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The Dead Zone, an area of oxygen so low that Gulf-bottom organisms are killed and fish and crabs swim away, covered 5,840 square miles of Gulf of Mexico seafloor along Louisiana’s coastline this summer, according to a survey by scientists based at the Louisiana Universities Marine Consortium.

That’s more than twice the measured area in 2012, and greater than the average size of the low-oxygen areas during the past five years, according to a news release by lead scientist Nancy Rabalais, who also is director of LUMCON.

The very low oxygen levels, called hypoxia, kill organisms that live in bottom sediment and cannot swim away, and dramatically reduce the amount of habitat available for shrimp and a variety of commercial and recreational fish species, including red drum, red snapper and croaker.

Organisms that live in the bottom sediment, including burrowing eels, lesser and small blue crabs and other swimming crabs, often are found floating on the surface above hypoxic areas.

This year’s five-year average for the low-oxygen area also is well above the goal of reducing the five-year average dead zone size to below 5,000 square kilometers, or about 1,930 square miles by 2015. That goal was set in 2008 by the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, whose members include five federal agencies, 12 states and Indian tribes located within the Mississippi and Atchafalaya river basins.

The hypoxia results from nutrient-rich freshwater from the Mississippi and Atchafalaya rivers entering the Gulf in the Spring, feeding blooms of phytoplankton, which sinks to the seabed when it dies and uses up oxygen in the lower water level when it decomposes.

The nutrients, including nitrogen and phosphorus, come mostly from runoff from agricultural lands in the Midwest. Some nutrients also come from treated wastewater from sewage treatment plants and sewage from septic systems that also moves downriver.