Eutrophication (from Greek eutrophos, "well-nourished") is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients. It has also been defined as "nutrient-induced increase in phytoplankton productivity".: 459 Water bodies with very low nutrient levels are termed oligotrophic and those with moderate nutrient levels are termed mesotrophic. Advanced eutrophication may also be referred to as dystrophic and hypertrophic conditions. Eutrophication in freshwater ecosystems is almost always caused by excess phosphorus.
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Prior to human interference, this was, and continues to be, a very slow natural process in which nutrients, especially phosphorus compounds and organic matter, accumulate in water bodies. These nutrients derive from degradation and solution of minerals in rocks and by the effect of lichens, mosses and fungi actively scavenging nutrients from rocks. Anthropogenic or cultural eutrophication is often a much more rapid process in which nutrients are added to a water body from any of a wide variety of polluting inputs including untreated or partially treated sewage, industrial wastewater and fertilizer from farming practices. Nutrient pollution, a form of water pollution, is a primary cause of eutrophication of surface waters, in which excess nutrients, usually nitrogen or phosphorus, stimulate algal and aquatic plant growth.
The visible effect of eutrophication is often nuisance algal blooms that can cause substantial ecological degradation in water bodies and associated streams. This process may result in oxygen depletion of the water body after the bacterial degradation of the algae.
Approaches for prevention and reversal of eutrophication include: minimizing point source pollution from sewage, and minimizing nutrient pollution from agriculture and other nonpoint pollution sources. Shellfish in estuaries, seaweed farming and geo-engineering in lakes are also being used, some at the experimental stage.