Modeling of decadal scale phosphorus retention in lake sediment under varying redox conditions

Phosphorus (P) releases from lake sediments are controlled in the long term by P burial into the deep sediment and on shorter time scales by the redox conditions at the sediment–water interface. In Lake Sempach (Switzerland), hypolimnetic oxygen concentration was increased by artificial aeration after two decades of nearly anoxic conditions. Using diagenetic reaction-transport modeling and sediment core analysis, we investigated the effects that this change, as well as variations in the organic carbon loadings, had on the long-term mobility of sediment P. During low-oxygen conditions, the reducible iron pool in the sediment was depleted, resulting in the release of previously accumulated P. The remobilization of iron-bound P affected phosphate effluxes from the sediment on the time scale of the sediment iron cycle (several years). On longer time scales, P effluxes followed the sedimentation fluxes of organic matter. Mass balance calculations indicate that, despite the dominance of internal P loading in Lake Sempach, over the long-term phosphorus content in the water column was controlled by the external P inputs. The results suggest that, whereas short-term decreases in sediment P releases may be achieved by preventing sediment anoxia, long-term solutions should involve reductions in the external P inputs.