfide (H<sub>2</sub>S) is released. This gas is characterized by a very unpleasant characteristic foul odor of rotten eggs.
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===Economic impacts===
 +
Due to the toxins produced by harmful algal blooms commercial fish and shellfish may become '''unsuitable for consumption''' resulting in potential economic and financial problems for the fishing industries. In extreme cases beaches are closed due to the presence of toxic algal blooms (Fig. 5). The annual economic costs related to illness and death caused by consumption of poisoned fish products is estimated at around 30 million US$ worldwide<ref>Kouakou, C.R.C. and Poder, T.G. 2019. Economic impact of harmful algal blooms on human health: a systematic review. Journal of Water and Health  17.4: 499-516</ref>. The fishing industry is suffering economic losses because consumers refrain from consuming fish and shellfish products due to the perceived danger of poisoning. For the USA, the loss related to this avoidance behavior is estimated at around 1 billion US$ per year<ref>Parsons, G.R., Morgan, A.O., Whitehead, J.C. and Haab, T.C. 2006. The Welfare Effects of Pfiesteria-Related Fish Kills: A Contingent Behavior Analysis of Seafood Consumers. Agricultural and Resource Economics Review 35(1)</ref>.
 +
 
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The Baltic Sea harbours one of the the largest dead zones in the world. Lost revenues from ecosystem services (fishing, recreation, but also non-use values) as a result of eutrophication amount to 3-4 billion euros, according to an estimate by HELCOM<ref>Hyytiainen, K., Hasler, B., Ericsdotter, S., Nekoro, M., Blyh, K., Artell, J., Ahlvik, L. and Ahtiainen, J. 2013. Worth it: Benefits outweigh costs in reducing eutrophication in the Baltic. BalticSTERN Summary Report for HELCOM 2013 Ministerial Meeting</ref>.
  
==== Economical impacts====
 
<P ALIGN="justify">In some specific cases local authorities must rely on eutrophic waters for production of drinking water. Infected waters increases the '''costs of water treatment''' in order to avoid taste, odor and toxin problems in the water. Due to the toxins produced by harmful algal blooms commercial fish and shellfish may become '''unsuitable for consumption''' resulting in potential economical and financial problems for the fishing industries. In extreme cases beaches are closed due to the presence of toxic algal blooms.</P>
 
  
  
 
==Related articles==
 
==Related articles==
* [[Eutrophication in coastal environments]]
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:[[Harmful algal bloom]]
* [[Threats to the coastal zone]]
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:[[Algal bloom dynamics]]
* [[What causes eutrophication?]]
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:[[ What causes eutrophication?]]
* [[OSPAR eutrophication assessment]]
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:[[Threats to the coastal zone]]
* [[European policy on eutrophication: introduction]]
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:[[Estuarine turbidity maximum]]
* [[Case studies eutrophication]]
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:[[Case studies eutrophication]]
* [[Coupled hydrodynamic - water quality - ecological modelling]]
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:[[Coupled hydrodynamic - water quality - ecological modelling]]
* [[Modelling marine and coastal eutrophication]]
 
* [[Continental Nutrient Sources and Nutrient Transformation]]
 
* [[In situ monitoring of eutrophication]]
 
  
 +
See also other articles in the [[:Category:Eutrophication]]
  
== References==
+
 
#Eutrophication and health. European Commission (2002). Office for Official Publications of the European Communities: Luxembourg. ISBN 92-894-4413-4.28 pp.
+
==References==
#The National Eutrophication monitoring Programme Implementation Manual (Murray et al., 2002).
+
<references/>
#Guiry, Michael D. (2013). Nitzschia Hassall, 1845. In: Guiry, M.D. & Guiry, G.M. (2013). AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=149045 on 2013-04-22.
 
#Guiry, Michael D. (2013). Gymnodinium Stein, 1878. In: Guiry, M.D. & Guiry, G.M. (2013). AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=109475 on 2013-04-22.
 
#Guiry, Michael D. (2013). Prorocentrum Ehrenberg, 1834. In: Guiry, M.D. & Guiry, G.M. (2013). AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=109566 on 2013-04-22.
 
#WoRMS (2013). Dinophysis Ehrenberg, 1839. In: Guiry, M.D. & Guiry, G.M. (2013). AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=109462 on 2013-04-22.
 
#Guiry, Michael D.; Moestrup, Ø. (2013). Alexandrium Halim, 1960. In: Guiry, M.D. & Guiry, G.M. (2013). AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=109470 on 2013-04-22.
 
  
  
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[[Category:Eutrophication]]
 
[[Category:Eutrophication]]

Latest revision as of 16:28, 15 November 2020

Introduction

Enhanced plant production and improved fish yields are sometimes described as positive impacts of eutrophication, especially in countries where fish and other aquatic organisms are a significant source of food[1]. However, detrimental ecological impacts can in turn have other negative consequences and impacts which are described below. The entire aquatic ecosystem may change with eutrophication. The diagram below gives an overview on the eutrophication process and its causes and consequences.

705


Ecological impacts

Increased biomass of phytoplankton resulting in algal blooms

Fig. 1. Envisat satellite image of an algal bloom captured with MERIS (Photo Credit: ESA, 2009)

Phytoplankton or microalgae are photosynthesizing microscopic organisms. They contain chlorophyll and require sunlight in order to live and grow. Most phytoplankton are buoyant and float in the upper part of the ocean where sunlight penetrates the water. In a balanced ecosystem they provide food for a wide range of organisms such as whales, shrimp, snails and jellyfish. Among the more important groups are the diatoms, cyanobacteria, dinoflagellates and coccolithophores (see: Marine Plankton). Phytoplankton species require inorganic nutrients such as nitrates, phosphates, and sulfur which they convert into proteins, fats and carbohydrates. When too many of these nutrients (by natural or anthropogenic cause) are available in the water, phytoplankton may grow and multiply very fast forming algal blooms (see Algal bloom dynamics). Algal blooms may occur in freshwater as well as marine environments. Only one or a small number of phytoplankton species are involved and some blooms discolor (green, yellow-brown or red) the water due to their high density of pigmented cells. Blooms in the ocean may cover a large area and are easily visible in satellite images (Fig. 1) – see the article Determining coastal water constituents from space.

Toxic or inedible phytoplankton species (harmful algal blooms)

Harmful algal blooms (HAB) are bloom events involving toxic or harmful phytoplankton. These cause harm through the production of toxins or by their accumulated biomass, which can effect co-occurring organisms and alter food web dynamics. Impacts include:

  • Human illness,
  • Mortality of fish, birds and mammals following consumption or indirect exposure to HAB toxins,
  • Substantially economic losses to coastal communities and commercial fisheries.

For more details on toxic algal blooms, see the article Harmful algal bloom.

Increased blooms of gelatinous zooplankton

Phytoplankton are the food source for numerous other organisms, especially the zooplankton. Zooplankton are heterotrophic plankton. They are primarily transported by ambient water currents but many can swim. Through their consumption and processing of phytoplankton and other food sources they play a role in aquatic food webs as a resource for higher trophic levels including fish. Zooplankton can be divided in two important groups: crustacean (copepods and krill) and gelatinous zooplankton. Gelatinous zooplankton have relatively fragile, plastic gelatinous bodies that contain at least 95% water and which lack rigid skeletal parts. The most well-known are the jellyfish. There is some evidence that gelatinous zooplankton organisms increase in relative importance versus crustacean zooplankton in areas where the natural species diversity has been affected by pollution, over-fishing and climate change[2].

Decreases in water transparency (increased turbidity)

The growth of phytoplankton causes increased turbidity or decreased penetration of light into the lower depths of the water column. In lakes and rivers this can inhibit growth of submerged aquatic plants and affect species which are dependent on them (fish, shellfish).

Dissolved oxygen depletion or hypoxia

Fig. 2. Fish kill (menhaden) due to severe hypoxia (Photo credit: Chris Deacutis, IAN Image library )

Oxygen is required for all life forms on the planet. Oxygen is produced by plants during photosynthesis. At night animals and plants, as well as aerobic micro-organisms respire and so consume oxygen which results in a decrease in dissolved oxygen levels. Algal blooms may cause strong fluctuations in dissolved oxygen levels. When the algae population is growing at a fast rate, it may block sunlight from reaching other organisms and cause a decrease of dissolved oxygen levels. When algae die, they are decomposed by bacteria which in this process consume oxygen so that the water can become temporarily hypoxic. Oxygen depletion, or hypoxia, is a common consequence of eutrophication, both in fresh water and seawater. The direct effects of hypoxia include fish kills, especially the death of fish that need high levels of dissolved oxygen (Fig. 2). There is some evidence that hypoxic conditions promote the growth of cyanobacteria as a consequence of enhanced phosphorus release[3]. Many cyanobacteria species produce toxins that are lethal to birds and animals.

Dead zones

Zones with extreme hypoxia are called dead zones. Upwelling of nutrient-rich waters may produce 'natural' dead zones in some ocean regions where water mass circulation is minimal. However, most severe dead zones occur as a result of anthropogenic eutrophication of stratified water masses. Stratification is mainly caused by temperature-induced density differences between surface waters and deeper waters in the ocean; stratification in estuaries and coastal waters is mainly due to the salinity-induced density difference between seawater and river water. When an algal bloom in the surface layer decays, the deeper water layers become laden with sinking dead algae that consume oxygen while decomposing. Stratification inhibits turbulent mixing of the oxygen-rich surface layer into the underlying water layer that therefore becomes depleted from oxygen.


Fig. 3. Areas with hypoxia in coastal seas (red) and in the oceans (blue). From IOC-UNESCO 2018[4] Creative Commons Licence.


Fig. 3 shows a map of coastal and ocean regions with frequently occurring hypoxia. The largest and most persistent dead zones are situated in the Baltic Sea, the Black Sea, the Arabian Sea and the Gulf of Mexico. The size of dead zones decreases during severe storms when strong wind-driven turbulent water motion partially neutralizes stratification.

Climate change is expected to worsen oxygen depletion in the ocean and coastal waters by increasing surface temperatures and stratification[5]; in turn, ocean oxygen depletion can contribute to climate change by increased microbial denitrification with production of the greenhouse gas NO2[4].

Species biodiversity decreases and the dominant biota changes

Fig. 4. Macroalgae washed ashore. Photo credit D. Ramirez.

Eutrophication leads to changes in the availability of light and certain nutrients to an ecosystem. This causes shifts in the species composition so that only the more tolerant species survive and new competitive species invade and out-compete original inhabitants. Examples are macroalgae and their massive biomass which inhibits the growth of other aquatic plants and algal blooms that consists of one type of phytoplankton species because other species are outcompeted.

Green tides

Many observations and experiments provide evidence that the occurrence of so-called 'green tides', consisting of marine seaweeds or macroalgae, is increased in eutrophicated coastal waters[6]. These blooms are recognizable by large blades of algae that may wash up onto the shoreline (Fig. 4). Green tides can asphyxiate aquacultures or disrupt traditional artisanal fisheries. Macroalgae can also be valorized as a raw material; harvesting could therefore be an effective solution[6].

Human health impacts

Harmful algal bloom species have the capacity to produce toxins dangerous to humans. Algal toxins are observed in marine ecosystems where they can accumulate in shellfish and more generally in seafood reaching dangerous levels for human as well as animal health. Examples include paralytic, neurotoxic and diarrhoeic shellfish poisoning. Several algal species able of producing toxins harmful to human or marine life have been identified in European coastal waters[7]. The table gives an overview of some species that are regularly observed and represent a risk for seafood consumers[8].


Disease Symptoms Species Carriers
Amnesic shellfish poisoning (ASP) Mental confusion and memory loss, disorientation and sometimes coma Diatoms of the genus Nitzschia Shellfish (mussels)
Neurotoxic shellfish poisoning (NSP) Muscular paralysis, state of shock and sometimes death Genus Gymnodinium Oysters, clams and crustaceans
Venerupin shellfish poisoning (VSP) Gastrointestinal, nervous and hemorrhagic, hepatic symptoms and in extreme causes delirium and hepatic coma Genus Prorocentrum Oysters and clams
Diarrhoeic shellfish poisoning (DSP) Gastrointestinal symptoms (diarrhoea, vomiting and abdominal pain) Genus Dinophysis and Prorocentrum Filtering shellfish (oysters, mussels and clams)
Paralytic shellfish poisoning (PSP) Muscular paralysis, difficulty in breathing, shock and in extreme causes death by respiratory arrest Genus Alexandrium and Gymnodinium Oysters, mussels, crustacean and fish


Finfish can also be a vector for toxins, as in the case of ciguatera, where it is typically predator fish whose flesh is contaminated with the toxins originally produced by dinoflagellates[8]. People are poisoned when consuming contaminated fish. Symptoms include gastrointestinal and neurological effects.

Socio-economic impacts

Nearly all of the above described impacts have a direct or indirect socio-economic impact.

Impact on recreation and tourism

The enrichment of nutrients to an ecosystem can result in a massive growth of macroalgae. The existence of such dense algal growth areas can inhibit or prevent access to waterways. The seaweed is harmless to humans but decreases the fitness for use of the water for water sports (swimming, boating and fishing).

Aesthetic impacts

Fig. 5. Beaches can be closed as a result of toxic algal blooms (Photo credit: Elizabeth Halliday, Woods Hole Oceanographic Institution).

Algal blooms are unsightly and can have unpleasant smells for example:

  • The appearance of a white yellowish foam on the beach in spring, for example on the shores along the North Sea. The foam is formed by the wind that sweeps up the decaying remains of Phaeocystis algal colonies. An extreme case is shown in Foam beach, Sydney.
  • When macroalgae or seaweed are decomposed by anaerobic bacteria hydrogen sulfide (H2S) is released. This gas is characterized by a very unpleasant characteristic foul odor of rotten eggs.

Economic impacts

Due to the toxins produced by harmful algal blooms commercial fish and shellfish may become unsuitable for consumption resulting in potential economic and financial problems for the fishing industries. In extreme cases beaches are closed due to the presence of toxic algal blooms (Fig. 5). The annual economic costs related to illness and death caused by consumption of poisoned fish products is estimated at around 30 million US$ worldwide[9]. The fishing industry is suffering economic losses because consumers refrain from consuming fish and shellfish products due to the perceived danger of poisoning. For the USA, the loss related to this avoidance behavior is estimated at around 1 billion US$ per year[10].

The Baltic Sea harbours one of the the largest dead zones in the world. Lost revenues from ecosystem services (fishing, recreation, but also non-use values) as a result of eutrophication amount to 3-4 billion euros, according to an estimate by HELCOM[11].


Related articles

Harmful algal bloom
Algal bloom dynamics
What causes eutrophication?
Threats to the coastal zone
Estuarine turbidity maximum
Case studies eutrophication
Coupled hydrodynamic - water quality - ecological modelling

See also other articles in the Category:Eutrophication


References

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  3. Funkey, C.P., Conley, D.J., Reuss, N.S., Humborg, C, Jilbert, T. and Slomp, C.P. 2014. Hypoxia Sustains Cyanobacteria Blooms in the Baltic Sea. Environ. Sci. Technol. 48: 2598−2602
  4. 4.0 4.1 Breitburg, D., Gregoire, M. Isensee, K. (eds.) 2018. The ocean is losing its breath: Declining oxygen in the world’s ocean and coastal waters. IOC-UNESCO, IOC Technical Series, No. 137 40pp. (IOC/2018/TS/137)
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  6. 6.0 6.1 Smetacek, V. and Zingone, A. 2013. Green and golden seaweed tides on the rise. Nature 504, doi:10.1038/nature12860
  7. Eutrophication and health. European Commission 2002. Office for Official Publications of the European Communities: Luxembourg. ISBN 92-894-4413-4.28 pp.
  8. 8.0 8.1 Berdalet, E., Fleming, L.E., Gowen, R., Davidson, K., Hess, P., Backer, L.C., Moore, S.K., Hoagland, P. and Enevoldsen, H. 2016. Marine harmful algal blooms, human health and wellbeing: challenges and opportunities in the 21st century. J. Mar. Biol. Assoc. U.K. 96: 61–91
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The main author of this article is Knockaert, Carolien
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Citation: Knockaert, Carolien (2020): Possible consequences of eutrophication. Available from http://www.coastalwiki.org/wiki/Possible_consequences_of_eutrophication [accessed on 2-12-2020]