Hurricane Hanna Responsible for Smaller Gulf of Mexico Dead Zone

Map of measured Gulf of Mexico hypoxia zone, July-August 2020. (LUMCON/NOAA)

(CN) — This year’s so-called dead zone in the Gulf of Mexico is the third smallest ever recorded thanks to Hurricane Hanna, scientists said Tuesday, despite a June prediction that it would be larger than usual.

The dead zone stretches from the mouth of the Mississippi River into Texas and is created by low oxygen levels in water, which is called hypoxia. The low oxygen is linked to high concentrations of nutrients found in agricultural fertilizers, such as nitrogen and phosphorous, and other nutrients carried into the Gulf of Mexico by the Mississippi River.

Though the dead zone is in the Gulf of Mexico, the contamination that creates hypoxia is largely the result of accumulated nutrients in sediment that wash into the river from midwestern farmland along its extensive watershed.

Once the nutrients land in the Gulf of Mexico, they feed blooms of algae. As the algae sink to the bottom, die and decompose, they use up the oxygen in the water. Oxygen levels below two parts per million can kill organisms on the gulf floor that are food sources for larger oceanic species, such as fish and shrimp.

This year’s dead zone measured 2,116 square miles, or 1.4 million acres, during the annual check at the end of July, according to the National Oceanic and Atmospheric Administration, or NOAA.

The June estimate placed it at 6,700 square miles, or roughly the size of Connecticut and Delaware combined. Though that sounds big, it would not have been as large as the biggest dead zone ever, which was recorded in 2017 at 9,776 square miles.

This year, Hurricane Hanna churned the waters, dispersing the large swaths of algae growth and intermixing oxygen from other waters. A similar effect happened last summer following Hurricane Barry. By estimates last year, the dead zone would have spanned 7,800 square miles or more, but because of the hurricane the actual number turned out to be 6,952.

David Kidwell, director of the competitive research program at NOAA’s National Centers for Coastal Ocean Science, said during a teleconference Tuesday that the reduced dead zone this year has to do entirely with the hurricane and “nothing has changed that dramatically agriculturally that you would see that much of an impact in the gulf.”

June’s forecast that predicted a larger-than-average hypoxic zone relied primarily on nutrient data supplied by the U.S. Geological Survey that indicated heavy discharge of nutrient runoff in the Mississippi River. 

A hypoxia task force that includes federal and state agencies as well as tribal governments has worked over the past 20 years to reduce agricultural runoff and emphasize water quality in an effort to address the problems contributing to the dead zone from state to state along the Mississippi and its tributaries. So far, the task force has reported much success even while the successes appear to have little ultimate effect on the volume of nutrient discharge in annual spring runoffs into the Mississippi River.

By 2035, the task force has said it would like the dead zone’s five-year average to measure 1,900 square miles, which is still 200 square miles less than this year’s unusually small dead zone.

Mike Naig of the Iowa Department of Agriculture and Land Stewardship, who co-chairs the task force, said during the teleconference that the dead zone is part of a natural flow and changes size from year to year.

Even while the size of the dead zone fluctuates each year, Naig said “each state in the task force is committed to and has committed funding to address” the issue of nutrient runoff.

Naig said his work as part of the task force in his state involves “empowering people to enjoy economic prosperity up and down the river” while also learning to conserve water and when to use fertilizer. He said improvements made now may take years to show an impact on the dead zone.

Environmental advocacy groups, headed by New Orleans-based Gulf Restoration Network, filed a petition in 2008 asking the Environmental Protection Agency to institute nutrient runoff standards for states along the Mississippi.

The EPA denied the petition three years later, saying that while agrees that “nutrient loadings to the Mississippi River and its tributaries are both harming upstream water quality and contributing significantly to hypoxia … in the Gulf of Mexico,” it “does not believe that the comprehensive use of federal rulemaking authority is the most effective or practical means of addressing these concerns at this time.”

An EPA spokesman said Tuesday that making slow and steady progress in agricultural practices from state to state that is voluntary, rather than forced, will have a longer lasting impact on diminishing the size of the dead zone.

“There is a reason that we set the goal for 2035. We will watch the global trend overtime. If we rush, we will get disinvestment from the people we need to help us. If you lose the partnership at the local scale, you will not get the help you need at a global scale,” he said.

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