What is the Gulf of Mexico ‘dead zone’ and how big will it be this summer

Weather Blog

TEXAS (KXAN) — During the summer in the Gulf of Mexico, a ‘dead zone’ or hypoxic area forms where little to no oxygen in the water kills fish and other types of marine life.

What is the reason there’s so little oxygen?

According to the National Oceanic Atmospheric Administration, 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 the 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.

Some fish, shrimp and crab swim out of this area, but not all animals or fish can get away and the lack of oxygen stresses or kills them.

For the last few years, NOAA has issued a forecast for the size of the hypoxic area leading into summer. Summer 2021 is expected to have a ‘dead zone’ of around 4,880 square miles. That’s actually slightly smaller than the five-year average size of 5,400 square miles.

The largest hypoxic zone was in 2017 when 8,776-squre-miles of the Gulf of Mexico had little to no oxygen. These measurements have been mapped since 1985, but this is only the fourth year NOAA has made a ‘dead zone’ forecast.

“Understanding the effects of hypoxia on valuable Gulf of Mexico resources has been a long-term focus of NOAA’s research,” said Nicole LeBoeuf, acting director of NOAA’s National Ocean Service. “These forecasting models inform us of the potential magnitude of the Gulf of Mexico hypoxic zone that might impact living marine resources and coastal economies.”

According to NOAA, river discharge in May and the associated nutrient load to the Gulf of Mexico has been shown to be a major contributing factor to the size of the dead zone which forms each summer. In May 2021, discharge in the Mississippi and Atchafalaya rivers was about 2% below the long-term average between 1980 and 2020. The United States Geological Survey estimates that this smaller-than-average river discharge carried 90,500 metric tons of nitrate and 15,600 metric tons of phosphorus into the Gulf of Mexico in May alone. These nitrate loads were about 32% below the long-term average, and phosphorus loads were about 9% below the long-term average.

The USGS uses more than 3,000 stream gauges and sensors to measure the nutrients leading into the Gulf of Mexico to help get an idea the impact on the hypoxic zone.

While agricultural sources are the largest nutrient sources to the Gulf, urban areas, waste treatment and with natural sources also contribute significant nutrients to the Gulf of Mexico.

Hurricanes and tropical storms can impact the size of the hypoxic zone, as those storms mix ocean waters and add oxygen to de-oxygenated areas. Generally the improvement is just temporary as those waters start losing oxygen soon after storms move on.

NOAA will survey the Gulf of Mexico to confirm the size of this year’s dead zone and test how accurately the ‘dead zone’ forecast was.

The Interagency Mississipi River and Gulf of Mexico Hypoxia Task Force has set a goal to reduce the hypoxic zone to a five-year average measured size of 1,900 square miles. In order for this goal to be reached, nutrient reducing methods along the Mississippi River watershed need to work.

“The Hypoxia Task Force plays a critical role in managing nutrient loads in the Mississippi River Basin to reduce over time the size of the hypoxic zone,” said John Goodin, director of U.S. Environmental Protection Agency’s (EPA) Office of Wetlands, Oceans and Watersheds. “Through state leadership in implementing nutrient reduction strategies, support from EPA and other federal agencies, and partnerships with basin organizations and research partners, we will continue to tackle the challenge of Gulf hypoxia. This annual forecast will continue to inform our collective efforts.”

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