Eutrophication

Post By Voon Chen Li

Eutrophication is a condition in an aquatic ecosystem where high nutrient concentrations stimulate blooms of algae (e.g., phytoplankton). The main cause of eutrophication is excess nitrogen run-off from farm fertilizers, sewage and industrial pollutants. Eutrophication reduces water clarity and depletes oxygen. Reduced water clarity can starve sea grasses and algae that live in corals from light, reducing their growth or killing them. While wind and waves aerate surface waters, the pycnocline—a layer of rapid change in water temperature and density—acts as a barrier to oxygen exchange in bottom waters.

Excess phytoplankton reduces water clarity and consumes oxygen. Phytoplankton need nutrients as well as the energy from the sun to survive but too many nutrients can cause algae blooms and, in turn, red tides (dying phytoplankton). In some regions (particularly near major rivers), excess nutrients can be added to the coastal zone as a result of fertilizer runoff, sewage, animal feedlot runoff, or air pollution. During the bloom, the phytoplankton consume nutrients and oxygen which, in turn, causes a decrease in the amount of dissolved nitrogen and phosphorus in the water body. As the nutrients become depleted, the algae can no longer survive. The dead phytoplankton sink to the bottom of the water column where they are consumed by decomposers. Since these decomposers require oxygen to break down the algae, dissolved oxygen levels will decrease during this time period. Resulting low oxygen levels can be detrimental to fish health; if dissolved oxygen drops to below 2 mg/l, mass fish kills can result. This is known as hypoxia. The areas in which hypoxia has occurred are known as 'Dead Zones.' Dead zones have been a factor in the Gulf of Mexico and Chesapeake Bay on the U.S. east coast, and are now spreading to other bodies of water, including the Baltic Sea, Black Sea, Adriatic Sea, Gulf of Thailand and Yellow Sea. There are now nearly 150 dead zones around the globe-- double the number in 1990, with some extending 27,000 square miles (United Nations Environment Programme, 2003). The article states that "Unless urgent action is taken to tackle the sources of the problem, it is likely to escalate rapidly."