Understanding Phosphorus Sources in Minnesota's Watersheds for Improved Water Quality Outcomes of Agricultural Management.

Excess phosphorus in streams, lakes and rivers causes widespread impairment of water quality and habitat for biota throughout Minnesota and much of the Midwest. While progress has been made in reducing P losses from sewage and field erosion sources, many waterways remain highly eutrophic. We are using intensive sampling and modeling to understand the source of P losses from agricultural watersheds in Southern Minnesota and identify strategies that will be most efficient and effective for improving P retention. Analyses of spatial datasets for large rivers in MN and intensive sampling in three agricultural watersheds show that soluble P dominates the total P concentrations and annual loads in a majority of sites. Soluble and particulate P appear to have different sources in the landscape. Soluble P levels often peak at snowmelt, but remain elevated in drainage networks throughout the year. Particulate P levels are closely related to TSS concentrations and have two major sources: lakes and stream banks. Lakes strongly increase particulate P concentrations while reducing soluble P, and appear to function as hotspots for P transport into rivers. Erosion of stream banks and bluffs is also an import source of particulate P in some watersheds. We are using a watershed-scale model (SWAT) to complement field sampling and integrate our understanding of all P sources and environmental controls in this agricultural landscape. Results will be used to assess and develop strategies for phosphorus management that target all drivers of P fluxes within a watershed. This will include both direct edge of field P losses via surface runoff as well as dissolved P in tile drainage, and indirect controls such as mobilization of local and downstream stores through hydrologic pathways during high flow conditions.