A newly released study by the United States Geological Survey shows that nitrate concentrations in the Mississippi River Basin did not consistently decline from 1980 to 2008. The increased nitrate levels directly affect the Gulf of Mexico, where they contribute to “dead zones,” or hypoxia.

Dead zones come about when oxygen levels are so low that marine life in bottom and near-bottom waters cannot be supported. The gulf’s dead zone has become notorious and has been, at times, an obstacle for Florida’s commercial fishing industry.

According to the Geological Survey study, “state and federal partners serving on the Mississippi River Gulf of Mexico Watershed Nutrient Task Force are striving to decrease nutrients transported to the Gulf to reduce the size of the hypoxic zone to less than 5,000 square kilometers (about 2,000 square miles) by 2015.”

Among the major findings of the study was that nitrate transport to the Gulf of Mexico was 10 percent higher in 2008 than 1980, which is in part due to the flow of the water.

From the study:

Nitrate transport during the spring is one of the primary determinants of the size of the Gulf hypoxic zone. At times of high spring streamflow during the period studied, the concentration of nitrate decreased at the study site near where the Mississippi River enters the Gulf of Mexico, indicating that some progress has been made at reducing nitrate transport during high flow conditions. However, during times of low to moderate spring streamflow, concentrations increased. The net effect of these changes is that nitrate transport to the Gulf was about 10% higher in 2008 than 1980.

Earlier this week, scientists from the National Oceanic and Atmospheric Administration announced that the dead zone wasn’t as large as had been previously estimated. But, at 6,765 square miles wide, it’s still larger than the state of Connecticut.

The gulf is responsible for about 40 percent of the seafood harvested in the lower 48 states, and the Oceanic and Atmospheric Administration has conservatively estimated (.pdf) that dead zones and harmful algal blooms cost U.S. coastal economies about $82 million a year.

The Geological Survey study examined concentrations and transport of nitrates at eight major study sites in the Mississippi. The results of the study are published in the journal Environmental Science and Technology.

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