Eutrophication which may be natural or 'man-made', is the response in water to overenrichment by nutrients, particularly phosphorus and nitrogen. 'Man-made' eutrophication, in the absence of control measures, proceeds much faster than the natural phenomenon and is one of the major types of water pollution. The resultant increase in fertility in affected lakes, reservoirs, slow-flowing rivers and certain coastal waters causes accelerated plant growth producing symptoms such as algal blooms, heavy growth of certain rooted aquatic plants, algal mats, deoxygenation and, in some cases, unpleasant tastes and odours. These often adversely affect the vital uses of water such as water supply, fisheries, or recreation, deplete surface oxygen in the water and impair aesthetic qualities. In short, man-made eutrophication of inland bodies of water has become synonymous with the deterioration of water quality and is frequently the cause of considerable cost increases.
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.
Early in the 1960s, it became obvious that a large number of lakes and reservoirs, particularly those located in industrialized countries, were rapidly changing in character and becoming increasingly fertile (eutrophication) because of addition of plant nutrients originating largely from human activities. The main nutrient sources were municipal and industrial wastewater and agricultural and urban runoffs. The incidence of these conditions, lakes covered at certain periods of the year with algal blooms, the water full of microscopic algae and weeds, with a resultant deoxygenation of the water and incomplete decomposition processes, are difficult to predict because of varying conditions (limnological, ecological or climatological). The change, however, can take place in 10 years or less, thus preventing the use of the lake or river for swimming and many recreational purposes. The use of eutrophied water for drinking or industrial purposes requires expensive treatment processes which may not be really successful in today's state of knowledge (for example, the water taste may not be acceptable). Eutrophic water is poisonous to many species of fish, especially the more desired game fish, and can kill animals drinking it.
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).