The process of heightened biological productivity in a body of water is call eutrophication. The major factors controlling eutrophication in a body of water, whether large, small, warm, cold, fast-moving, or quiescent, are nutrient input and rates of primary production. Not all lakes experience eutrophication. Warmth and light increase eutrophication, (which in Greek means "well nourished") if nutrient input is high enough. Cold dark lakes may be high in nutrients, but if rates of primary production are low, eutrophication does not occur. Lakes with factors that limit plant growth are called oligotrophic. Lakes with intermediate levels of biological productivity are called mesotrophic.
Many lakes around developed areas experience cultural eutrophication, or an accelerated rate of plant growth, because additional nitrates and phosphates (which encourage plant growth) flow into the lakes from human activities. Fertilizers, soil erosion and animal wastes may run off from agricultural lands, while detergents, sewage wastes, fertilizers, and construction wastes are contributed from urban areas. These nutrients stimulate the excessive growth of green plants, including algae. Eventually these plants die and fall to the bottom of the lake, where decomposer organisms use the available oxygen to consume the decaying plants. With accelerated plant growth and subsequent death, these decomposers consume greater amounts of available oxygen in the water; other species such as fish and mollusks thus are affected. The water also becomes less clear as heightened levels of chlorophyll are released from the decaying plants. Native species may eventually be replaced by those tolerant of pollution and lower oxygen levels, such as worms and carp.
While at least one-third of the mid-sized or larger lakes in the United States have suffered from cultural eutrophication at one time or another over the past 40 years, Lake Erie is the most publicized example of excessive eutrophication. Called a "dead" lake in the 1960s, the smallest and shallowest of the five Great Lakes was inundated with nutrients from heavily developed agricultural and urban lands surrounding it for most of the twentieth century. As a result, plant and algae growth choked out most other species living in the lake, and left the beaches unusable due to the smell of decaying algae that washed up on the shores. New pollution controls for sewage treatment plants and agricultural methods by Canada and the United States led to drastic reductions in the amount of nutrients entering the lake. Almost forty years later, while still not totally free of pollutants and nutrients, Lake Erie is a biologically thriving lake, and recreational swimming, fishing, and boating are strong components of the region's economy and aesthetic benefits.