Scientists predict that a “dead zone” in the Gulf of Mexico could expand to between 8,500 and 9,421 square miles this year, roughly the size of New Jersey and Delaware combined. Dead zones are oxygen-depleted, lifeless expanses of water. The dead zone in the Gulf, 60 miles off the coasts of Louisiana and Texas, is one of the largest in the world and has more than doubled since the 1980s. In 2002, a staggering 8,400 square miles of the Gulf was “hypoxic” or lacked sufficient oxygen for most marine life to live.The Gulf’s dead zone is largely caused by agricultural waste like nitrate- and phosphate-containing fertilizer from the Midwest Corn Belt, which is carried by rainwater into the Mississippi River and flushes into the Gulf. Other toxins in the dead zone’s runoff include sewage, animal waste and car exhaust. “The Mississippi is a drainage for a third of the country and there are all kinds of sewage-treatment plants and factories discharging chemicals into the river,” Charles Jagoe, an environmental toxicologist at Florida A&M University, said in a National Geographic News story.
Record flooding along the Mississippi this spring has caused twice the amount of average runoff to drain into the Gulf, as this year’s high corn prices encouraged additional planting that resulted in excess amounts of fertilizer. And the U.S. Geological Survey (USGS) said the 164,000 tons of nitrogen that reached the Gulf this May was 35% higher than average.
“Stream flows were nearly double normal during May, delivering massive amounts of nutrients to the Gulf, and that’s what drives the dead zone,” Donald Scavia, director of the University of Michigan Graham Sustainability Institute, told the school’s paper. The University of Michigan contributed to a team of National Oceanic and Atmospheric Association (NOAA)-supported scientists in predicting this year’s dead zone expansion in the Gulf, as did the http://www.lumcon.edu”>Louisiana Universities Marine Consortium (LUMCON) and Louisiana State University.The Mississippi’s warm freshwater, combined with the nitrogen and phosphorous runoff, sits on top of cooler saltwater in the Gulf of Mexico preventing oxygen in the atmosphere from reaching the deeper water. Strong sunlight during the warmer spring and summer months, meanwhile, promotes the growth of algae blooms and dead zones peak. Since the water is unable to support life, the algae remains uneaten, sinks to the bottom and sucks up oxygen while it decomposes.
The massive dead zone poses serious threats for the future of Gulf fisheries and marine life populations already suffering from the ongoing effects of the massive Deepwater Horizon oil spill in 2010. “There’s a huge area where you just can’t catch anything for a long period of time,” Nancy Rabalais of LUMCON told The New York Times. A study conducted during the summers of 2006 and 2007 found that nearly a quarter of female Atlantic croaker fish caught in the Gulf’s dead zone had developed deformed, testes-like organs instead of ovaries. Researchers weren’t sure how long the fish were living in the low-oxygen dead zone before they developed these sexual defects, but lab experiments showed that the changes could happen in as little as 10 weeks of exposure.
Female croakers caught during this period were found to have a decreased level of aromatase, the key chemical needed to produce estrogen and ovaries. Some of the female “testes” even contained sperm, but were not able to fertilize normal eggs. “If the oxygen levels go down, it affects the brain and the neurohormones and neuropeptides that it produces,” says M.S. Rahman, a marine biologist at the University of Texas in Austin’s Marine Science Institute. Male fish have also shown sexual defects from low oxygen levels. Those caught in the dead zone had smaller than average testes and lower sperm counts. Reproduction rates that should average a 40-80% hatching rate were instead only producing a 10% hatching rate.
Croakers are a “pretty typical Gulf fish,” says Prosanta Chakrabarty, a fish biologist at Louisiana State University in Baton Rouge. “I wouldn’t be surprised if these findings could be generalized.”
Though the Gulf dead zone continues to expand, future restoration and recovery is possible. Such a turnaround has been seen in the Black Sea, which contained the largest dead zone in the world during the 1980s. Following the collapse of the Soviet Union, fertilizers became too costly to use. Phosphorus applications were cut by 60% and nitrogen use was halved. By 1996, the dead zone was absent for the first time in 23 years.
“The evidence suggests that if the spigot of nutrients can be turned off, coastal systems can recover,” NOAA Administrator Dr. Jane Lubchenco told The San Francisco Chronicle. “Doing it can be accomplished by using fertilizers more efficiently, preventing human and animal sewage from entering rivers, and replanting vegetation [along riverbanks] to absorb excess nutrients.”