Eutrophication

Eutrophication

The process by which a body of water becomes overly productive is called eutrophication. The major factors controlling eutrophication, which in Greek means “well nourished,” are nutrient input, light and

temperature. All three of these factors are required for the increased growth rates of phytoplankton that lead to eutrophication. Bodies of water with factors that limit plant growth are called oligotrophic. Those with intermediate levels of biological productivity are called mesotrophic. Eutrophication may occur in freshwater or marine environments.

Water bodies around developed areas experience cultural eutrophication, or an accelerated rate of plant growth, because additional nitrates and phosphates that encourage plant growth flow into the water 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 planktonic algae. Eventually these plants die and fall to the bottom, where decomposers use the available oxygen in the process of breaking down the plant material. With accelerated plant growth and subsequent death, these decomposers consume greater amounts of available oxygen in the water. Animal species such as fish and mollusks are harmed by low concentrations of oxygen in the water. The water also becomes less clear as the concentration of algae and bacteria increases. 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. Pollution controls for sewage treatment plants and improved agricultural restrictions by Canada and the United States led to drastic reductions in the amount of nutrients entering the lake. By the turn of the millennium, Lake Erie had again become a biologically thriving lake. Recreational swimming, fishing, and boating were again strong components of the region’s economy and aesthetic benefits.