WASHINGTON - Floodwaters loaded with farm runoff are heading down the Mississippi River, and scientists fear the deluge will dramatically increase this summer's "dead zone" in the Gulf of Mexico, covering an area the size of Maryland.

The dead zone is a region of the gulf that becomes starved for oxygen during much of the summer and cannot support fish or other sea life.

There are hundreds of dead zones around the world that wreak havoc with marine ecology and cut off vast areas for commercial fishing. The zone in the gulf is the largest in the Western Hemisphere.

"It's going to be a very interesting summer out there just because of this," said Steven DiMarco, a professor of oceanography at Texas A&M University. "The last time something like this happened, we did see a huge difference" in the size of the dead zone from one year to the next.

The zone off the Louisiana and Texas coasts was first seen in 1972. Its size varies each year, but it has tended to grow over the decades, with a major jump in 1993, after the last big Mississippi River flood.

That flood made the oxygen problem substantially worse, which may happen again this year, DiMarco said Friday.

Even before the flooding, scientists had predicted that the gulf this summer would see its largest-ever dead zone -- more than 10,000 square miles. Now experts say it's likely to be even bigger.

Oxygen in the dead zone is depleted by excess nutrients, mostly nitrates from farm fertilizer runoff, that cause algae blooms. After the algae die, bacteria on the bottom feast on the remains, removing crucial oxygen from the water.

The dead zone in the gulf forms in early summer and lasts through early fall.

This year's massive floods will bring a heavier load of fertilizer into the gulf, DiMarco said.

But it's more than the nitrates.

The trillions of gallons of floodwater help trap the oxygen-depleted water near the gulf floor. The fresh water, which stays at the surface because it is less dense, forms a physical barrier that keeps oxygen in the air from mixing with the water covering the dead zone area, DiMarco said.

Scientists are just starting to study how the increasing size of the dead zone is affecting fish.

Think of a giant corridor from Des Moines to Chicago and "you took a great big piece of Saran Wrap over all that area and sucked all the oxygen out," said Nancy Rabalais, executive director of the Louisiana Universities Marine Consortium. "You would have a big problem."