NOAA-supported scientists have found the size of this year's Gulf of Mexico oxygen-free 'dead zone' to be the fourth smallest since mapping of the annual hypoxic, or oxygen-free area began in 1985. Measuring approximately 2,889 square miles, the 2012 area is slightly larger than Delaware.
The survey also found a patchy distribution of hypoxia across the Gulf differing from any previously recorded. This is in stark contrast to last year, when flood conditions, carrying large amounts of nutrients, resulted in a dead zone measuring 6,770 square miles, an area of the state of New Jersey. The last time the dead zone was this small was in 2000 when it measured 1,696 square miles, an area slightly smaller than Delaware.
"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, Ph.D., executive director of the Louisiana Universities Marine Consortium (LUMCON) who led the survey cruise. "What wasn't expected was how the scattered distribution of hypoxia areas differed from any others documented in the past. Confirmed, however, is the strong relationship between the size of the hypoxic zone and the amount of fresh water and nutrients carried to the Gulf by the Mississippi River."
The smallest recorded dead zone to date measured 15 square miles in 1988. The largest dead zone, also called a hypoxic zone, measured to date occurred in 2002 encompassing more than 8,400 square miles. 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.
Hypoxia is fueled by nutrient runoff from agricultural and other human activities in the Mississippi River watershed, which stimulates an overgrowth of algae that sinks, decomposes and consumes most of the life-giving oxygen supply in bottom waters.
The hypoxic zone off the coast of Louisiana and Texas forms each summer and threatens valuable commercial and recreational Gulf fisheries. In 2010, the dockside value of commercial fisheries in the Gulf was $639 million. More than 4.6 million recreational fishers took an estimated 22 million fishing trips in 2010, further contributing to the Gulf economy."
Earlier this summer, NOAA-sponsored forecast models developed by Donald Scavia, Ph.D., of the University of Michigan, and R. Eugene Turner, Ph.D. of the Louisiana State University had issued conflicting forecasts for the hypoxic zone, ranging from a small 1,197 square miles to a moderate 6,213 square miles. The forecast of the larger zone hinged on the possibility that organic matter stored in Gulf sediments from large algal blooms during the 2011 flood would act as an additional carbon source for the development of hypoxia this year. The small size of this year's hypoxic zone suggests only a limited role for this "carryover effect" in hypoxia formation under the current low flow conditions.
Prior to the LUMCON cruise, two surveys in June, one led by a NOAA-supported Texas A&M team and another by NOAA's Southeast Fisheries monitoring and assessment program's summer survey, found very little hypoxia in the Gulf. Texas A&M will be conducting a follow-up cruise in mid-August to provide an update on the size of the dead zone.
NOAA has been funding monitoring and research for the dead zone in the Gulf of Mexico since 1985 and currently oversees the two national hypoxia research programs authorized by the Harmful Algal Bloom and Hypoxia Research and Control Act.