Modeling Nutrients and Multiple Algal Groups Using AQUATOX: Watershed Management Implications for the Braden River Reservoir, Bradenton Florida

As part of a comprehensive watershed management plan (WMP) for the Braden River, Florida, watershed, a series of models were calibrated and validated for the river system which includes the Braden River (Bill Evers or Ward Lake) water supply reservoir. The models consisted of separate quantity and quality models. The primary water quality concern in the Bill Evers Reservoir is the increasing difficulty with algae blooms. Treatment of the blooms has lead to concerns over increased copper concentrations. Modeling of the various forms of nitrogen and phosphorus is an important aspect of this phase. A goal of this study was to determine the potential impacts to water quality that may be attributed to changes in the watershed since the early to mid 1980's.Water quality analysis of the Evers Reservoir was performed using the USEPA ecosystem model AQUATOX using water loadings derived from the SWRA water quantity model and constituent concentrations derived from the Florida Department of Environmental Protections' (FDEP) Impaired Water Rule Database (IWR) using the USGS empirical model MVUE (Minimum Variance Unbiased Estimator). Simulation of nutrient loads and corresponding ecosystem response for Evers Reservoir was achieved using AQUATOX. The model was run for the calibration period from 1997 through 2001 and the verification run performed for the years 1992 through 2005. The model was also run for two management scenarios, simulating a decrease of 15% phosphorus loading and a decrease of 35% phosphorus loading, as predicted from changing land use conversions. The model reasonably predicted nutrient loads, algal biomass as indicated by chlorophyll a and green and blue green algal dynamics within the Evers Reservoir system. The model also predicted the responses of blue green algae to chemical controls (algaecides: copper sulfate and peroxide).