Eutrophication of water can produce conditions in lakes and slow moving water previously only associated with the stagnant village pond; namely water filled with plant life and little else, evil smelling from lack of oxygen to purify decaying products and generally unacceptable for any recreational or functional use except perhaps by ducks. Furthermore, it is undesirable, and perhaps dangerous, to dump unwanted products of industry, agriculture or sewage into such water since the oxygen deficiency prevents the natural purification processes from operating.
The exact cause of the change from clear water conditions to an explosive growth of algae is unknown. Certainly there must be an excess of nutrients of which nitrates and phosphates from sewage, chemical fertilisers and detergents account for some 90% or more of the man-added nutrient burden. Other chemical or bio-chemical substances present in effluents and land drainage can act as micro-nutrients. Irrespective of the nature of the trigger action which initiates eutrophication, the development of this undesired state in water is certainly made possible by the existence in water of a large added nutrient burden.
Eutrophication is reported to be widespread in Europe's seas and fresh water; this affects fish stocks as well as human health and recreational use. Except for rivers in the Nordic countries, 68% of sites surveyed in all European rivers had an average nitrate concentration greater than 1 mg/litre between 1992 and 1996. The main source of nitrate is generally diffuse pollution from agriculture. Since the 1970s the concentration of organic pollution has been generally reduced and the oxygen concentration increased, especially in the previously most impacted rivers.
Rising nitrogen loads combined with phosphorous have led to exuberant and unwanted plant and algal growth in many freshwater habitats and coastal areas throughout the world. In the United States, eutrophication - rapid plant growth in water resulting in oxygen deprivation for other species - accounts for about half of the impaired lake area and 60 per cent of the impaired river reaches (Carpenter and others 1998).