Heavy rains earlier this summer might be a contributor to a dead zone in the Gulf of Mexico that is larger than last year’s, the National Oceanic and Atmospheric Administration said this week.
The “dead zone” is an area of low to no oxygen that is at 6,474 square miles this year, compared to 5,052 last year. The size is also larger than the 1,900 square mile target proposed by the Gulf of Mexico / Mississippi River Watershed Nutrient Task Force (Hypoxia Task Force).
Also called hypoxia areas, dead zones are caused by nutrient runoff from agricultural and other human activities in the watershed and are “highly affected” by river discharge and nitrogen loads, NOAA said.
Excess nutrients stimulate algae growth, which later sinks, decomposes, and consumes oxygen. It’s estimated that there are more than 550 dead zones occurring annually worldwide.
A suite of NOAA-sponsored models forecasted a range of 4,633 to 5,985 square miles based on May nitrogen loading data provided by USGS.
“Since the models are based largely on the May nitrogen loads from the Mississippi River, 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, Ph.D. executive director of the Louisiana Universities Marine Consortium.
NOAA and EPA fund the annual measurement mapping of the dead zone, which is completed by ship.
It provides a record of the trend of hypoxia in the Gulf and is the primary measure of progress used by the Hypoxia Task Force to determine whether efforts to reduce nutrient loading upstream in the Mississippi River Basin are yielding results.
“This information ultimately informs the best strategies to reduce the size and the impacts of the dead zone, which will help improve the sustainability and productivity of our coastal economy,” said Holly Bamford, Ph.D., assistant NOAA administrator for the National Ocean Service.
The largest previous Gulf of Mexico dead zone was in 2002, encompassing 8,497 square miles. The smallest recorded 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, nearly three times the 1,900 square mile goal set by the Hypoxia Task Force in 2001 and reaffirmed in 2008.