Midwest droughts cited as factor in smaller dead zone

Drought in the Midwest contributed to what has been measured as the fourth smallest “dead zone” in the Gulf of Mexico since 1985, according to scientists with the Louisiana Universities Marine Consortium and the National Oceanic and Atmospheric Administration.

This year’s dead zone measures about 2,889 square miles compared with a dead zone of 6,770 square miles last year.

The size of the dead zone can fluctuate based on river flow, amount of nutrients in the water and if any storms pass through mixing higher oxygen waters with lower oxygen waters.

The dead zone is a low-oxygen area that forms off the coast of Louisiana every summer when nutrients from agriculture and urban runoff flow down the Mississippi River into the Gulf.

These nutrients feed microscopic organisms that use up oxygen when they die and decompose on the water bottom.

During summer months, the upper layer of freshwater from the river does not mix with the saltier layer of water underneath where organisms are using up oxygen.

That results in a layer of water along the bottom where oxygen levels fall to a point where it cannot sustain marine life.

The last time the dead zone was this small was in 2000 when it measured 1,696 square miles, according to a news release Friday from NOAA, the agency that funds the annual surveys in the Gulf.

“The smaller area was expected because of drought conditions and the fact that nutrient output into the Gulf this spring approached near the 80-year record low,” said Nancy Rabalais, executive director of LUMCON who led the survey cruise.

The average size of the dead zone over the past five years has been 5,684 square miles, more than twice the 1,900 square mile goal set by the Gulf of Mexico/Mississippi River Watershed Nutrient Task Force.

More information is available at http://www.gulfhypoxia.net.