Mark Schleifstein | NOLA.com 3 June 2021

The 2021 summertime “dead zone” along the Louisiana and Texas coast lines — an area of low to no oxygen that can kill fish and bottom-living organisms — is predicted to cover 4,880 square miles of the Gulf of Mexico’s water bottom, according to NOAA scientists.

That’s almost as large as the five-year average size of the low oxygen zone, which is 5,400 square miles. But it’s significantly less than the record-setting 8,776 square miles of hypoxic water measured in 2017 by scientists with Louisiana State University and the Louisiana Universities Marine Consortium.

Researchers will conduct their annual oxygen measurement cruise in the last week of July and the first week of August to determine the dead zone’s size. If tropical storms don’t churn up the Gulf of Mexico and mix oxygen back in before then, this year’s dead zone will be almost three times as large as the state of Rhode Island.

The low-oxygen area is directly linked to the nutrients that run into the Mississippi and Atchafalaya rivers and are carried into the Gulf. The nutrients include nitrogen and phosphorus that are used as fertilizer on farmland and that come from untreated sewage in the 32 states and parts of two Canadian provinces in the watershed.

When the freshwater carrying the nutrients reaches the Gulf, it creates a layer over more dense salt water. The nutrients trigger blooms of algae that eventually die and sink to the bottom, where the use up oxygen as they decompose.

That low-oxygen condition, called hypoxia, will kill bottom-living organisms, such as crabs and worms, 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.

In estimating the size of the dead zone, scientists measured the amount of nutrients in the river water and the amount of water expected to enter the Gulf.

This year’s estimated dead zone is expected to be about 2 1/2 times larger than the goal set by the federal government and states along the Mississippi River in an effort to reduce environmental damage.

The Interagency Mississippi River and Gulf of Mexico Hypoxia Task Force first set the goal at 1,900 square miles when it was first created in 1998, and renewed that goal ten years later.

However, despite millions of dollars spent by the federal government and states on voluntary nutrient reduction measures, there has been no significant reduction in the average amount of nutrients carried by the river.

“The concentration of nitrate has been declining over the last 20 years, but the increase in river discharge means that the total loading has remained the same in recent decades or is increasing,” said LSU researchers Eugene Turner and Nancy Rabalais in their own preseason hypoxia forecast, also released on Thursday.

The NOAA forecast relies on preseason forecast information gathered by its own scientists and those of the U.S. Geological Survey, and from teams at LSU, University of Michigan, the Virginia Institute of Marine Sciences, North Carolina State University and Dalhousie University.

Rabalais has headed the annual monitoring cruise effort since 1985.

In 2020, the cruise measured the dead zone at only 2,117 square miles, the third-smallest since mapping began. But that low size was the result of Hurricane Hanna mixing oxygen-loaded surface water into the bottom waters.

The May 2020 estimate by NOAA was for a low oxygen area of 6,400 square miles, while LSU’s team predicted it would be 7,769 square miles.

The tropical depression that became Hanna formed on July 23 about 210 miles south southeast of the mouth of the Mississippi, and moved west towards a landfall near Padre Island, Texas, on July 25 as a hurricane with top winds of 90 mph.

That was just as last year’s cruise was beginning.

https://www.nola.com/news/environment/article_67b485c0-c487-11eb-aaaf-3b9808de3fd0.html