http://www.nytimes.com/2011/06/03/science/earth/03runoff.html?hpw

> Dead zones are areas of the ocean where low oxygen levels can stress or kill bottom-dwelling organisms that cannot escape and cause fish to leave the area. Excess nutrients transported to the gulf each year during spring floods promote algal growth. As the algae die and decompose, oxygen is consumed, creating the dead zone. The largest dead zone was measured in 2002 at about 8,500 square miles, roughly the size of New Jersey. Shrimp fishermen complain of being hurt the most by the dead zones as shrimp are less able to relocate — but the precise impacts on species are still being studied.

> The United States Geological Survey has found that nine states along the Mississippi contribute 75 percent of the nitrogen and phosphorus. The survey found that corn and soybean crops were the largest contributors to the nitrogen in the runoff, and manure was a large contributor to the amount of phosphorus.

> There are many other factors, of course, that determine what elements make it from crops into river water, for example, whether watersheds are protected by wetlands or buffer strips of land.

> John Downing, a biogeochemist and limnologist at Iowa State University, said structural issues were also to blame. Many farms in Iowa, he said, are built on former wetlands and have drains right under the crop roots that whisk water away before soils can absorb and hold on to at least some of the fertilizer.