eutrophication

Excessive enrichment of rivers, lakes, and shallow sea areas, primarily by municipal sewage, by sewage itself, and by the nitrates and phosphates from fertilizers used in agriculture. These encourage the growth of algae and bacteria which use up the oxygen in the water, making it uninhabitable for fish and other animal life. In this way eutrophication is responsible for a particular type of water pollution.

The dissolved fertilizers cause the rapid growth of water plants, especially green algae, which can clog up waterways and prevent light reaching plants below the surface. As the algae die, aerobic bacteria bring about decay, using up oxygen in the water as they do so. Anaerobic bacteria take over and convert part of the dead matter into smelly decay products. Slowly a layer of dead plant material builds up on the bottom of the lake or river. Deprived of oxygen, fish and other life forms die and decay, leaving putrid, poisonous water.