by Michael Totten

Created on: September 21, 2010

Dead zones in the Gulf of Mexico are large regions of water where the oxygen has been depleted to less than 2-3 parts per million. This condition is known as hypoxia. At these levels, there is not enough oxygen to sustain a full ecology. Many dead zones are no longer able to sustain any animal life at all.

The first dead zone in the Gulf of Mexico was discovered in the 1970s. It follows the effluent of the Mississippi and Atchafalaya Rivers, spreading out along the immediate shoreline

as far west as Houston and outward as far as the mid-continental shelf. It currently covers a maximum of about 8,500 square miles, larger than New Jersey, although it can also drop to as little as 1,500 square miles. This is the best known of the Gulf of Mexico dead zones, but it is not the only one.

The Mississippi River/Gulf of Mexico dead zone is caused by upstream runoff of agricultural fertilizers and sewage, aggravated by soil erosion. The Mississippi River watershed is a very large one, which includes all of the major Midwest farming states. Corn and soybean production is responsible for half of all nitrogen and a quarter of all phosphorus in the Mississippi River.

When the concentrated nitrogen and phosphorus reach the warm waters of the Gulf of Mexico, they promote algae growth far beyond the normal ecology of the continental shelf. Although algae normally produce oxygen, the rapid growth of these algae blooms and their subsequent decay reduces dissolved oxygen in the region.

If oxygen reaches critically low levels, fish and other animal life cannot survive. Bottom-dwelling creatures travel too slowly to escape into safer waters. Fish stray into the zone, fall unconscious due to lack of oxygen, and die. Their decaying bodies absorb even more of the dissolved oxygen. A few forms of algae are themselves toxic, which increases the overall rate of decay.

In a normal ecology, these kinds of concentrations rarely happen, because nitrogen is normally a limiting factor in growth. Plants which are hungry for nutrients take up naturally occurring nitrogen and phosphorus before most of it can reach the river, so there is normally very little nitrogen in runoff.

However, modern fertilizers and fertilization methods artificially inflate the amounts of nitrogen and phosphorus beyond what plants can take up. Thus, dead zones usually fluctuate with the agricultural season so that they are largest in mid-summer, just before the hurricane season disperses them. The Mississippi River/Gulf

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