Eutrophication is a form of pollution which is expressed as an excessive supply of nutrients in an aquatic environment (river, lake, pond). It occurs when this environment receives too many nutritional materials which are assimilated by plants. The result is the proliferation of the latter. The nutrients of which there is most frequently a surcharge are phosphorous (carried by phosphates found for example in washing products or fertilisers) and nitrogen (contained for example in nitrates found in fertilisers and liquid manure). This supply encourages the growth and proliferation of certain plants in surface waters. The death of these plants provokes an increase in biodegradable organic matter, which in turn encourages the proliferation of aerobic bacteria at the bottom of the ecosystems. These bacteria end up exhausting the oxygen available within these depths, which leads to an accumulation of organic material which is no longer degraded.