Eutrophication

Eutrophication is the enrichment of lakes, reservoirs, rivers, and coastal seas by plant nutrients, otherwise in short supply, and the increased mass of aquatic plant life that the additional nutrients can sustain.

“Eutrophic” means “well-feeding”: ecologists use the term to describe relatively productive habitats and communities having good nutrient supplies and to separate them from unproductive “oligotrophic” ones, characterized by a nutrient deficiency. In 1919 the Swedish limnologist Einar Naumann distinguished oligotrophic lakes as being generally deep, in mountainous catchments of resistant rocks, transparent, and supporting little plant or animal life in the water with, typically, a predominance of salmonid fishes (such as salmon, trout, or char).

Eutrophic lakes were correspondingly shallow, often in lowlands; they received water altered by contact with soft eroding rocks and soils; they supported abundant microscopic plant life (chiefly the algae and cyanobacteria comprising the phytoplankton), and sometimes also copious reed swamps and submerged plant beds at their margins; coarse fish (such as perch, carp, bream, roach, and pike) often thrived.

Many studies have shown that these characteristics are critically determined by the biological availability of nitrogen and, especially, phosphorus. Nitrates are derived principally from the activities of nitrifying bacteria in soil. Being very soluble, nitrates leach easily into drainage water, if terrestrial plants fail to take them up first. Phosphates, on the other hand, are sparingly soluble; the little which escapes to drainage is generally in the form of particles. It was once assumed that lakes became naturally more eutrophic through time. However, evidence strongly indicates that most recent changes are due to the increase in nutrients coming from the land in consequence of human activities (such as forest clearance, ploughing, and fertilizing).

The supply of dissolved phosphorus to lakes and rivers is greatly increased by domestic and industrial sewage disposal, unless steps are taken to remove it from the final effluent. Polyphosphate-based detergents may also contribute a significant proportion. As the turbidity of water (its murkiness, caused by suspended nutrients) increases so does the production of phytoplankton: greater rates of bacterial decomposition remove dissolved oxygen from deep water faster than it can be replaced from the atmosphere, leaving less of the water habitable for fish. Lakes are less attractive and reservoir water requires more expensive treatment to become drinkable. Enhanced production of toxic cyanobacteria is sometimes a further consequence of eutrophication.

Eutrophication can be reversed by cutting back the phosphorus loads, either by diversion from sensitive waters or by chemical precipitation with iron salts (“phosphate stripping”) at such point-sources as the effluents of sewage-works. Successful schemes include those of Lake Washington, Wahnbach Talsperre (Germany), and Lake Windermere (United Kingdom).

Contributed by:
Colin Stanley Reynolds, C.Biol., B.Sc., Ph.D., D.Sc., F.I.Biol.
Senior Scientist, Freshwater Biological Association. Author of The Ecology of Freshwater Phytoplankton; and co-editor of Archiv Fnr Hydrobiologie.

These charts show the steady rise over several decades in the concentrations of phosphorus in Lake Windermere (in Cumbria, England), with a consequent increase in the concentrations of chlorophyll, indicating increased algal growth. In 1992 measures were taken to remove phosphorus from sewage effluent entering the lake, and the dramatic decrease in both phosphorus and chlorophyll levels can be seen to start at that time.

Many human activities cause additional nutrients, such as phosphates and nitrates, to enter rivers and lakes—a process known as eutrophication. This promotes the growth of algae and other types of plankton, thus depleting the water's oxygen and making it difficult for fish to survive, as well as making waterways unsightly. Eutrophication can be reversed by reducing the amount of nutrients entering the water, for instance by removing them from treated sewage.