Nitrogen Flux and Sources in the Mississippi River Basin by Donald A. Goolsby, William A. Battaglin, Brent T. Aulenbach, and Richard P. Hooper ABSTRACT Nitrogen from the Mississippi River Basin is believed to be at least partly responsible for the large zone of oxygen-depleted water that develops in the Gulf of Mexico each summer. Historical data show that concentrations of nitrate in the Mississippi River and some of its tributaries have increased by factors of 2 to more than 5 since the early 1900s. We have used the historical streamflow and concentration data in regression models to estimate the annual flux of nitrogen (N) to the Gulf of Mexico and to determine where the nitrogen originates within the Mississippi Basin. Results show that for 1980- 96 the mean annual total N flux to the Gulf of Mexico was 1,568,000 metric tons per year (t/yr). The flux was about 61% nitrate as N, 37% organic N, and 2% ammonium as N. The flux of nitrate to the Gulf has approximately tripled in the last 30 years with most of the increase occurring between 1970 and 1983. The mean annual N flux has changed little since the early 1980s, but large year-to-year variations in N flux occur because of variations in precipitation. During wet years the N flux can increase by 50 percent or more due to flushing of nitrate that has accumulated in the soils and unsaturated zones in the basin. The principal source areas of N are basins in southern Minnesota, Iowa, Illinois, Indiana, and Ohio that drain agricultural land. Basins in this region yield 800 to more than 3,100 kilograms total N per square kilometer per year (kg/km 2 /yr) to streams, several times the N yield of basins outside this region. Assuming conservative transport of N, streams draining Iowa and Illinois contribute on average about 35% of the total N discharged by the Mississippi River to the Gulf of Mexico. In years with high precipitation they can contribute a larger percentage. INTRODUCTION The Mississippi and Atchafalaya Rivers are the primary riverine sources of freshwater and nutrients discharged to the Gulf of Mexico. The combined mean annual streamflow for the Mis- sissippi and Atchafalaya Rivers since 1980 is nearly 22,000 cubic meters per second and represents about 80 percent of the estimated freshwater discharge from the United States to the Gulf (Dunn, 1996). These two rivers also account for an estimated 90 percent of total nitrogen (N) flux discharged annually to the Gulf (Dunn, 1996). Nitrogen along with other nutrients leads to increased production of algae in the Gulf and has been implicated as a possible cause of hypoxia (dissolved oxygen less than 2 milligrams per liter) in a large zone of bottom water in the Gulf along the Louisiana-Texas coast ( Rabalais and others, 1996, 1998). The seasonal decrease in dissolved oxygen develops each year during late spring following inflows of freshwater and nutrients to the Gulf. Hypoxia reaches a maximum in late summer and disappears each fall. Estimates of the size of the hypoxic zone during 1985-1992 averaged about 7,000-9,000 square kilometers (km 2 ). Following the 1993 Mississippi River flood, the hypoxic zone doubled to about 17,000 km 2 , an area twice the size of Chesapeake Bay. In the summers of 1994- 97, the hypoxic zone was reported to be as large as or larger than during the summer of 1993 (Rabalais and others, 1998; Rabalais, 1997). Because of the public concern that has developed over the hypoxia issue, the White House Committee on Environment and Natural Resources (CENR) was asked in 1997 to conduct a scientific assessment of the causes and