Too high levels of nutrients in water may cause eutrophication. Eutrophication denotes the enrichment of a water body by input of organic waste containing nutrients, chiefly nitrates and phosphates. Many nutrients result from the natural disintegration of rocks and from mineralization of organic matter. Natural eutrophication is a very slow process, taking often a period of over hundred years. But artificial eutrophication which results from human activities is a dramatically fast process. This happens when domestic waste, agricultural residues, land drainage and industrial wastes reach a water body. The problem of eutrophication arises from nutrients released from organic wastes by the activity of aerobic bacteria in presence of oxygen. These nutrients induce changes in the ecosystem balance and composition of aquatic life.
All the nutrients fertilize microscopic water plants Called algae, as well as larger water plants, such as duckweed and water hyacinth. More of these plants grow as a result of additional nutrients. As more plants grow, more also die. They also decay. Both these processes consume oxygen resulting in oxygen deficit. Nutrients and organic wastes added by people imbalance the cycle, addition of nutrients at an increased rate increases the rate of growth of algae. As the algae die, they add to wastes. Bacteria sometimes use so much oxygen converting large amounts of wastes into nutrients that few fish survive because of depletion of oxygen. Such a lake is said to be eutrophied. It smells offensively as BOD rises and its aesthetic value goes down. Eutrophication leads to an increase in the growth of aquatic plants and often to algal blooms, ultimately the water body disappears.