Modeling phosphorus flux in the sediments of Onondaga Lake: Insights on the timing of lake response and recovery

External (i.e. point source and tributary) phosphorus loads are an appropriate target for lake managers concerned with eutrophication control. However, internal loads associated with phosphorus that has accumulated in lake sediments can retard the response to reductions in external loading. This paper describes the development, calibration, confirmation and application of a model for phosphorus exchange at the sediment-water interface of a eutrophic, urban system, Onondaga Lake, New York. The model is based on the principles of mass balance and accommodates the delivery of labile particulate phosphorus to the sediment surface, diagenesis of that phosphorus within the sediment (conversion to the soluble form), redox-mediated adsorption and desorption, and diffusion and release of soluble phosphorus to the water column. Values for most model inputs and coefficients were determined through a suite of field and laboratory measurements and experimentation, establishing the credibility of the tool for site-specific application. The model was calibrated to measured sediment profiles of soluble and labile particulate phosphorus, limiting coefficient adjustment to experimentally defined bounds. The confirmation process included comparison of model-predicted and measured rates of sediment phosphorus release with no further coefficient adjustment. The model was then applied in the prediction of the sediment response to changes in the phosphorus content of sedimenting particulate matter, changes which would accompany reductions in external phosphorus loads to the lake. It is demonstrated that steady state rates of sediment phosphorus release, i.e. those in equilibrium with newly established rates of labile particulate phosphorus deposition, are achieved over a period of 19-26 years. These findings have important implications with respect to the natural recovery of eutrophic systems and will assist water quality managers in developing appropriate public expectations for the timing of lake restoration. © 2007 Elsevier B.V. All rights reserved