A Spatial Analysis of Phosphorus in the Mississippi River Basin.

Nutrients, especially nitrogen (N) and phosphorus (P), in the Mississippi River Basin (MRB) have been determined to have a major effect on hypoxia in the Gulf of Mexico. Although N has been extensively studied and attributed to be the main nutrient causing the Gulf hypoxic zone, the presence of P in waterways has increasingly been recognized as having more of an effect on Gulf hypoxia than was previously thought and to impact local water quality. Both agriculture and point sources (mainly sewage treatment plants) contribute P to the Mississippi River. Our objective was to analyze the spatial extent of P inputs and outputs in the MRB as they relate to riverine P loads to the Gulf of Mexico, including determining the counties in the MRB that are contributing the most P to the Mississippi River, and examining the causes of this P loss. A ten year data set from 1997 through 2006 for each county in the MRB of factors contributing P to the environment was constructed, as well as watershed riverine P loads to help determine the most critical factors. This dataset included information on fertilizer application, major crop acreage and harvest, animal numbers (with manure calculated), and people. Landscape characteristics such as tile drainage, soil type, and slope were used to determine how inputs and P balances related to stream loads of P. With this dataset, a leakiness value of P for each county was calculated and P losses visualized throughout the MRB. We found fertilized crops on sloping land (surface runoff), fertilized crops with tile drainage, and people (through sewage effluent) could explain much of the spatial pattern in stream P loads. This analysis of P in the MRB helps to point out specific areas where P reductions are needed to limit the occurrence of Gulf hypoxia.