Average-sized Dead Zone Forecasted for the Gulf of Mexico

NOAA scientists are forecasting that this summer’s Gulf of Mexico hypoxic zone or ‘dead zone’ – an area of low to no oxygen that can kill fish and other marine life – will be approximately 5,780 square miles, approximately the size of Connecticut. 

The 2018 forecast is similar to the 33-year average Gulf dead zone of 5,460 square miles and is smaller than the 8,776 square mile 2017 Gulf dead zone, which was the largest dead zone measured since mapping began in 1985.

Even though NOAA is predicting an average dead zone this summer, the dead zone remains three times larger than the long-term target set by the Interagency Mississippi River and Gulf of Mexico Hypoxia Task Force, a group charged with reducing the Gulf dead zone.

The Gulf’s hypoxic zone is caused by excess nutrient pollution, primarily from human activities in the watershed, such as urbanization and agriculture. The excess nutrients stimulate an overgrowth of algae, which then sinks and decomposes in the water. The resulting low oxygen levels near the bottom are insufficient to support most marine life.

Studies have also shown a multitude of other impacts associated with high nutrient concentrations within watersheds. Such impacts include high nitrates in groundwater, higher drinking and wastewater treatment costs and wasted fertilizer applications. 

“The Gulf’s recurring summer hypoxic zone continues to put important habitats and valuable fisheries at risk,” said Steve Thur, Ph.D., director of NOAA’s National Centers for Coastal Ocean Science. “Although there has been some progress in reducing nutrients, the overall levels remain high and continue to strain the region’s coastal economies.”

NOAA issues a dead zone forecast each year, and refines the models used by the Hypoxia Task Force to set nutrient reduction targets and better understand the link between hypoxia and nutrients. The forecast is based on nitrogen runoff and river discharge data from the U.S. Geological Survey. The forecast assumes typical weather conditions, but the dead zone could be disrupted by hurricanes and tropical storms. A NOAA-supported monitoring survey will confirm the size of the 2018 Gulf dead zone in early August.

Higher river discharge in May carries a larger nutrient load into the Gulf of Mexico, one factor that contributes to a larger hypoxic zone. This past May, discharge in the Mississippi and Atchafalaya rivers was about 4 percent above the long-term average (between 1980 and 2017). The USGS estimates that this near-average river discharge carried 115,000 metric tons of nitrate and 18,500 metric tons of phosphorus into the Gulf of Mexico in May. These nitrate loads were about 13 percent below the long-term average, and the phosphorus loads were about 10 percent above the long-term average.

The USGS operates more than 3,000 real-time stream gauges, 60 real-time nitrate sensors, and 35 long-term monitoring sites throughout the Mississippi-Atchafalaya watershed, which drains parts or all of 31 states.

The USGS recently completed the largest-ever assessment of water-quality trends in the nation's streams and rivers, using monitoring data from the USGS and 73 other federal, state, tribal, and local organizations.

“While recent trend results show nutrient loads decreasing in some areas of the Mississippi-Atchafalaya watershed, on balance, there has been little change in loading to the Gulf in recent decades,” said Don Cline, associate director for the USGS Water Mission Area.

This is the first year NOAA is conducting the Gulf dead zone forecast independently. It is the culmination of a multi-year academic-federal partnership to develop a suite of NOAA-supported hypoxia forecast models. The partnership included teams of researchers at the University of Michigan, Louisiana State University, Virginia Institute of Marine Sciences/College of William and Mary, North Carolina State University and the USGS. Some of these groups are also developing independent forecasts, released in coordination with NOAA and using the USGS data.

NOAA and its partners continue to develop additional hypoxia forecast capabilities. For example, new forecasts are being used to evaluate the impacts of reducing phosphorus on dead zone size, and to better estimate the timing and location of hypoxia occurrence. NOAA also provides information to farmers through its Runoff Risk Forecasts, which are aimed at reducing nutrient pollution by giving farmers information about when to apply fertilizers to their fields. Such forecasts can help farmers ensure that fertilizer stays on fields, instead of washing off into waterways.