NOAA forecasting below-average Gulf of Mexico dead zone

Mark Milam


HOUSTON (ICIS)–The National Oceanic and Atmospheric Administration (NOAA) is forecasting the 2023 seasonal Gulf of Mexico hypoxic zone, frequently referred to as the ‘dead zone’, to be below average, with the estimate at approximately 4,155 square miles.

The average over the 36-year history of dead zone measurements is at 5,364 square miles and since records began, the largest hypoxic zone measured was 8,776 square miles in 2017.

The US Geological Survey (USGS) provided Mississippi river discharge and nutrient loading data for the month of May, which were both key factors used to estimate the size of the dead zone.

Last month, discharge in the Mississippi and Atchafalaya rivers was about 33% below the long-term average between 1980 and 2022, and the nitrate and phosphorus loads were about 42% and 5% below the long-term averages, respectively.

The Gulf’s hypoxic (low oxygen) and anoxic (oxygen-free) zones are caused by excess nutrient pollution, which researchers attribute to being primarily from human activities such as agriculture and wastewater occurring in the watershed.

It was first documented in 1985 off the coast of Louisiana and there has been research that consistently identifies farmland fertilizer run-off as being the main cause of the dead zone.

Yet there is further evidence which demonstrates that urban areas, human waste treatment, precipitation and atmospheric dust as well as natural sources also contribute large amounts.

With excess nutrients in the ocean there then becomes an overgrowth of algae, which sinks and decomposes. This causes low oxygen levels which are then insufficient to support most marine life and habitats.

“NOAA hypoxia forecasts aim to provide coastal managers and stakeholders with the information they need to take proactive action to mitigate the impacts of hypoxic events,” said Nicole LeBoeuf, NOAA Assistant Administrator of National Ocean Service.

“These forecasts also help managers set nutrient reduction targets necessary to reduce the frequency and magnitude of future dead zones.”

The Interagency Mississippi River and Gulf of Mexico Hypoxia Task Force have set a goal of reducing the dead zone to 1,900 square miles by 2035.

“With new investments from the Bipartisan Infrastructure Law, the Hypoxia Task Force is taking action to accelerate nutrient load reductions from the Mississippi River Basin and reduce the size of the hypoxic zone,” said Brian Frazer, US Environmental Protection Agency (EPA) director of the Office of Wetlands, Oceans, and Watersheds.

“Together the task force will continue to tackle the challenge of Gulf Hypoxia. This annual forecast is a key metric that informs our collective efforts.”

To confirm the size of the hypoxic zone and refine the forecast models, a NOAA-supported monitoring survey is conducted each summer, with results released in early August.


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