WASHINGTON The Gulf of Mexico's "dead zone" -- a swath of water with such low levels of oxygen that marine life can be threatened or killed -- could be the largest since measurements began in 1985, scientists said on Tuesday.
The dead zone, which recurs each year off the Texas and Louisiana coasts, could stretch to more than 8,500 square miles
this year -- about the size of New Jersey -- compared with 6,662 square miles in 2006 and nearly double the annual average since 1990 of 4,800 square miles.
Scientists from the U.S. National Oceanic and Atmospheric Administration, Louisiana Universities Marine Consortium and Louisiana State University forecast the expanded dead zone in a statement.
The dead zone is fed by melted snow and spring flooding along the Mississippi and Atchafalaya rivers, carrying farm chemicals and other runoff into the Gulf of Mexico, the scientists said.
Substances in this runoff include the nutrients nitrogen and phosphorus, which can stimulate the growth of algae. These algae settle and decay in the bottom waters of the Gulf, and the bacteria that decompose them gobble oxygen faster than it can be replenished from the surface, which means lower levels of dissolved oxygen in the water.
"This hypoxic area is of particular concern because of its potential to affect the valuable Gulf fishery," NOAA said in a statement.
The dead zone starts to form in the spring and usually reaches its peak by the end of July or early August, said David Whitall, a coastal ecologist with NOAA who worked on the prediction. A research ship will survey the area to measure the zone by July's end, Whitall said in a telephone interview.
Tropical storms can disrupt the dead zone, and NOAA has predicted an active hurricane season for 2007, but if no strong storms occur, this year's dead zone could exceed the record largest zone of 2002.
"We are predicting that it's going to be bigger than 2002, but not a lot bigger," Whitall said.
In 2002, the dead zone stretched in a wide band for 8,495 square miles from the waters off New Orleans to the central Texas coast.
This year's expected large dead zone may be due to more intensive farming, including crops that produce biofuels, on more land. While agriculture is the main source of the chemicals that contribute to the dead zone, these substances also come from wastewater treatment plants and pollution that gets into rain water, Whitall said.