Marine ‘dead zones’ are hypoxic (low-oxygen) areas in the world’s oceans and large lakes, caused by excessive nutrient pollution from human activities coupled with other factors that deplete the oxygen required to support most marine life in bottom and near-bottom marine environments. There are more than 500 dead zones around the world, with the northern Gulf of Mexico dead zone being the second largest human-caused coastal hypoxic area in the world.
Scientists have predicted that this dead zone in the northern Gulf of Mexico will become larger than the state of Connecticut by the end of July. It will cover about 6,620 square miles of the bottom of the continental shelf off Louisiana and Texas.
Although this forecast has been the average size for the past 31 years, it is more than three times larger than the goal outlined by the Hypoxia Action Plan, which is about 1,930 square miles.
The cause for this huge dead zone results from nutrient run-off from the Mississippi River watershed, particularly nitrogen and phosphorus, fertilizing 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. The Mississippi River and its tributaries drain 41 percent of the United States, carrying the nutrients that cause the Gulf’s dead zone.
Efforts to reduce the nitrate loading into the Gulf have not yet demonstrated success at the watershed scale. The National Oceanic and Atmospheric Administration, or NOAA, which funded the scientists’ research, estimates that the dead zone costs U.S. seafood and tourism industries $82 million a year. The impact could be devastating to the Gulf’s seafood industry, which accounts for more than 40 percent of the nation’s seafood. Louisiana is second in seafood production only to Alaska.
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. The new report and forecast will be available June 7.
“The oceans warm a little more each year and currents change, making new observations a necessity. Model calibration is not a fixed phenomenon,” Turner said.
“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 6,316 square miles.
In river basins throughout the Mississippi River watershed, organizations like the Nature Conservancy are working with farmers, university scientists and departments of agriculture to promote more effective and efficient use of fertilizers and reduce the amount of nutrients that enters America’s rivers from other sources. In many cases better nutrient management also leads to better soil management, which reduces surface runoff and flooding and maintains or enhances agricultural productivity.
Satellite image showing runoff from Mississippi watershed into the Gulf of Mexico. Credit: NASA Scientific Visualization Studio.