SO, WHAT'S THE ISSUE?
Cultural eutrophication is a form of water pollution caused by fertilizer of plant nutrients. When nitrates and phosphates from fertilizers, animal waste, and detergents enter a waterway, they can cause algal blooms that reduce water clarity and diminish oxygen levels. The resulting lack of oxygen creates a Hypoxic Zone (aka a Dead Zone) where fish start dying. When concentrations of molecular oxygen may reach levels of less than about one milligram per litre, a threshold for various biological and chemical processes is crossed, causing issues. Much of the phosphorus in streams and lakes comes from agriculture, from both soil erosion and fertilizer runoff. Nitrogen from municipal sewage treatment plants and the direct runoff from animal feedlots are also common causes. At normal levels, these nutrients feed the growth of an organism called cyanobacteria, or blue-green algae. With too many nutrients, however, cyanobacteria grows out of control, causing the dead zone.
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Algal blooms prevent light and oxygen from reaching the benthic (bottom) layer of water bodies, hurting organisms that dwell there.
Algal blooms also cause larger-scale problems, such as human illness. Shellfish, such as oysters, are filter feeders. As they filter water, they absorb microbes associated with algal blooms. Many of these microbes are toxic to people. People may become sick or even die from shellfish poisoning.
Human activities have emitted nearly twice as much nitrogen and three times as much phosphorus as natural emissions. Different regions of the world emit different levels of these nutrients. In developed countries, such as the United States and nations in the European Union, heavy use of animal manure and commercial fertilizers in agriculture are the main contributors to eutrophication. Runoff from large agricultural fields enters creeks and bays because of rain or irrigation practices.
In developing countries of Latin America, Asia, and Africa, untreated wastewater from sewage and industry mainly contribute to eutrophication. Factories and sewage facilities are less regulated than they are in developed countries, and sometimes wastewater is simply dumped into creeks, rivers, lakes, or the ocean.
