Gulf of Mexico 'Dead Zone' larger than usual this year

The annual Gulf of Mexico "dead zone" — a region of oxygen-depleted water off the Louisiana and Texas coasts that is harmful to sea life — is larger than average this summer, federal scientists reported Tuesday.

This year's zone is 6,474 square miles, an area about the size of Connecticut and Rhode Island combined. It's bigger than average likely because of heavy June rains throughout the Mississippi River watershed, the National Oceanic and Atmospheric Administration said.

A dead zone occurs at the bottom of a body of water when there isn't enough oxygen in the water to support marine life. Also known as hypoxia, it's created by nutrient runoff, mostly from over-application of fertilizer on agricultural fields during the spring.

Nutrients such as nitrogen can spur the growth of algae, and when the algae die, their decay consumes oxygen faster than it can be brought down from the surface, NOAA says. As a result, fish, shrimp and crabs can suffocate.

There is also an annual dead zone in the Chesapeake Bay.

"The heavy rains that came in June with additional nitrogen and even higher river discharges in July are the possible explanations for the larger size,” said Nancy Rabalais, executive director of the Louisiana Universities Marine Consortium, who led a recent scientific cruise to study the region.

The largest Gulf of Mexico dead zone was in 2002, when it was 8,497 square miles, NOAA said. The smallest dead zone measured 15 square miles in 1988. The average size of the dead zone over the past five years has been about 5,500 square miles,

Weather, including wind speed, wind direction, precipitation and temperature, affect the size of the dead zone, which usually fades away by autumn.

Annual measurements of the Gulf dead zone began in 1985, though it likely started in the late 1950s, NOAA reported.