Image: Map of dead zone
The areas in red are the most oxygen-depleted areas detected in the Gulf from July 24 to Aug. 2, 2010. staff and news service reports
updated 8/2/2010 5:41:10 PM ET 2010-08-02T21:41:10

This year's low-oxygen "dead zone" in the Gulf of Mexico is probably the largest on record and overlaps areas hit by oil from BP's well disaster, scientists report.

The area of hypoxia, or low levels of oxygen, covered more than 7,722 square miles of the bottom of the Gulf and extended far into Texas waters, researchers from the Louisiana Universities Marine Consortium said in a statement after surveying the waters.

The area would have included a section off Galveston, Texas, but bad weather forced the surveyors to turn back early.

"The total area probably would have been the largest if we had enough time to completely map the western part," said Nancy Rabalais, the consortium's executive director.

The largest dead zone measured since surveys began in 1985 was just over 8,000 square miles in 2001.

The annual summer "dead zone" in the Gulf is fueled by farm chemicals carried by the Mississippi and Atchafalaya rivers. Nitrogen and phosphorus in agricultural runoff stimulates algae growth in the Gulf.

When these tiny plants or fecal matter from animals that eat them settles to the bottom waters, decomposition of this organic material by bacteria consumes oxygen in the water, the consortium said.

The result, the researchers said, is oxygen depletion that forces many types of fish, shrimp and crabs to leave the area or suffocate. Animals that live in the sediments that can survive with little oxygen will die if the oxygen level falls toward zero.

To be considered hypoxic, oxygen content in the bottom waters of the Gulf must reach the level of 2 parts per million or less.

By late July, large patches of the northern Gulf had reached that level, including one swath off Galveston Bay.

The area of the BP oil spill overlaps some parts of the "dead zone," Rabalais said, and microbes that eat the oil can deplete oxygen in the water.

But the researchers couldn't say there is a definite connection between the spill and the dead zone's size.

"It would be difficult to link conditions seen this summer with oil from the BP spill in either a positive or negative way," Rabalais said.

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