Aquatic Sciences Meeting, Albuquerque 2001

SS31 Integrated Approaches to Drainage Basin Nutrient Inputs and Inland/Coastal Eutrophication (Science and Society Connections)

Date: Thursday, February 15, 2001, Time: 11:15:00 AM

Location: Cimarron

Battaglin, W, A, U.S. Geological Survey, Lakewood, USA, wbattagl@usgs.gov

Goolsby, D, A, U.S. Geological Survey, Lakewood, USA, dgoolsby@usgs.gov

NITROGEN FLUX IN THE MISSISSIPPI RIVER BASIN: LONG-TERM TRENDS AND RELATION TO NITROGEN INPUTS

The flux of freshwater and nitrogen (N) from the Mississippi River Basin (MRB) in part controls the annual development of the hypoxic zone in the Gulf of Mexico. During the last 20 years, the Mississippi River has discharged annually an average of 700 billion cubic meters of water and 1.55 million metric tons (t) of N into the Gulf of Mexico. The bulk of the N originates in the agricultural lands of the upper Midwest (parts of Iowa, Illinois, Indiana, Minnesota, Nebraska, and Ohio). Basins in the upper Midwest commonly yield 1 to 3 metric tons of N per square kilometer per year (t/km2/yr), whereas yields for most of the rest of the MRB are less than 0.5 t/km2/yr. Total N inputs also are highest in the agricultural lands of the upper Midwest, commonly in excess of 10 t/km2/yr, whereas N inputs are less than 5 t/km2/yr for most of the rest of the MRB. Regression models were used to relate total N yields to N inputs in 42 large watersheds in the MRB. Results indicate that the N from fertilizer plus the N mineralized from the soil inorganic N pool together account for about 50% of the annual N flux, animal manure accounts for about 15%, other non-point sources including atmospheric deposition and soil erosion account for 24%, and municipal and industrial point sources account for 11%. About 61% of the total N discharged from the MRB is in the form of nitrate (NO3). NO3, the most biologically available from of N, is what is fuels algal growth in near surface waters of the Gulf. NO3 concentrations in the lower Mississippi River have increased from an average of 0.56 milligrams per liter (mg/L) in 1905-06 to an average of 1.45 mg/L in 1980-98. N concentration and stable isotope data suggest that little of the N that enters the mainstem Mississippi River is lost to denitrification of other transformation before discharging to the Gulf.

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Aquatic Sciences Meeting, Albuquerque 2001

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