Gulf of Mexico hypoxic ‘dead zone’ forecasted to exceed 17 250 square km (6 660 square miles)

The June 2018 forecast of the size of the hypoxic zone also known as the 'dead zone' in the northern Gulf of Mexico for late July 2018 is that it will cover 17 250 km² (6,660 mi²) of the bottom of the continental shelf off Louisiana and Texas. The 95% confidence interval is that it will be between 14 628 and 19 727 km² (5 648 and 7 617 mi²). While there are more than 500 dead zones around the world, the northern Gulf of Mexico dead zone is the second largest human-caused coastal hypoxic area in the world.

Hypoxia, or low oxygen, is an environmental phenomenon where the concentration of dissolved oxygen in the water column decreases to a level that can no longer support living aquatic organisms. Hypoxic areas, or 'Dead Zones,' have increased in duration and frequency across our planet’s oceans since first being noted in the 1970s. The largest hypoxic zone currently affecting the United States, and the second largest hypoxic zone worldwide, is the northern Gulf of Mexico adjacent to the Mississippi River.

The latest estimate is based on the assumption that there are no significant tropical storms occurring in the two weeks before the monitoring cruise, or during the cruise. If a storm does occur, then the size of the zone is predicted to be 70% of the predicted size without the storm, equivalent to 12 075 km² (4 662 mi²). The predicted hypoxic area is slightly larger than the State of Connecticut (14 357 km² / 5 543 mi²) and 4% larger than the average of 16 357 km² (6 315 mi²)

Although this forecast has been the average size for the past 31 years, it is about three and a half times larger than the goal outlined by the Hypoxia Action Plan, which is about 4 998 km² (1 930 mi²). Efforts to reduce the nitrate loading have not yet demonstrated success at the watershed scale.

Every year LSU Department of Oceanography & Coastal Sciences professors and Louisiana Universities Marine Consortium scientists Nancy Rabalais and Eugene Turner conduct a research cruise to measure the dead zone, which has little or no oxygen in the bottom waters. Turner and Rabalais then use these data in their computer models to predict its size in the summer.

Nutrients from the Mississippi River watershed, particularly nitrogen and phosphorus, fertilize the Gulf of Mexico's surface waters to create excessive amounts of algae. When the algae decomposes in the deepest parts of the ocean, it leads to oxygen distress and can even kill organisms in the Gulf of Mexico's richest waters. These low oxygen conditions threaten living resources including fish, shrimp and crabs, which humans depend upon for food and industry.

This image from a NOAA  Environmental Visualization Lab animation shows how runoff from farms (green areas) and cities (red areas) drains into the Mississippi. This runoff contains an overabundance of nutrients from fertilizers, wastewater treatment plants, and other sources. This nutrient pollution eventually ends up in the Gulf of Mexico, leading to the formation of hypoxic "dead zones."

"This means that the impacts of water quality changes upstream in the Midwest affect our coast – directly," Rabalais said.

The dead zone occurs year-round, but it is most persistent and severe in spring and summer. Various computer models use the May nitrogen load of the Mississippi River as the main driving force to predict the size of this hypoxic zone in late July. If a storm occurs, then the size of the dead zone is predicted to be reduced to about 16 358 km² (6 316 mi²).

Featured image: This image from a NOAA Environmental Visualization Lab animation shows how runoff from farms (green areas) and cities (red areas) drains into the Mississippi. This runoff contains an overabundance of nutrients from fertilizers, wastewater treatment plants, and other sources. This nutrient pollution eventually ends up in the Gulf of Mexico, leading to the formation of hypoxic "dead zones."