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Larger-Than-Average Gulf of Mexico ‘Dead Zone’ Measured

Today, NOAA-supported scientists announced that this year’s Gulf of Mexico “dead zone”— an area of low to no oxygen that can kill fish and marine life — is approximately 6,334 square miles, or equivalent to more than four million acres of habitat potentially unavailable to fish and bottom species.

The average hypoxic zone over the past five years is 5,380 square miles, which is 2.8 times larger than the 2035 target set by the Hypoxia Task Force. Since records began in 1985, the largest hypoxic zone measured was 8,776 square miles in 2017.

Scientists at Louisiana State University and the Louisiana Universities Marine Consortium led the annual dead zone survey during a research cruise from July 25 to August 1 aboard the R/V Pelican.

“The distribution of the low dissolved oxygen was unusual this summer,” said Nancy Rabalais, Ph.D., professor at Louisiana State University and LUMCON who is the principal investigator. “The area from the Mississippi River to the Atchafalaya River, which is usually larger than the area to the west of the Atchafalaya, was smaller. The area to the west of the Atchafalaya River was much larger. The low oxygen conditions were very close to shore with many observations showing an almost complete lack of oxygen.”

Surface waters were low in salinity across most of the study area, indicating that high freshwater runoff from the Mississippi River watershed affected the area. River discharge measured above normal for the three weeks prior to the research cruise.

The role of chief scientist was shared by Rabalais, Cassandra Glaspie, Ph.D., of LSU, and Gina Woods, LUMCON research associate. The 2021 shelf wide cruise had a full complement of scientists, which was curtailed in 2020 due to COVID restrictions.

In June, NOAA forecasted an average-sized hypoxic zone of 4,880 square miles, based primarily on the Mississippi River discharge and nutrient runoff data from the U.S. Geological Survey. With average- to below-average discharge and nutrient loading this spring, the models predicted an average hypoxic zone to form during the time of the cruise. The forecast models assume typical weather conditions and do not have capacity to factor in storms or other wind events, which can disrupt the hypoxia zone around the time of the cruise. While the model results underestimated the measured size of the zone this year, they were within the expected margin of uncertainty for the forecast and provide further evidence of the robustness of the models to relate nutrient inputs to observed hypoxia size in the summer.

Excess nutrients stimulate algae growth

Each year, excess nutrients from cities, farms and other sources in upland watersheds drain into the Gulf and stimulate algal growth during the spring and summer. The algae eventually die, sink and decompose. Throughout this process, oxygen-consuming bacteria decay the algae. The resulting low oxygen levels near the bottom are insufficient to support most marine life, rendering the habitat unusable and forcing species to move to other areas to survive. Exposure to hypoxic waters has been found to alter fish dietsgrowth rates, reproduction, habitat use, and availability of commercially harvested species like shrimp.

“NOAA and its partners use data from this cruise to help refine models and more accurately simulate how river discharge, nutrient loads, and oceanographic conditions influence hypoxic conditions in the Gulf and affect living resources,” said Nicole LeBoeuf, Assistant Administrator for NOAA’s National Ocean Service. “By understanding the scale and effects of these hypoxia events, we can better inform the best strategies to reduce its size and minimize impacts to our coastal resources and economy.”

Read more at NOAA

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