The study describes where within the Mississippi/Atchafalaya River Basin nutrients originate and identifies the relative importance of different nutrient sources—e.g., fertilizers, manure, wastewater treatment plants, or atmospheric deposition—throughout the Basin. Nutrients exported from the Basin contribute to hypoxia in the Gulf of Mexico. Results of this study will assist states in the Mississippi River/Gulf of Mexico Hypoxia Task Force—an interagency group working to reduce Gulf hypoxia—to implement their nutrient reduction strategies.

Using the SPARROW (SPAtially Referenced Regression On Watershed attributes) model, the study determined that catchments in the Corn Belt and along the Mississippi River had the highest yields (mass per unit area) of N and P. Agricultural activities were the largest source of N and P, but local inputs from wastewater treatment plants in some cases also were large contributors, and natural sources were sizable contributors of P.

Results of this study are based on catchment activities updated to reflect 2012 (the most recent data available) and a much finer spatial scale than previous studies—the mean catchment size now is ~2 km² as opposed to ~300 km² in earlier models.  An associated online mapping tool can be used by water-quality mangers to identify where the largest sources of nutrients are throughout the Mississippi/Atchafalaya River Basin and to guide actions to reduce nutrient loading to the Gulf of Mexico. The updated data and finer spatial resolution will better inform these management actions.