The Gulf of Mexico's Dead Zone is among the world's largest—and corn is one of the culprits

For the hundreds of thousands of people in the Gulf of Mexico who depend on commercial and sport fishing, directly and indirectly, the assault on sea life from the BP oil disaster has been a serious blow. But it's hardly unfamiliar.

That's because even before the spill, up to 8,000 square miles of gulf waters would turn every year into dead zones—vast areas of the coast so depleted of oxygen that shrimp, crabs and other marine animals could no longer live.

Now scientists fear the BP spill will make a bad situation worse. Despite blithe predictions that the gulf fishing industry will bounce back like Alaska's after the Exxon Valdez disaster, the already-expanding Gulf Dead Zone presents a uniquely dangerous scenario that many fear poses a long-term threat to ocean life.

First noted by scientists in the 1960s, dead zones are formed when huge amounts of nutrients—such as those found in agricultural fertilizers, municipal sewage and other wastes—overload the water, leading to explosive algae growth that ultimately robs oxygen from marine life below.

A 2008 study found more than 400 dead zones around the world, and the Gulf of Mexico's is one of the largest. Snaking along the Louisiana and Texas coasts, the expanding Gulf Dead Zone has drastically reduced seafood stocks and pushed fishers further out to sea.

The primary culprit? Nitrate-laced runoff from agricultural operations along the Mississippi River, which eventually drain into gulf waters. One study found that 51 percent of the Mississippi's nitrogen load was from commercial fertilizer, with livestock manure, human sewage and runoff from other crops contributing to the mix.

Fall weather brings cooling air and churning waters that dampen algae growth, but scientists are now seeing a "legacy" effect where lingering decomposed organic matter continues to steal away oxygen. This means that even if nitrate levels hold steady or decline, past pollution can still cause dead zones to expand in the future.

Now add the BP disaster. Gushing oil is the most immediate threat to marine life, but scientists see another looming danger: methane, which BP itself has estimated constitutes 40 percent of what's flooding out of the Deepwater Horizon drill site.

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