CHANGE IN NUTRIENT TRANSPORT FROM THE UPPER AND MIDDLE MISSISSIPPI BASINS SINCE THE BASELINE PERIOD (1980-1996)

Nutrients originating from the Midwestern United States have been identified as a major contributing factor in the formation of hypoxic regions in the Gulf of Mexico (Gulf). The Gulf Watershed Nutrient Task Force proposed a revised interim target of 20 percent reduction in annual nitrogen and phosphorus transport by 2025, relative to baseline period of 1980 to 1996. The final 45 percent reduction goal would be achieved by 2035.

To better manage reduction strategies and to assess progress, there is a need to better understand current nitrogen and phosphorus transport from the upper and middle Mississippi River Basin and to quantify this transport in relation to the 1980-1996 average (baseline) loading. Annual total nitrogen (TN) and phosphorus (TP) loads for 1980-2016 were calculated with LOADEST from a limited number of rivers in the upper and middle Mississippi River basins where monthly nutrient data were available. The average annual TN load in many rivers in 1997-2016 was slightly less than during the baseline (1980-1996) period in all rivers, but the reduction was not significant. In contrast, average TP loads significantly increased during the 1997-2016 period in three rivers; the Ohio, Illinois, and Grand Rivers than during the baseline period.

Five-year running average loads are more sensitive to emerging trends, but also exhibit large year-to-year variability. For example, the Iowa and Illinois River five-year running average loads in 2015-2016 are approaching the 20-percent reduction for TN, but TP loads are substantially greater than baseline. These results provide information on reaching the nitrogen and phosphorus load targets. Based on the 2015 5-year average, the total phosphorus load from the Iowa River will need to be reduced by 74 percent and the total phosphorus load from the Illinois River will need to be reduced by 54 percent to meet the 2025 reduction target.