Net and gross sedimentation in relation to the phosphorus budget of Eau Galle Reservoir, Wisconsin

We compared P retention with direct measures of P sedimentation, estimated from sediment cores (annual P sedimentation) and sediment traps (daily P sedimentation), to quantify P sedimentation in Eau Galle Reservoir, Wisconsin. Mean annual P retention was similar to mean annual P sedimentation, as estimated from sediment core rates integrated over the entire lake basin, indicating that annual P mass balance approximated annual net P sedimentation in this reservoir. However, sediment trap P rates, measured over the summer stratified period, overestimated P retention rates determined over the same period, suggesting substantial deposition of internally-derived P. Inclusion of measured internal P loadings from a variety of sources in Eau Galle Reservoir in a P mass balance only accounted for an additional 24% of the summer sediment trap P rate, indicating substantial uncertainty in the overall P budget. Imbalances in the P budget may also suggest deposition of sediment from other as yet unquantified internal sources. Potential internal sources of P include sediment resuspension and chemical release and direct uptake of P from the sediment by phytoplankton.