The dead zone starts in Midwestern corn country when farmers fertilize their fields with nitrogen. The fertilizer run-off flows down the Mississippi River into the Gulf of Mexico, making algae bloom on the surface and cutting oxygen to creatures that live on the bottom.

The low levels of oxygen in the zone make it difficult for crustaceans and bottom-feeding fish to survive, said Simon Donner, who worked on the study published the Proceedings of the National Academy of Sciences.

Crustaceans will likely struggle to stay alive, Donner said by telephone. Fish will swim out of the zone, potentially devastating local fisheries, he said.

"We're already at a point where recommendations have been made that nitrogen levels in the Mississippi River have to decrease by up to ... 55 percent in order to shrink the dead zone," said Donner, of the University of British Columbia in Vancouver.

"And now with this incentive to produce more corn and use more fertilizer, we're pushing in the other direction," Donner said. "The two policies are just completely incongruous."

A recent US Senate energy policy proposal recommended the manufacture of 15 billion to 36 billion gallons (68 to 164 billion liters) of renewable fuels by the year 2022, Donner's team found.

To reach that goal with corn-based ethanol would increase nitrogen pollution in the Mississippi River by 10 to 18 percent, Donner said.