By Jeff Franks

HOUSTON, Aug 3 (Reuters) - Researchers have found 9,650 square miles (24,990 sq km) of "dead zones," or oxygen-depleted water, in the Gulf of Mexico this summer, the biggest area since tracking of the annual phenomenon began.

They say humans are mostly to blame for the dead waters, and that increased planting of corn to make ethanol is adding to the problem.

The dead zones, which have been appearing each summer since at least 1970, threaten marine life and over time have altered the gulf's ecology, scientists say.

Usually researchers, who began measuring the dead zones in 1985, find only one large zone each year, just off the Louisiana coast where the Mississippi River empties into the gulf.

But this summer, for the first time, a separate zone has developed off Texas, Texas A&M University oceanographer Steve DiMarco said this week.

Recent measurements taken in separate studies show the Louisiana dead zone covered about 7,900 square miles (20,461 sq km), while the Texas zone was 1,750 square miles (4,532 sq km), for a total of 9,650 (24,990 sq km).

The previous largest amount was 8,495 square miles (22,002 sq km) found in 2002, Nancy Rabalais, chief scientist for the Louisiana Universities Marine Consortium, said on Friday.

The Louisiana dead zone is caused mostly by nitrogen-based fertilizers carried by the Mississippi from America's farm belt into the Gulf, she said. The nitrogen feeds the growth of algae, which depletes oxygen from the water.

The U.S. Department of Agriculture said in March corn planting would rise 15 percent this year to feed increased demand for ethanol, a motor fuel distilled from corn and now promoted as an alternative to gasoline.

Corn needs more fertilizer than other crops, which is probably why tests have found more nitrogen in the Mississippi this year, Rabalais said.

She said only the Baltic Sea had a larger "man-made" dead zone than the Gulf, but it is about four times bigger.

The Texas dead zone was caused not by fertilizer, but by heavy rains that filled the Gulf with fresh water, said DiMarco.

The fresh water, he said, sits on top of salt water "like oil and water" and prevents it from being oxygenated by air.

Water in the dead zones cannot support most life, DiMarco said. There were already signs of problems on the Texas coast.

"I'm getting reports there have been some fishkills," he said.

Rabalais said dead zones reduce the amount and variety of marine life and, as a result, "have already changed the Gulf of Mexico ecosystem, and possibly in a permanent way."

The dead zones form in the calm summer waters and break up when the summer doldrums end or a hurricane churns through the gulf, she said.

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