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Eutrophication describes the biological effects of an increase in the concentration of nutrients. The collective term ‘nutrients’ refers to those elements that are essential for primary production by plants or other photosynthetic organisms.
Eutrophication is most often caused by increases in the availability of nitrogen and phosphorus, commonly present in soil and water in the form of nitrate and phosphate, respectively. However, altered concentrations of any plant nutrient may have a recognizable biological effect. Eutrophication can occur in any aquatic system (freshwater or marine), and the term is also used to describe the process whereby terrestrial vegetation is affected by nutrient-enriched soil water.
The term ‘eutrophication’ came into common usage from the 1940s onwards, when it was realized that, over a period of years, plant nutrients derived from industrial activity and agriculture had caused changes in water quality and the biological character of water bodies.
In England and Wales, eutrophication has been a particular concern since the late 1980s, when public awareness of the problem was heightened by widespread toxic blue-green bacterial (cyanobacteria) blooms (commonly, but incorrectly, referred to as algal blooms) in standing and slow-flowing freshwaters.
Cyanobacteria are not typical bacteria, not only because some of them are photosynthetic, but also because some of them can be multicellular, forming long chains of cells. Nonetheless, cyanobacteria clearly belong to the kingdom Bacteria because of their internal cellular structure.
The levels of nutrients present determine the trophic state of a water body, where trophic means ‘feeding’. The adjective eutrophe (literally ‘well fed’) was first used by the German botanist Weber in 1907, to describe the initially high nutrient conditions that occur in some types of ecosystem at the start of secondary succession.
Scientists studying lakes at the beginning of the 20th century identified stages in plant community succession that appeared to be directly related to trophic state or nutrient status. They described a series of stages:
‘oligotrophic - mesotrophic - eutrophic - hypertrophic’
where oligotrophic meant ‘low in nutrients’, mesotrophic ‘with intermediate nutrient concentration’, eutrophic ‘high in nutrients’ and hypertrophic ‘very high in nutrients’.
At the time, the definitions of 'oligotrophic', 'eutrophic' etc. were derived from comparative estimates between water bodies with different nutrient status, judged according to their phytoplankton communities.
Phytoplankton is a collective term for the free-floating photosynthetic organisms within the water column. It encompasses both algae (from the kingdom Protoctista) and photosynthetic members of the kingdom Bacteria.
Thus an oligotrophic lake would have clear water with little phytoplankton, whereas a eutrophic lake would be more turbid and green from dense phytoplankton growth, and a mesotrophic lake would be intermediate between the two.
The trophic state of water bodies and rivers varies depending on a number of factors, including position in the landscape and management of surrounding land. In general, upland areas are more likely to have nutrient-poor (oligotrophic) water, characterized by relatively fast-flowing rivers and lakes that have clear water with limited higher plant communities.
By contrast, lowland waters in more fertile river catchments tend to be nutrient-rich(eutrophic), and lakes in lowland areas are more likely to be turbid with lush fringing vegetation. Lowland rivers have slower flow and are likely to be more nutrient rich as a result of soluble compounds having been washed into them. They are likely to have fringing vegetation and some floating and submerged aquatic plants.
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