NOAA Predicts Larger Gulf ‘Dead Zone’

The National Oceanic and Atmospheric Administration (NOAA) is forecasting the Gulf of Mexico hypoxic area or “dead zone”—an area of low to no oxygen that can kill fish and other marine life—will extend to an area of 6,700 square miles this summer. That’s larger than the long-term average measured size of 5,387 square miles, but substantially less than the record of 8,776 square miles set in 2017, NOAA said in the June 3 announcement. The annual prediction is based on U.S. Geological Survey river-flow and nutrient data.

The annual Gulf of Mexico dead zone is primarily caused by excess nutrient pollution from human activities in urban and agricultural areas throughout the Mississippi River watershed. When excess nutrients reach the Gulf, they stimulate an overgrowth of algae, which eventually die and decompose, depleting oxygen as they sink to the bottom. The resulting low oxygen levels near the bottom of the Gulf cannot support most marine life. Fish, shrimp and crabs often swim out of the area, but animals that are unable to swim or move away are stressed or killed by the low oxygen. The Gulf of Mexico dead zone occurs every summer.

“Not only does the dead zone hurt marine life, but it also harms commercial and recreational fisheries and the communities they support,” said Nicole LeBoeuf, acting director of NOAA’s National Ocean Service. “The annual dead zone makes large areas unavailable for species that depend on them for their survival and places continued strain on the region’s living resources and coastal economies.”

A major factor contributing to this year’s above-average hypoxic zone are the high river flows and nutrient loads delivered to the Gulf this spring, primarily from the Mississippi and Atchafalaya rivers, NOAA said. Discharge in the Mississippi and Atchafalaya rivers in May was about 30 percent above the long-term average between 1980 and 2019. The USGS estimates that this larger-than-average river discharge carried 136,000 metric tons of nitrate and 21,400 metric tons of phosphorus into the Gulf of Mexico in May alone. These nitrate loads were about 2 percent above the long-term average, and phosphorus loads were about 25 percent above the long-term average.

The USGS operates more than 3,000 real-time stream gauges, 50 real-time nitrate sensors and 35 long-term monitoring sites to measure nutrients in rivers throughout the Mississippi-Atchafalaya watershed. Data from these networks are used to track long-term changes in nutrient inputs to the Gulf and to help build models of nutrient sources and hotspots within the watershed.

While the hypoxic zone forecast assumes typical coastal weather conditions, the measured dead zone size could be disrupted and its size could be changed by major weather events, such as hurricanes and tropical storms, which mix ocean waters, as occurred in 2018 and 2019. A NOAA-supported monitoring survey, scheduled for later this summer, will confirm the size of the 2020 dead zone, and it is a key test of the accuracy of NOAA’s models.

The Mississippi River/Gulf of Mexico Hypoxia Task Force, a group working to reduce the Gulf dead zone through state led nutrient reduction strategies and targets across the Mississippi River watershed, has set a five-year average measured size target of 1,900 square miles. The NOAA models help predict how hypoxia in the Gulf of Mexico is linked to nutrients coming from throughout the Mississippi River Basin.