NOAA-supported scientists have determined this year’s Gulf of Mexico “dead zone”— an area of low to no oxygen that can kill fish and marine life — is approximately 2,116 square miles, or equivalent to 1.4 million acres of habitat potentially unavailable to fish and bottom species.
The measured size of the dead zone is the third smallest in the 34-year record of surveys. The average hypoxic zone over the past five years is 5,408-square miles, which is 2.8 times larger than the 2035 target set by the Hypoxia Task Force.
The annual dead zone survey was led by scientists at Louisiana State University and the Louisiana Universities Marine Consortium during a research cruise from July 25 to August 1 aboard the R/V Pelican.
This year, Hurricane Hanna passed through the central and western Gulf days prior to the research cruise and mixed the water column, disrupting the hypoxic zone which forms in the coastal ocean west of the Mississippi River delta. While the size of the hypoxic zone fluctuates naturally throughout the summer, it usually forms again within days or weeks after the passage of storms. Due to the close proximity of the storm to the survey cruise, the hypoxia area was only able to partially reform before the end of the monitoring cruise, resulting in a patchy distribution across the Gulf.
“The passage of Tropical Storm/Hurricane Hanna across the central Gulf generated 5- to 6- and occasional 8-foot waves along the inner shelf, and mixed the water column down to about 15 to 20 meters,” said Nancy Rabalais, Ph.D., professor at Louisiana State University and LUMCO who is the principal investigator. “The consistent winds from the south generated downwelling favorable conditions, and the remaining low oxygen was further offshore, in deeper water than normally. Vertically uniform temperature, salinity and dissolved oxygen data across the broad area mapped is not the norm for a July shelf-wide hypoxia cruise.” Cassandra Glaspie, Ph.D., of LSU was the chief scientist on board the Pelican, due to an injury to Dr. Rabalais, who attended stations virtually and guided the small science crew, which was limited due to Covid-19 restrictions.
In June, NOAA forecasted a larger-than-average hypoxic zone of 6,700 square miles, smaller than the record size of 8,776 square miles set in 2017. The forecast relies primarily on the Mississippi River discharge and nutrient runoff data from the U.S. Geological Survey. With elevated discharge and nutrient loading this spring, the models predicted a larger than average hypoxic zone to form during the time of the cruise. The forecast models assume typical weather conditions and only have a limited capacity to factor in storms such as Hurricane Hanna and other wind events, which can disrupt the hypoxia zone around the time of the cruise.