The Gulf of Mexico's "dead zone," a region depleted of oxygen to the point where fish and other marine life can die, covers 6,474 square miles this year, federal scientists say.
That is above the yearly average and much larger than had been forecast for 2015, the National Oceanic and Atmospheric Administration says.
A "dead zone," also known as an hypoxia area, is the result of the runoff of nutrients from agriculture and other human activity carried by rivers into the ocean. There, those nutrients accelerate an overabundance of algae that then sinks to the bottom where it decomposes, consuming the oxygen needed to support marine life, the agency explains.
This year's dead zone in the Gulf is the size of Rhode Island and Connecticut combined, scientists say, with nutrients flowing from the Mississippi River affecting coastal resources and marine habitats in the Gulf.
NOAA had earlier used computer models to predict a summer 2015 dead zone off the coasts of Texas and Louisiana in a range between 4,633 and 5,985 square miles.
"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," says Nancy Rabalais, executive director of the Louisiana Universities Marine Consortium.
The largest Gulf dead zone ever recorded was in 2002, at 8,497 square miles; the average for the last five years has been around 5,500 square miles.
The annual dead zone presents an economic challenge for the Gulf region fishing industry, since it depletes the food supply that fish and shrimp in the area depend on.
The marine consortium conducted a survey cruise of the affected area from July to Aug. 3 as part of a program funded by NOAA and the Environmental Protection Agency to monitor and record hypoxia trends in the Gulf.
"The importance of having continued and sustained coastal observations are foundational in helping us better understand the size and impacts of the Gulf dead zone," says Holly Bamford, assistant NOAA administrator for the National Ocean Service. "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."
Other dead zones in the U.S. caused by agricultural runoff include areas in the Chesapeake Bay and Lake Erie.
The Gulf of Mexico dead zone is the second-largest in the world after the widespread algal blooms in the Baltic Sea.
