This year’s area of low oxygen in the Gulf of Mexico is shrinking — but not as fast as hoped, the National Oceanic and Atmospheric Administration (NOAA) said on Wednesday.

The “Dead Zone” is approximately 3,058 square miles, as measured last week.

That’s about a quarter smaller than predicted at the beginning of the summer, but is larger than the target goal set by the Mississippi River/Gulf of Mexico Hypoxia Task Force, a federal-state-tribal partnership launched 26 years ago to reduce the size and severity of this low-oxygen zone.

But experts say the slight decrease was not likely due to ongoing efforts to reduce the harmful runoff carried downstream by the Mississippi River. Instead, it’s largely due to low water flow on the Mississippi and to fluctuating ocean temperatures. 

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

One of the biggest offenders is fertilizer that runs off farmers’ fields into the Mississippi and Atchafalaya Rivers, which carry the nutrients to the Gulf, where they stimulate an overgrowth of algae that can stretch 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. It also alters the distribution of commercially harvested species like shrimp. This year, because of the Dead Zone, almost two million acres of habitat are potentially uninhabitable for fish and other species. 

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

The current five-year average is 4,347 square miles. That has to be cut in half in the next 12 years to meet the goal set by the Hypoxia Task Force: 1,900 square miles or less, by the year 2035. 

This year’s Dead Zone 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 reduced by 48% to meet the 2035 goal, Sprague said. The task force’s interim goal is to cut nitrogen and phosphorus loads by 20% in the river by 2025. 

There has been progress since measurements began. Total nitrogen loads have decreased by 23% since the 1980s, according to the U.S. Geological Survey. However, phosphorus loads have increased.

Water temperature also affects the level of oxygen dissolved in the Gulf of Mexico. Warmer waters cannot hold as much oxygen as cooler waters, said Nancy Rabalais, Ph.D. professor at Louisiana State University and LUMCON, the co-principal investigator on the survey cruise. 

Despite record high ocean temperatures off the coast of Florida, the water temperatures were slightly lower in the deeper parts of the Gulf where the Dead Zone occurs.

“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.” 

Reducing nitrogen runoff, from fertilizer — and refineries.

The Environmental Protection Agency (EPA) is restricted in its ability to control nutrient pollution because Congress exempted agriculture from the Clean Water Act of 1972. Since the 1950s, levels of nitrogen – a key component of fertilizer – in the Mississippi River watershed have tripled. 

U.S. farmers apply about 21 million tons of fertilizer annually, more than half of which is nitrogen. Plants can only use half of what is applied. The rest runs off. 

A key part of the task force’s Gulf Hypoxia Action Plan is reducing nutrient runoff. Federal infrastructure legislation provides $60 million to the plan’s implementation: $12 million per year over five years, to be divided between 12 states in the Mississippi River basin. 

Some states, including Louisiana, plan to use the federal money to institute more sustainable farming practices. Over the next five years, Louisiana plans to spend its $4.1 million to cut and manage nutrients, according to a 2022 annual report.

But nitrogen in Louisiana also comes from the fossil-fuel industry. 

In April, 13 environmental groups, including Healthy Gulf in Louisiana, filed a federal lawsuit against the EPA, alleging that the agency’s water-pollution-control standards are outdated. Standards for oil refineries have not been updated since 1985, according to the suit, while the standards for fertilizer plants have not been updated since 1986. 

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, 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. 

The CF Industries plant is the largest source of industrial greenhouse gas emissions in Louisiana

Two more ammonia plants are projected to go online in coming years: expansions of CF Industries and the Air Products ammonia plant, said Matt Rota, senior policy director at the environmental advocacy group Healthy Gulf. 

And that could increase future Dead Zones. “These facilities are going to be discharging into the Mississippi River,” Rota said, “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