Environment, Runoff and Eutrophication

Eutrophication, or cultural eutrophication, is the enhancement of the natural process by which streams, lakes, reservoirs, and estuaries become enriched with nutrients (phosphorous and nitrogen), enabling the ecosystem to support higher rates of production as measured by biomass or energy per unit area over time. This natural process of enrichment can take thousands of years and result in the succession of a glacial lake into a bog and, eventually, a prairie. However, this process can be greatly accelerated by human activities; while natural eutrophication occurs on geologic timescales, a reservoir undergoing cultural eutrophication can evolve into a bog in less than a hundred years.

This accelerated enrichment can have many detrimental ecological, aesthetic, and human health effects. Excess phosphorous and nitrogen can disrupt the natural balance of the aquatic ecosystem by spurring population explosions of nuisance algae and aquatic plants. As these algal populations sink and die, they create an oxygen demand in the underlying water [Page 311]where bacteria deplete oxygen supplies to decompose the dead algae. The oxygen-depleted bottom waters become poor fish habitat. In freshwater ecosystems, sudden, short-term episodes of low oxygen levels can cause fish kills, and extended periods of low bottom-water oxygen concentrations can cause a shift in fish populations from desirable sport fish to low-oxygen-tolerant species such as carp. In marine ecosystems, this phenomenon is seen in the Gulf of Mexico's “dead zone,” where nitrogen from the Mississippi and Atchafalaya Rivers stimulates high rates of algal growth in the upper water layers, which cause oxygen depletion in the underlying waters.

Nutrient enrichment has also been cited as the cause of coral reef destruction, degrading both the ecological and recreational value of these marine resources. In freshwater ecosystems, nuisance algal blooms can diminish the aesthetic and recreational value of the water body by forming surface scums and producing earthy and musty tastes and odors, which can persist in finished drinking water. In addition to ecological and aesthetic degradation, some algal blooms can pose risks to human health. These harmful algal blooms (HABs) can produce potent toxins, and scientists have implicated them in wildlife, livestock, and pet deaths after the animals had drunk contaminated water. Long-term low dose exposures of a hepatotoxin, microcystins-LR produced by HABs of Microcystis, is suspected to contribute to high rates of liver cancer in certain parts of China. A short-term acute poisoning of the neurotoxin anatoxin-a, produced by an HAB of Anabaena, was the likely cause of death for a Wisconsin teenager in July 2002. In marine ecosystems, HABs have resulted in human exposures to the neurotoxins brevetoxin and saxi-toxin, which are produced by the algae Karenia brevis and Alexandrium fundyense, a known cause of paralytic fish poisoning.

The causes of nutrient enrichment can be categorized into two main sources: point source pollution (i.e., from a pipe) and non-point source (i.e., diffuse) pollution. As all streams, lakes, reservoirs, and estuaries receive water from their respective watersheds (area of land that drains into a water body), any upstream or up-watershed sources of pollution can become pollution sources to the receiving water body. In the United States, the point source release of plant (and algae) nutrients, phosphorus and nitrogen, into the environment is controlled by the National Pollutant Discharge Elimination System (NPDES), a provision of the Clean Water Act.

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