Jonathan Evatt Online

While the BP oil spill has been labeled the worst environmental catastrophe in recent U.S. history, a biofuel is contributing to a Gulf of Mexico "dead zone" the size of New Jersey that scientists say could be every bit as harmful to the gulf.

Each year, nitrogen used to fertilize corn, about a third of which is made into ethanol, leaches from Midwest croplands into the Mississippi River and out into the gulf, where the fertilizer feeds giant algae blooms. As the algae dies, it settles to the ocean floor and decays, consuming oxygen and suffocating marine life.

Known as hypoxia, the oxygen depletion kills shrimp, crabs, worms and anything else that cannot escape. The dead zone has doubled since the 1980s and is expected this year to grow as large as 8,500 square miles and hug the Gulf Coast from Alabama to Texas.

As to which is worse, the oil spill or the hypoxia, "it's a really tough call," said Nathaniel Ostrom, a zoologist at Michigan State University. "There's no real answer to that question."

Some scientists fear the oil spill will worsen the dead zone, because when oil decomposes, it also consumes oxygen. New government estimates on Thursday indicated that the BP oil spill had gushed as much as 141 million gallons since an oil-rig explosion and well blowout on April 20 that killed 11 workers. Corn is biggest culprit

The gulf dead zone is the second-largest in the world, after one in the Baltic Sea. Scientists say the biggest culprit is industrial-scale corn production. Corn growers are heavy users of both nitrogen and pesticides. Vast monocultures of corn and soybeans, both subsidized by the federal government, have displaced diversified farms and grasslands throughout the Mississippi Basin.

"The subsidies are driving farmers toward more corn," said Gene Turner, a zoologist at Louisiana State University. "More nitrate comes off corn fields than it does off of any other crop by far. And nitrogen is driving the formation of the dead zone."

The dead zone, he said, is "a symptom of the homogenization of the landscape. We just have a few crops on what used to have all kinds of different vegetation."

In 2007, Congress passed a renewable fuels standard that requires ethanol production to triple in the next 12 years. The Department of Agriculture has just rolled out a plan to meet that goal, including building ethanol refineries in every state. The Environmental Protection Agency will decide soon whether to increase the amount of ethanol in gasoline blends from 10 percent to 15 percent.

A 2008 National Research Council report warned of a "considerable" increase in damage to the gulf if ethanol production is increased.