Mark Schleifstein / NOLA.com 4 August 2020

The 2020 summer low-oxygen “dead zone” along the Louisiana Gulf coastline covered only 2,117 square miles, an area just a bit larger than the state of Rhode Island and the third-smallest dead zone since mapping began in 1985, according to researchers with the Louisiana Universities Marine Consortium and the National Oceanic and Atmospheric Administration.

Give the winds and waves of Hurricane Hanna credit for mixing oxygen-rich surface water into the coastal area’s bottom waters this year, resulting in the smaller dead zone, said Nancy Rabalais, a Louisiana State University marine researcher who oversees the annual mapping program.

The smaller low-oxygen area did not result from less nutrient pollution being carried down the Mississippi River to the Gulf this spring, though the river’s flow did slacken just before the monitoring cruise after several months of high flow, she said.

With the drop, the five-year average dead zone size is 5,408 square miles, which is still 2.8 times larger than the goal set by the federal-state Hypoxia Task Force to reduce the size of the dead zone to a five-year average of 1,930 square miles by 2035.

That five-year average could have been significantly higher were it not for Hanna. It’s the second year in a row that a Gulf tropical system is believed to have reduced the size of the low-oxygen area.

In May, NOAA scientists had estimated the dead zone would cover 6,400 square miles of Gulf water bottoms, while an LSU team estimated it would be 7,769 square miles, both based on data collected by the U.S. Geological Survey showing the amount of nutrients in the Mississippi River as it enters Louisiana in the spring.

The low-oxygen condition is caused by nutrient-rich freshwater collected by the river’s vast watershed — which includes parts of 32 states and two Canadian provinces — and delivered to the Gulf of Mexico during the spring and summer.

The freshwater creates a layer that sits atop the salty Gulf waters until it’s mixed by tropical storms in the summer or frontal systems in the fall. The nutrients — mostly nitrogen and phosphorus from Midwest agriculture and from sewage wastes — feed huge blooms of algae that grow in surface waters but eventually die and sink to the bottom, where they decompose and use up oxygen in the saltier, deeper water.

The result is often huge areas of Gulf bottom waters containing from zero to 2 parts per million of oxygen, a condition called hypoxia. It kills bottom-living organisms that form the base of the Gulf food chain, and forces fish, crabs and shrimp to flee.

USGS scientists estimated that in May, the nutrient-rich Mississippi and Atchafalaya rivers were discharging 30% more water than the long-term average between 1980 and 2019. The nitrate in that water was about 2% above the long-term average, while phosphorus represented a 25% increase.

But then Hanna formed in the Gulf of Mexico on July 23 and moved west towards a July 25 Texas landfall across the northern Gulf of Mexico, with its winds and waves moving across the Louisiana coastline both before and after that landfall, mixing the water column along the coast.

The research cruise measured oxygen in the water column at numerous locations in the Gulf between July 25 and Aug. 1.

The 2019 dead zone, which was partly disrupted by Hurricane Barry, but was fed by the longest modern high-water period in the lower Mississippi River’s history, measured 6,952 square miles. It was the eighth-largest in the history of the cruises.

In attempting to reduce the low-oxygen area’s size, the federal-state task force relies on voluntary programs run by the states within the river watershed, sometimes supported by federal conservation grants. The U.S. Environmental Protection Agency has not adopted stringent regulations setting limits on the amount of nutrients allowed to enter Mississippi tributaries.

In addition to the economic effects to commercial fisheries in the Gulf, the nutrients carried by the river cause similar low-oxygen zones in inshore areas, including in wetlands in St. Bernard and Plaquemines parishes when the Bonnet Carre Spillway is opened.

Commercial fishers opposing Louisiana’s efforts to build major sediment diversions below New Orleans to rebuild wetlands also warn that the use of those diversions will add nutrient pollution to those areas.

In a Tuesday telephone news conference about the cruise sponsored by NOAA, which funds the research, officials with NOAA, the EPA and the secretary of the Iowa Department of Agriculture stressed that this year’s cruise results represent only a single data point in the task force’s efforts.

“Models are models; estimates are estimates,” said Mike Naig, the Iowa agriculture secretary. He said the partnerships between farmers and federal and state regulators aimed at reducing the nutrients are beginning to result in successes, although the reduction in the dead zone’s size is still many years away.

Indeed, the task force’s 1,930-square-mile goal has been a moving target. In 2008, the task force said the goal would be met in 2015. In 2016, it pushed back the date to 2035, and added a provision calling for a 20% reduction in nitrogen and phosphorus in the river by 2025.

In 2012, environmental groups filed suit against the EPA, charging the agency had failed to conduct a study it believed would result in new regulations setting limits on “non-point” pollution — runoff from farmland, rather than emissions from a pipe — and a federal judge in New Orleans agreed. But in 2015, the U.S. 5th Circuit Court of Appeals overruled the lower court judge, finding that the EPA had the discretion to skip the study.

“We’re a long way from where we want to be, ultimately,” said Naig, a Republican, who is co-chair of the task force. “But waving a regulatory wand will not solve the issue, either.”

The problem with using the size of the dead zone to justify more regulations is the risk of losing the cooperation of both the farmers and state officials participating in the task force nutrient reduction process, said David Ross, the EPA’s assistant administrator for water and the task force’s other co-chair.

“We will watch the global trend over time and we are confident you will see a reduction,” Ross said.

“If we continue to focus only on a single endpoint and not on successes at the local scale, we’ll risk disenfranchisement of the folks we need to solve the problem,” Ross said. “If you lose the partnership at the local scale, we will not get the results we need on the local scale.”

https://www.nola.com/news/environment/article_02d39f24-d66c-11ea-9828-e32433b3b786.html