NEW ORLEANS - The Gulf of Mexico ''dead zone'' is one of the largest ever recorded. A survey of the area measured the dead zone to be roughly the size of Massachusetts, about 7,722 square miles.

The dead zone forms every year when bacteria feeds on algae blooms and uses up oxygen. The blooms are caused by the nutrient-rich waters from rivers that carry runoff to the Gulf.

The large area of low oxygen chokes marine life. Scientists are concerned because the large dead zone comes in addition to the BP oil spill. The researchers who measured the dead zone said they couldn't link the spill to the dead zone's size.

"It's a classic case of what we call multiple stressors," said Paul Montagna, a marine scientist at the Harte Research Institute for Gulf of Mexico Studies at Texas A&M University-Corpus Christi. "When you have multiple stressors this adds additional problems for animals."

The largest dead zone ever measured was just over 8,000 square miles in 2001.