Every June the U.S. Geological Survey predicts the size of the Dead Zone in the northern Gulf of Mexico. The Dead Zone is a hypoxic zone in the gulf, the place where levels of oxygen drop so low that it becomes inhospitable to fish and shellfish.
Hypoxia happens naturally every summer, when the Mississippi pours its floods of fresh water into the gulf. The lighter freshwater floats on top of the heavier salt water. Algae, nourished by nutrients in the fresh water, grow and die and sink to the flour of the gulf, where they decay, soaking up the oxygen. Bottoming-dwelling creatures must leave the Dead Zone or die. Those that can’t do. Fish and shellfish, which breed in coastal wetlands and migrate to the gulf as juveniles, can’t pass through the Dead Zone.
The nutrients, ammonium nitrate or nitrate nitrogen, come from Midwestern corn and soybean fields, from golf courses and carefully tended lawns.
Last year the USGS predicted that the Dead Zone would grow to the size of New Jersey, 8,800 square miles. It didn’t, because Hurricane Dolly mixed in a bit of oxygen and reduced it to 7,889 square miles. The agency attributed its prediction to the size of the corn crop, which was destined for ethanol plants. This year the Dead Zone will be a little smaller.
The National Corn Growers objected this year. They have done research and say that their growers are balancing the corns nutritional needs with the amount of nitrates, which are not washing off their fields and into the rivers.
By the way, before the nitrates even get to the Gulf of Mexico, they are causing problems in the Upper Mississippi, where the “lakes” , the pools, behind the dams suffer from low-oxygen conditions, making it hard on fish.
Update, June 4, 2010: BP oil and the dead zone.
An article in the Scientific American discusses how the BP oil slick riding on the surface of the Gulf of Mexico and streaming into the bays behind barrier island like Timbalier Island is going to affect the annual Dead Zone described in the above posting. It could make the oxygen-deprived areas of the shallow in the Gulf worse or it could make them better.
In short BP has forced scientists into a giant science experiment and they can only guess at the outcome.