Dictionary of Ichthyology  

eutrophication = the deterioration of the life-supporting features of a lake or estuary caused by excessive pollutants or fertilisation from effluents high in phosphorus, nitrogen and organic growth substances. More decomposing organic matter is produced by the excessive growth of algae and plants than self-purification processes can overcome. It is also the natural aging process of a lake, leading to its disappearance as plants and sediments fill it in.

Wikimedia Commons: http://commons.wikimedia.org/wiki/File:Scheme_eutrophication-en.svg

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