Seasonal and animal farm size influences on in-stream phosphorus transport in an agricultural watershed

Excess phosphorus (P) is generally responsible for the eutrophication of Midwestern surface waters and causes algal blooms, fish kills, and other detrimental ecological effects. In-stream transport of dissolved and particulate P (PP), under both stormflow and baseflow conditions, was studied in an agricultural Wisconsin watershed with diverse sizes of animal farm operations. Trends in P losses were compared both on an individual event and seasonal (spring, summer, fall) basis. In-stream P transport was characterized by high PP losses in the spring during periods of relatively little crop cover and high dissolved P losses in the fall, when surface coverage was still present. The majority of P (64–71%) was transported in the spring season during periods of high flow. Presence of high animal density at lower portions of the watershed resulted in increasing downstream P concentration and load (i.e., concentration dilution effects with increasing drainage area were absent). The baseflow TP concentrations were always above the USEPA eutrophication threshold [Ecoregion VII, level III-53: 0.08 mg L−1; (USEPA in Ambient water quality criteria recommendations. Information supporting the development of state and tribal nutrient criteria. Rivers and stream in nutrient ecoregion VII. EPA 822-B-00-018. https://www.epa.gov/sites/production/files/documents/rivers7.pdf, 2000)]. While upland conservation practices are necessary for non-point source pollution control, it is important to ensure that these practices afford protection during the spring time periods when agricultural fields are most vulnerable for offsite movement of sediments and associated constituents.