Mark Schleifstein / 7 June 2020

The summertime low-oxygen “dead zone” in the Gulf of Mexico is expected to cover at least 6,700 square miles along the Louisiana and eastern Texas coasts at the end of July, according to a federal forecast based on estimates developed by five research teams studying the effects of fertilizer and other nutrients on Gulf waters.

The estimates by the National Oceanic and Atmospheric Administration predict as few as 6,551 and as many as 7,889 square miles of Gulf ocean floor will have oxygen levels of 2 parts per million or less. The high-end estimate would cover an area larger than the state of New Hampshire.

That low-oxygen condition, called hypoxia, will kill bottom-living organisms, such as crabs, and force creatures that can escape, including many commercial fish species, into deeper water. Scientists have also found that such low levels of oxygen reduce some fish species’ reproductive capability and can reduce the average size of shrimp.

LSU oceanography and coastal sciences professor Eugene Turner and marine science professor Nancy Rabalais, who make up one of the research teams, estimated the dead zone will be 7,769 square miles, four times larger than a long-standing governmental goal to shrink it to an average of 1,900 square miles.

Estimates of the dead zone’s size are largely based on the amount of nitrogen and phosphorus and other nutrients measured in Mississippi River water above Baton Rouge in May. The nutrients include agricultural fertilizer entering the river in rainwater runoff from Midwestern farms, animal wastes from fields and from concentrated feedlot operations, and sewage from urban treatment systems and private septic tanks in the huge Mississippi River watershed.

The river’s watershed includes parts of 31 states and two Canadian provinces and comprises about 40 percent of the land in the continental United States.

When the nutrient-rich freshwater flows into the Gulf of Mexico in the spring and early summer from the mouths of the Mississippi and Atchafalaya rivers, it forms a separate lighter layer of water atop saltier, heavier Gulf waters.

Once there, the nutrients feed huge blooms of tiny algae that eventually die and sink to the Gulf floor. There, they decompose, using up the oxygen in the lower layer of salt water. Unless a tropical storm or hurricane comes along to stir up the water, hypoxia conditions usually remain in place until late summer or fall, when east-moving frontal systems mix the layers.

Since 1985, Rabalais has overseen an annual cruise during the last week of July and first days of August that maps the dead zone by taking water samples and measuring the oxygen, nutrient and algae content at various locations along the coast.

The dead zone has been a focus since soon after Rabalais’ cruises outlined its effects on Louisiana’s Gulf Coast in the mid-1980s, linking its increased size to the dramatic increase in the use of modern fertilizers that began in the early to mid-20th century.

Since 1997, a federal-state hypoxia reduction task force has struggled to reduce nutrient pollution through a variety of measures, including education programs and computer-aided farming techniques aimed at reducing fertilizer use, and grants used to build wetlands or grassy nutrient collection strips along the Mississippi’s vast network of tributaries to capture and consume the nutrients.

In 2001, the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force released an action plan that set a goal of reaching a five-year average of only 1,900 square miles of low-oxygen waters by 2015 through voluntary efforts.

In 2015, when the dead zone was nearly 6,500 square miles, the task force announced it would extend the time for reaching its goal to 2035, but set an interim target of reducing the river’s nutrient load by 20 percent by 2025.

But changing patterns in crop production, including federal support for the use of corn to support the development of biofuels, have resulted in uneven success in limiting fertilizer use. That, plus a six-year period of unusually high water, has meant little change in the size of the dead zone.

Under the federal Clean Water Act, states are required to identify whether the amount of nutrients in individual segments of water bodies, including tributaries, meet human health or environmental regulatory limits.

If not, the states are supposed to identify the sources of the nutrients and get them to reduce their releases.

Between 2009 and 2016, nearly $3.8 billion in federal grants were provided to task force states to assist in their nutrient reduction programs.

But measurements by the U.S. Geological Survey through 2020 show the target is nowhere near being met, although both total nitrogen and total phosphorus levels have been falling.

This year, the May total nitrogen loads were about the same as the average for all years between 1980 and 2019, while total phosphorus loads were 25% above their average.

In Louisiana and Mississippi, the Mississippi River’s nutrient pollution levels during a record extended flood season in 2019 also have led to dramatic fisheries losses because of river water diverted through the Bonnet Carre Spillway into Lakes Pontchartrain and Borgne in Louisiana and the Mississippi Sound.

In their forecast, Turner and Rabalais point out that such dead zones have become a worldwide problem, with more than 500 low-oxygen areas now being monitored by scientists.

But, they said, “The dead zone off the Louisiana coast is the second largest human-caused coastal hypoxic area in the global ocean.”

In a 2019 scientific review of the hypoxia problem, Rabalais and Turner said lowering the hypoxia levels will require more successful efforts to reduce the use of artificial fertilizers, reducing the use of single or dual-crop growing strategies for farms, reduced use of intensified animal husbandry measures, reducing the release of untreated wastewater and curbing consumption of fossil fuels.

“There are no easy societal shifts to a less consumptive lifestyle, nor is it an easily achieved political outcome,” they said. But they also pointed to recent successes by several foreign countries in reducing nutrient pollution in the Baltic and North seas and several states in reducing nutrients entering Chesapeake Bay.

“Recovery pathways may take years or even decades to reverse the severity and size of hypoxic conditions adjacent to the Mississippi River outflow,” they wrote. “This calls for a serious commitment to improve coastal water quality (including improvement of hypoxic conditions) on an individual, community, and political basis.”