An average "dead zone" is likely in the Gulf of Mexico, where a large area of water holding too little oxygen to keep marine animals alive forms every summer, the National Oceanic and Atmospheric Administration said Thursday.
A hurricane or tropical storm in the weeks before the July measurement cruise could shrink that considerably by stirring up the water and mixing in oxygen, NOAA noted.
It said its analysis of five university models indicates that this summer's low-oxygen, or hypoxic, area will cover about 4,880 square miles (12,600 square kilometers).
That's close to the five-year average of 5,400 square miles (14,000 square kilometers), NOAA said. It's also bigger than the nation of Vanuatu, nearly double the size of the state of Delaware and 2.5 times a goal set in 2001 by the Gulf of Mexico Hypoxia Task Force.
The Mississippi River drains 40% of the continental United States, including all or part of 32 states.
The dead zone is created as calm weather lets water from the Big Muddy form a layer above the Gulf's salt water, which is denser. Fertilizer and other nutrients carried downriver feed algae, which die and then decompose on the sea floor, using up oxygen from the bottom up. Fecal pellets from grazing zooplankton also contribute, said an analysis by the Louisiana Universities Marine Consortium's Nancy Rabalais, who has been measuring the dead zone since 1985.
"The dead zone off the Louisiana coast is the second largest human-caused coastal hypoxic area in the global ocean," according to the report from Rabalais and Louisiana State University's R. Eugene Turner.
Agriculture, much of it in the upper Midwest and along the Mississippi, is the largest source of nutrients, said Don Cline, associate director for the USGS Water Resources Mission Area.
"But urban areas, human waste treatment, precipitation and atmospheric dust, and natural sources also contribute large amounts," he said.
The tropical storm scenario played out last year, when Hanna -- which became a hurricane before hitting Texas -- came through. The 2020 dead zone was slightly less than one-third the forecasted area and the third-smallest ever measured.
The largest dead zone was 8,776 square miles (22,700 square kilometers) in 2017.
NOAA bases its prediction on forecasts from the University of Michigan, LUMCON and LSU, North Carolina State University, William & Mary's Virginia Institute of Marine Science and Dalhousie University in Halifax, Canada. Those in turn are based on river-flow and nutrient data from the U.S. Geological Survey.