‘Dead zone’ volume more important than area to fish, fisheries

Dubravko Justic, the Texaco Distinguished Professor in the LSU Department of Oceanography & Coastal Sciences, and Research Associate Lixia Wang recently co-authored a study suggesting that measuring the volume rather than the area of the Gulf of Mexico’s dead zone, is more appropriate for monitoring its effects on marine organisms.

The dead zone, a hypoxic zone, is a region of low oxygen that results from runoff of high nutrients, such as nitrogen and phosphorus, often found in fertilizer, flowing from the Mississippi River into the coastal ocean. It is the largest recurring hypoxic zone in the U.S., occurring most summers, and is located off the coast of Louisiana. This nutrient pollution, coupled with other factors, is believed to have a negative impact on fisheries because it depletes the oxygen required to support most marine life in bottom and near-bottom waters.

Since 2001, stakeholders have used hypoxic area measurements to set goals for limiting or reversing its size, but this new study shows that the hypoxic volume appears more responsive to reductions in nitrogen flowing into the northern Gulf of Mexico than the hypoxic area. The researchers’ model simulations indicate that even with a 25 percent nitrogen load reduction, the thickness of the hypoxic layer in the northern Gulf of Mexico decreases markedly, and hypoxia remains localized to a relatively thin layer near the bottom that most fish and other mobile organisms can more effectively avoid.

Justic believes this should be considered when reviewing and potentially setting new hypoxia management goals.

“Understanding variability in hypoxic volume is relevant to assessing the effects of hypoxia on fish and fisheries, such as enhanced susceptibility to fishing due to an increased aggregation of fish avoiding hypoxic waters,” Justic said.

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LSU is at the forefront of coastal research and education with more than 200 faculty exploring and collaborating on every continent. This study was published in the journal Environmental Research Letters with co-authors from the University of Michigan, North Carolina State University and NOAA Southeast Fisheries Science Center in addition to the LSU College of the Coast & Environment. The full study is available here: https://iopscience.iop.org/article/10.1088/1748-9326/aaf938/meta.