The states of Illinois, Iowa, Indiana, Missouri, Arkansas, Kentucky, Tennessee, Ohio and Mississippi make up only one-third of the 31-state Mississippi River drainage area, but contribute more than 75 percent of nitrogen and phosphorus to the Gulf, U.S. Geological Survey, USGS, researchers report.
Excess amounts of these nutrients have resulted in a zone of low dissolved oxygen or hypoxia, caused by the growth of large amounts of algae. This can stress and cause death in bottom-dwelling organisms in the Gulf, so it is commonly called the dead zone.
The study found that agricultural nonpoint sources contribute more than 70 percent of the nitrogen and phosphorus delivered to the Gulf. By comparison, only about nine to 12 percent originates from urban sources.
Corn and soybean cultivation is the largest contributor of nitrogen to the Gulf. Animal manure on pasture and rangelands and crop cultivation are the largest contributors of phosphorus.
These are among the new findings in the report, "Differences in Phosphorus and Nitrogen Delivery to the Gulf of Mexico from the Mississippi River Basin," by the U.S. Geological Survey, USGS, published in the journal "Environmental Science and Technology. "
"This study is important because it reveals new details about sources of phosphorus," said Richard Alexander, USGS scientist and lead investigator. "The report shows that animal manure on pasture and range lands contribute nearly as much phosphorus as cultivated crops, 37 versus 43 percent."
The study suggests that phosphorus associated with the wastes of unconfined animals is a much larger source of phosphorus in the Mississippi River Basin than previously recognized. Current animal manure management emphasizes controlling nutrients primarily from confined animal facilities.
Delivery of nutrients to the Gulf was found to be highest from watersheds in the central and eastern portions of the Mississippi River Basin that are drained by large, fast flowing rivers with very little natural removal.
Runoff entering Fall Creek in Indianapolis, Indiana (Photo by Charles Crawford courtesy USGS)
Alexander concludes that nutrient reductions in the Gulf may be most efficiently achieved by managing nutrients in watersheds drained by large rivers.
Reservoirs, particularly common in the Tennessee Valley and along the Missouri River, are effective at removing phosphorus from watersheds in the Mississippi Basin, but Alexander and his team found that this creates water quality issues in the reservoirs themselves.
The joint federal-state Gulf of Mexico Hypoxia Task Force is evaluating recommendations by the U.S. Environmental Protection Agency's Science Advisory Board to set reduction targets of at least 45 percent for both nitrogen and phosphorus.
The targets would be set in an effort to shrink the size of the dead zone in half to 5,000 square kilometers by 2015.
USGS Associate Director for Water Dr. Robert Hirsch says "on-the-ground" water monitoring is essential to provide credible, comparable data to verify computer modeling predictions across large regions, such as the Mississippi River Basin.
Yet surface water quality monitoring stations are disappearing. Only 35 stations exist today, down from about 425 stations in the early 1990s, Hirsch said.
Hirsch says this study shows that nutrient issues are complex and management of animal and crop production, control of nutrient sources in close proximity to large rivers, and consideration of reservoir effects on phosphorus all are necessary to reduce the nutrient burden flowing in the Mississippi River Basin.
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