Nine states in the Mississippi River Basin contribute the majority of nutrients to the Northern Gulf of Mexico, threatening the economic and ecological health of one of the nation's largest and most productive fisheries.
Excessive 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.
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.
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.
Nutrient Sources and Gulf of Mexico Hypoxia
"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.
The study reports that 66% of nitrogen originates primarily from cultivated crops, mostly corn and soybean, with animal grazing and manure contributing only about 5 percent. Atmospheric contributions also are important, accounting for 16% of nitrogen.
The USGS findings help fill important gaps in information on sources of phosphorus in the Mississippi River Basin at a time when recognition of phosphorus is expanding as an important contributor to Gulf hypoxia. The joint federal-state Gulf of Mexico Hypoxia Task Force is evaluating recommendations by
the Environmental Protection Agency's Science Advisory Board to set reduction targets of at least 45 percent for both nitrogen and phosphorus in an effort to shrink the size of the "hypoxic zone" in half (to 5000 square kilometers) by 2015. States and EPA may therefore need to adopt a dual nutrient management strategy in order to reduce the size of the hypoxia zone in the Gulf.
Other new findings include:
- Agricultural nonpoint sources contribute more than 70 % of the nitrogen and phosphorus delivered to the Gulf, versus only about 9 to 12 % from urban sources.
- Reservoirs, particularly common in the Tennessee Valley and along the Missouri River, are very effective at removing phosphorus from watersheds in the Mississippi Basin and cause a large reduction in the amount of phosphorus reaching the Gulf. While these findings indicate that phosphorus inputs upstream of reservoirs may have limited impact on Gulf hypoxia, these inputs are known to create water-quality issues in the reservoirs themselves.
- Delivery of nutrients to the Gulf is 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. Nutrient reductions in the Gulf may thereby be more efficiently achieved through nutrient management in watersheds drained by large rivers.
The study used innovative geo-spatial modeling that integrates long-term monitoring data with spatially extensive geographic maps of hydrologic and watershed characteristics and contaminant sources.
"The key to predictive modeling is the continuation of critical "on-the-ground" water monitoring which provides credible, comparable and comprehensive data," said USGS Associate Director for Water, Dr. Robert Hirsch. These data can be used to verify predictions across large regions, such as the Mississippi River Basin.
Surface-water quality monitoring has declined significantly over time. The USGS network of long-term monitoring stations available for use in the USGS model has declined from about 425 to 35 stations from the early 1990s to today.USGS continues to work closely with the EPA, U.S. Department of Agriculture, and the States to assure that monitoring and modeling provide useful information for managing nutrients in watersheds throughout the Mississippi River Basin. As the study has shown, nutrient issues are complex and therefore a multitude of management approaches are necessary to reduce the nutrient burden flowing in the Mississippi River Basin. Some examples include, attention to the management of animal and crop production, controlling nutrient sources in close proximity to large rivers, and considering reservoir effects on phosphorus.