Exploring the dynamics and fate of total phosphorus in the Florida Everglades using a calibrated mass balance model

The Everglades protection area, which encompasses five Water Conservation Areas (WCA), Everglades National Park (ENP), and a network of canals, levees, structures, and pump stations, exhibits elevated nutrient concentrations in the water and sediments, primarily as a result of phosphorus loads in agricultural runoff. A mass balance model was developed to predict phosphorus fate and transport in the Everglades Protection Area that could result from proposed phosphorus reduction strategies. The modeled area is about a 7000 km2 region that is divided into 642, 3.2×3.2 km cells, plus additional cell areas for canals. Phosphorus is transported between model cells and canals in accordance with output from a regional hydrology model. Simulated water column phosphorus dynamics within each cell and canal is further controlled by a simple, apparent net settling rate coefficient that integrates the effects of chemical, biological, and physical processes, and leads to net deposition of phosphorus in the sediments. After specification of external phosphorus loads (surface water and atmospheric wet and dry deposition) and system boundary conditions, the model was calibrated to available field data. The calibration procedure consisted of varying the apparent net settling rate coefficients in the WCA and the ENP. The goodness of fit of predicted water column total phosphorus concentrations varied temporally and spatially. Sediment phosphorus net deposition rates calculated by the model matched well with in situ observations where available. The model indicates that phosphorus in seasonal rainfall is a dominant influence on water column phosphorus dynamics in remote areas of the Everglades, whereas phosphorus dynamics in cells directly downstream of runoff inputs exhibit well-documented, nutrient gradients in receiving waters and sediments that could not be caused by rainfall alone. The model suggests that reductions of phosphorus concentrations leaving agricultural areas at the north end of the system will lead to lower concentrations entering ENP at the south end of the system.