Gulf of Mexico 'dead zone' smaller than expected, still covers more than 3,000 square miles

This year's oxygen-starved zone in the Gulf of Mexico is smaller than forecast, but still larger than the goal set by a coalition of upstream governments. The National Oceanic and Atmospheric Administration (NOAA) announced on Wednesday that this "dead zone" is approximately 3,058 square miles, about 25% smaller than what was predicted in June.

The Mississippi River/Gulf of Mexico Hypoxia Task Force, a partnership formed in 1997 between federal and state agencies and the National Tribal Water Council, aims to reduce the dead zone to 1,900 square miles by 2035.

Experts say this year's decrease is largely due to low water flow on the Mississippi River and fluctuating ocean temperatures, rather than ongoing efforts to reduce harmful runoff.

The "dead zone," known scientifically as a hypoxic area which is unable to sustain life, occurs every summer due primarily to excess nutrient pollution from human activities throughout the Mississippi River watershed.

The Mississippi and Atchafalaya Rivers carry nutrients from fertilizer runoff used in agriculture down to the Gulf of Mexico, which stimulates an overgrowth of algae. The area stretches from the mouth of the Mississippi River westward and can reach as far as the Texas border.

As this algae eventually dies and decomposes, it sinks to the bottom and depletes oxygen from the deeper areas of the Gulf. The low oxygen levels kill bottom-dwelling sea creatures, such as clams and burrowing crabs, that cannot escape. It also alters the distribution of commercially harvested species like shrimp.

This year's hypoxic zone is the seventh smallest area on record since scientists began measuring the phenomenon in 1985. The largest zone was measured at 8,776 square miles in 2017.

The measurements were made during an annual survey cruise led by a team of scientists from Louisiana State University and the Louisiana Universities Marine Consortium.

Streamflow in the Mississippi and Atchafalaya Rivers during May was about 33% below the average since monitoring began in the 1970s, according to Lori Sprague, a program manager with the U.S. Geological Survey. Drought in the Midwest is one contributing factor for the smaller than average size of this year's hypoxic zone. Sprague said the nitrogen load was 42% below average while the phosphorus load was 5% below average.

Recent science, however, indicates that both nitrogen and phosphorus loads will need to be cut nearly in half to meet the 2035 goal, Sprague said. The task force has established an interim goal to reduce nitrogen and phosphorus loads by 20% in the river by 2025. The U.S. Geological Survey has shown that total nitrogen loads have decreased by 23% since the 1980s. However, phosphorus loads have increased.

Water temperature also affects the level of oxygen that is dissolved in the Gulf of Mexico. Nancy Rabalais, Ph.D. professor at Louisiana State University and LUMCON, who is the co-principal investigator on the survey cruise, said that warmer waters cannot hold as much oxygen as cooler waters. Despite record high ocean temperatures off the coast of Florida, the water temperatures in the deeper parts of the Gulf where the dead zone occurs were slightly lower.

"Surface waters were about 88 to 90 degrees Fahrenheit, which is a little bit warmer than last year, said Rabalais. "But the bottom waters, where the hypoxia occurs, were about 75 degrees."

Who has the authority to mitigate nutrient pollution?

The Environmental Protection Agency is restricted in its ability to control nutrient pollution because Congress exempted agriculture from the Clean Water Act of 1972. Since the 1950s, the amount of nitrogen in the Mississippi River watershed has tripled.

Nitrogen fertilizer is used to help increase crop yields but only half of it is taken up by the crops. U.S. farmers apply about 21 million tons of fertilizer annually, more than half of which is nitrogen. Federal funding from the Bipartisan Infrastructure Law will bring a total of $60 million to the task force's Gulf Hypoxia Action Plan to reduce nutrient runoff, to be divided between 12 states in the basin.

While some states plan to use their portion of this funding to institute more sustainable farming practices, the results of these new efforts won't be measurable for years. Louisiana will receive $4.1 million in funds over the next five years to implement state nutrient reduction and management strategies, according to a 2022 annual report.

"States across the Mississippi River watershed are focused on implementing their nutrient reduction strategies," said Iowa Secretary of Agriculture Mike Naig, who also serves as co-chair of the task force. "Hypoxia Task Force states are making water quality progress, but we are far from satisfied."

Programs are introduced on a state-by-state basis, such as in Missouri, which is establishing limits for facilities that discharge phosphorus into the river.

Improving the management of nutrient fertilizers so that they better align with crop needs has also been shown to decrease the amount of surplus phosphorus in the basin.

Agricultural runoff, however, is only part of the problem. In April, 13 environmental groups, including Healthy Gulf in Louisiana, filed a federal lawsuit to try to force EPA to strengthen water pollution limits. The suit alleges standards for fertilizer plants have not been updated since 1986, while the standards for oil refineries have not been updated since 1985.

The Environmental Integrity Project released a report in January that found ExxonMobil Baton Rouge Refinery released 182,238 pounds of nitrogen pollution into the Mississippi River in 2021. During the same year, 14 refineries across Louisiana released a total of 2.4 million pounds of nitrogen into waterways, according to the lawsuit.

In Donaldsonville, La., along the west bank of the Mississippi River, CF Industries operates the world's largest ammonia production facility, a 1,400-acre nitrogen fertilizer plant that produces nearly eight million tons of nitrogen products annually. This plant alone is the largest source of industrial greenhouse gas emissions in Louisiana.

"And we're seeing two more ammonia plants that are projected to go online: the expansion of CF Industries and the Air Products ammonia plant," said Matt Rota, senior policy director at the environmental advocacy group Healthy Gulf. "These facilities are going to be discharging into the Mississippi River, putting more nitrogen pollution into the river."

This story is a product of the Mississippi River Basin Ag & Water Desk, an independent reporting network based at the University of Missouri in partnership with Report for America, funded by the Walton Family Foundation. Sign up to republish stories like this one for free.