Dead Water in the Gulf of Mexico
The Mississippi River receives about 1.82 million metric tons of nitrogen each year from farms, sewage plants, and even air pollution, and delivers it to the Gulf of Mexico. This is three times the amount of nitrogen in the river only forty years ago. Nitrogen in the water is a plant fertilizer which causes rapid growth or blooms of marine algae in the Gulf in the summer when the water is relatively warm and calm. When the algal blooms die, they sink and rot, and the decomposition process depletes the water of oxygen to the point that marine animals must either leave the area or die. The water remains depleted of oxygen until the fall when storms mix the water and reoxygenate it.
Fishers have long known there were areas of dead water in the Gulf, but systematic measurements of the extent of oxygen depleted water only began in the 1980s. In 1989 the dead zone covered about 9,000 square kilometers (about 3,500 square miles) off the Mississippi delta. The summer after the 1993 floods the dead zone increased to 16,000 square kilometers, and has decreased to about 12,000 square kilometers since then.
We spoke with Dr. Nancy Rabalais, one of the first to document the dead zone in the Gulf, about her work.