Eutrophication is the biological response of water to overenrichment by plant nutrients, particularly nitrogen and phosphorus. Public concern began to rise in the 1960s (although the term "eutrophication" is older), when nutrient enrichment was rapidly making many bodies of water increasingly fertile. This eutrophication was mainly caused by the addition of plant nutrients from human activities, called, in this context, artificial or anthropogenic eutrophication. The phenomenon is a consequence of society's municipal, industrial, and agricultural use of plant nutrients and their subsequent disposal.
Lakes and reservoirs have a finite life span. They may pass through periods in their existence when they become more or less fertile, according to different factors--principally their geographical position or the climatic conditions (Moss, 1988). The process of eutrophication has been used deliberately as a way to fertilize and thus to increase phytoplankton production and, indirectly, the population of fish within a lake or reservoir. What is new in the past few decades, however, is the extent of enrichment of lakes and rivers throughout the world as a result of the growing human population, more intensive agricultural and industrial activities, and the development of large sewage systems associated with large metropolitan areas. Until recently, a relative lack of control over the sources of the nutrients or over their effect upon the aquatic ecosystems has resulted in changes occurring within decades rather than over the centuries--or longer--in which such changes would appear naturally Many studies of lake s around the world have provided evidence of human-induced changes. Good examples of such studies are those carried out on the Great Lakes (Beeton & Edmondson, 1972; Sly, 1991).
In the United Kingdom, eutrophication has been identified as an extremely widespread problem and has been blamed for damaging many aquatic sites in England known as Sites of Special Scientific Interest, despite government claims that only a few surface waters have been affected (Carvalho & Moss, 1995). In a study commissioned by English Nature, a statutory conservation agency in England, it was found that 79 Sites of Special Scientific Interest showed signs of eutrophication. As a result, English Nature has called for a large-scale investment program to deal with the eutrophication problem in aquatic wildlife sites (English Nature, 1997).
Anthropogenic eutrophication appears to be the main problem. Excessive fertility in lakes and reservoirs results in heavy growth of phytoplankton, particularly of blue-green algae (cyanobacteria), that may form thick mats at the water surface and thus spoil the appearance of the lake. Some species of cyanobacteria may produce substances that are highly toxic to fish, birds, or mammals. In some cases, dense blooms of algae have resulted in fish kills by causing the hypolimnion to become anaerobic. Increased crops of phytoplankton often clog the filters of water treatment plants and make the treatment of water more costly Furthermore, some unwanted organic substances produced by the algae can pass through the filters at water treatment plants and cause unpleasant tastes and odors, or may even be toxic to human consumers. Eutrophication thus can not only impair aesthetic qualities of the water, but also affect the use of water for water supply, fisheries, and recreation.
The essential elements required by living cells to sustain growth and reproduction are carbon, oxygen, hydrogen, other macronutrients, and trace elements. Of these, carbon is the most important, the main reservoir being atmospheric carbon dioxide. Carbon is easily soluble in water and is thus unlikely to be a limiting factor for algae growth, except during intense blooms. Oxygen and hydrogen are freely available in the water in most circumstances.
The most important macronutrients are calcium, magnesium, potassium, phosphorus, nitrogen, sulfur, iron, and silicon. Phosphorus is important because it is the only nutrient whose proportional abundance is lower in the lithosphere than in plant tissue. It is thus a prime candidate to become a limiting factor in algae growth. The main reservoir of nitrogen is atmospheric dinitrogen, which is not available to plants directly, consequently nitrogen might be a limiting factor as well.
Trace elements, including boron, chlorine, cobalt, copper, manganese, molybdenum, zinc, and, in some cases, vitamin complexes, are required in very small quantities.
The "law of the minimum," which was first formulated by Justus von Liebig, states that growth is limited by whatever is in shortest supply (Gibson, 1971; Welch, 1980). For the reasons stated above, phosphorus and nitrogen are said to be "key nutrients"; in some circumstances, they may become limiting. Therefore, they are in most cases the nutrients that control algae growth, though some diatom species may be limited by silica. Other factors, such as light, may also limit algal productivity.
Supply of Phosphorus and Nitrogen to Lakes
Phosphorus is the 11th most abundant element in the earth's crust, and it is geochemically classed as a trace element. In nature, phosphorus exists almost exclusively as phosphate, a great part of which is sorbed to soil particles or incorporated into soil organic matter. Phosphate deposits occur in the earth's crust principally as the mineral apatite: [Ca.sub.5](F,C1,OH,1/2[CO.sub.3])[([PO.sub.4]).sub.3]. The initial natural source of phosphorus is weathering of such rocks. Weathering liberates phosphate from the mineral, and the phosphate can then enter the biosphere through uptake by plants.
The initial source of nitrogen is the atmospheric reservoir of gaseous dinitrogen. Nitrogen gas is chemically very stable. It must be converted by nitrogen fixation, by microorganisms living principally in the soil but also in aquatic environments, before it is available to most living organisms. In natural water, nitrogen is present as dissolved dinitrogen, ammonia, and salts of the nitrate and nitrite ions; in addition, there are nitrogen-containing organic compounds primarily attributable to the presence of life.
In a natural, undisturbed environment, nutrient sources are the drainage of the catchment, the direct atmospheric deposition (rainfall and dry depositions) onto the water surface, and the internal recycling from lake sediments. Ahl estimates the background phosphorus input to be in the range of 3 to 10 kilograms (kg) of phosphorus per square kilometer per year, depending on the size and the characteristics of the basin (Ahl, 1988). He also estimates the …