A simple model of internal loading of phosphorus in Lake Okeechobee

Models of nutrient dynamics in Lake Okeechobee have not adequately simulated the lake's reduced capacity, over time, to remove phosphorus. These models lack processes to either simulate phosphorus saturation in sediments or internal loading to the water column from the sediments. We developed a relatively simple mass balance model that incorporates nonlinear Langmuir sorption dynamics previously used by Pollman (1983) to describe the partitioning of inorganic phosphorus between sediment aqueous and solid phases. This partitioning resulted in progressive saturation of sediment phosphorus exchange sites. The resulting model represents an improvement over earlier efforts but still yielded only modest agreement between observed and predicted annual values (r2= 0.38). Model residuals were significantly correlated with lake-water column calcium concentrations; therefore, the model was refined to include a predictive submodel for calcium, based on ambient loadings and precipitation to, or dissolution from, the sediments. The latter process was modeled kinetically, based on the deviation of water column concentrations from a calibrated equilibrium value. The submodel assumed that as particulate calcium dissolves it releases attached soluble reactive phosphorus. Although this additional source of phosphorus (P) to the lake was small (11% compared to external inputs), it was significant during years when appreciable calcium dissolution occurred. As a result, the revised model performance was improved (r2= 0.48). The model indicates that internal fluxes of P exceed external inputs on average by a factor of 2.6. This internal flux will compromise the efficacy of load reduction measures taken to improve the water quality of Lake Okeechobee.