Eutrophication

Eutrophication, 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. Cause and effect were linked in the working definitions of the original scientific studies into the problem (by the Organization for Economic Cooperation and Development and the United States National Academy of Sciences). “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 (the salts of nitric acid) 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 (the salts of phosphoric acid), 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). This increase caused by human beings is known as anthropogenic eutrophication. 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.

There are, unfortunately, many examples of lakes being damaged in this way. Some of the best-documented cases come from Sweden (Lake Norrviken, Lake Trummen), Central Europe (lakes Zurich and Constance), and the United States (Lake Washington). In the worst cases, lakes have lost their clarity (with the increase in suspended algae) and their deep-water oxygen, and their water has become tainted with unpleasant tastes and odours. Apart from aesthetic problems, fisheries have been damaged, the costs of drinking-water treatment have increased, and recreational activities have been impaired.

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). Shallow lakes take longer to restore because they recycle phosphorus much more efficiently than deep lakes, and methods to stimulate alternative food webs (“biomanipulation”) are used to overcome the symptoms of eutrophication. Where nutrient sources are diffuse and difficult to control, the use of artificial mixing systems can be considered as a means to reduce algal growth. Several main storage reservoirs serving London and receiving nutrient-rich water from the Thames are maintained in this way.