The so-called dead zone, an area of low oxygen that forms each summer in the Gulf of Mexico off the Louisiana coast, measured 6,765 square miles this year, lower than predicted.

Some of that reduction, researchers said Monday, was because of the turbulence cause by last week’s passage of Tropical Storm Don.

Researchers from the Louisiana Universities Marine Consortium, LSU and the University of Michigan said a previous June forecast predicted the dead zone would cover between 8,500 and 9,421 square miles.

“The prediction was it was going to be quite high because of high water in the Mississippi River,” said Nancy Rabalais, executive director of the consortium and leader of the dead zone mapping work.

The actual size turned out to be smaller than expected because of the wind and wave action produced by Don, Rabalais said.

“It’s still quite large,” she said during a news conference Monday to release this year’s survey results. “It’s over the average, but it’s not one of the biggest.”

The 2011 dead zone is the 11th largest measured in the 26 years of mapping, Rabalais said.

Last year, the dead zone was 7,722 square miles, but its size fluctuates depending on Gulf weather as well as how much fresh water flows down the Mississippi River.

The low-oxygen area forms when nutrients from agriculture and urban runoff flow from the river into the Gulf and feed microscopic organisms.

The upper layer of fresh water from the Mississippi River doesn’t mix with deeper, saltier layers of water where organisms’ use of oxygen creates areas with too little oxygen to support marine life.

This year, Don made landfall Saturday and stirred up Gulf waters and likely mixed low-oxygen water with oxygenated water.

In addition, wind and current conditions could have pushed some of the low oxygen areas to the east where the dead zone gets denser.

In some of these areas, such as near Grand Isle and Port Fourchon, mud-dwelling eels were seen swimming at the surface to get away from the oxygen-depleted zone, she said.

In other areas, researches smelled hydrogen sulfide — similar to the smell of rotten eggs — which means the oxygen levels had gotten near or at zero, she said.

Rabalais said when the oxygen levels get low enough, the chemistry where the sediment and water meet changes and releases hydrogen sulfide, which is toxic.

“So it’s a double jeopardy for the organisms that live in the sediment and can’t escape,” Rabalais said.

In addition, this year LUMCON received additional funding from the National Oceanic and Atmospheric Administration to survey east of the river as well.

Low-oxygen areas were found in areas east of the Chandeleur Islands, although it’s hard to know whether it’s worse or better than previous years.

“I’ve never mapped that area before so I don’t know how it compares,” she said.

There have been some studies in the area, however, and hypoxia has been found there when there’s high water or water that’s been diverted from the Mississippi River, she said.