For 35 years scientists have monitored the size of the Gulf of Mexico dead zone or hypoxic zone, which forms every spring. National Oceanic and Atmospheric Administration (NOAA) scientists are forecasting this summer’s hypoxic area or “dead zone” to be approximately 6,700 square miles, 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. The annual prediction is based on U.S. Geological Survey river-flow and nutrient data.
According to NOAA the hypoxic zone is caused by excess nutrient pollution, primarily from human activities, such as agriculture, wastewater treatment and urbanization occurring throughout the Mississippi River Watershed. The excess nutrients stimulate an overgrowth of algae, which eventually die, then sinks and decomposes in the water. 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.
“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.”
In a press release NOAA explained that 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. In May 2020, discharge in the Mississippi and Atchafalaya rivers was about 30% 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% above the long-term average, and phosphorus loads were about 25% above the long-term average.
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 5-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. The Task Force uses them to help inform overall nutrient reduction targets across the watershed.
Photo courtesy of NOAA