Seasonal Shift in the Relationship between Phosphorous and Suspended Solids Suggests Differences in Landscape Sources of Phosphorous.

Seasonal shift in the relationship between phosphorous and suspended solids suggests differences in landscape sources of phosphorous

Jamie Kraklow

The St. Croix River is a highly valued natural resource in the upper Midwest. A recent Total Maximum Daily Load (TMDL) management plan was initiated to decrease the amount of phosphorous deposited into the river and its tributaries to reduce eutrophication in the system. Because soil-bound phosphorus is considered to be the greatest fraction of the total non-point source phosphorus inputs to the St. Croix, most Best Management Plans (BMPs) developed in support of the TMDL suggest maintaining continuous living cover and leaving crop residue on agricultural lands in the watershed. Recent research in the Kinnickinnic watershed, an important sub-basin in the St. Croix watershed dominated by agricultural land uses, suggests that dissolved phosphorus may contribute a greater proportion of non-point source phosphorus than previously assumed. This study will further investigate the relationship between total phosphorous and suspended solids in the Kinnickinnic River and its tributaries to support BMP development and implementation in the St. Croix watershed. Baseflow and storm event samples will be collected from the Kinnickinnic River during the spring and summer seasons at sites along main river and in its tributaries. Water samples will be analyzed for dissolved and particle-bound phosphorous. Phosphorus content will be compared among samples of living cover and ground residue in various states of decay collected from different landscapes near the river system, including tilled ag land, no-till ag land, and CRP land to determine potential dissolved phosphorus losses from decaying plant material. These data will help to differentiate sources of phosphorus to the Kinnickinnic River and suggest whether current BMPs in the St. Croix watershed will be sufficient to meet TMDL goals.