Lakes are especially vulnerable to pollution in the form of eutrophication, which is excessive plant growth that occurs when water becomes artificially enriched with nutrients. This is sometimes caused by runoff of chemical fertilizers. Eutrophication produces bad tastes and odors and unsightly green scums of algae, as well as oxygen depletion in the deeper waters and bottom sediments of lakes. Another problem is acid rain, which has left many lakes in the northeastern United States and Canada totally devoid of life. The major sources of water pollution can be classified as municipal, industrial, and agricultural.
Municipal pollution consists of wastewater from homes and commercial establishments.
Industrial wastewater differs markedly both within and among industries, with its impact depending on characteristics such as biochemical oxygen demand, the amount of suspended solids, and the content of specific inorganic and organic substances. Agriculture is also the source of many organic and inorganic pollutants. These contaminants include both sediment from the erosion of cropland and compounds of phosphorus and nitrogen that partly originate in animal wastes and commercial fertilizers. The combination of toxic substances, the rapid uptake of contaminants by marine organisms, and heavy deposits of materials on the bottom environment near the shore all have very serious consequences for the marine environment. Oil spills are another major cause of marine pollution.