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Last edited 12 Feb 2019
Eutrophication (from Greek ‘eutrophos’ meaning ‘well nourished’ and pronounced ‘you-tro-fi-kay-shun’) is the process by which aquatic environments (rivers, lakes, coastal estuaries and shallow sea areas) become excessively enriched by minerals and nutrients which induce excessive growth of plants and animal life. This enrichment will be harmful if it leads to the death of plant and animal life and can occur in two ways:
- Cultural eutrophication
- Natural eutrophication
 Natural eutrophication
This is part of the ageing process of freshwater ecosystems and is a common phenomenon, particularly in lakes, although some may escape it if they receive sufficient warmth and light. Scientists recognise that a lake’s natural productivity may be regulated by factors such as climate change, geology and other external factors.
The fertility of such freshwater bodies changes over time if they are fed rich nutrients by a stream or river, leading to abundant plant or animal life; the lake or pond is then classed as ‘eutrophic’. However, if excessive it could adversely affect aquatic life. Although the process is extremely slow and occurs over millennia, it can be speeded up by cultural eutrophication.
 Cultural eutrophication
Cultural eutrophication occurs primarily as a result of human activity (industrial, domestic and agricultural) when nutrients, such as fertilizer chemicals (nitrates and phosphates), detergents and nutrients from municipal sewage, are conveyed into a water body by rainwater run-off. This causes higher rates of algal and bacterial growth. As the algal bloom decomposes and dies, it consumes oxygen which reduces the concentration in the water and may lead to the extinction of fish and other life. Once it sinks to the bottom, bacteria begin to decompose the algal remains, using oxygen in the process. Eventually, as levels of nutrients rise to an unacceptable level, the water body can sustain no life, fish suffocate and plants die.
Eutrophication is often characterised by the bright green algal bloom floating on the surface. If this is in sufficient quantities, it may block sunlight from penetrating to the bottom of the water body, thereby depriving plants of the ability to photosynthesise.
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