Predicting Nutrient Effects on Secchi Depth using the Tampa Bay Water Clarity Model (TBWCM)

The Tampa Bay Water Clarity Model was developed to predict the impact of changing nutrient loads on water clarity as measured by secchi depth. The model combines a physical mixing model with an irradiance model and nutrient cycling model. A 10 segment bifurcated box model based on salt and water balance, which assumed complete mixing of each segment at each time step, was used to physically mix the Bay. The irradiance model predicted light levels just below the Bay surface from atmospheric conditions appropriate for Tampa Bay. The nutrient cycling model was primarily based on the growth of phytoplankton, their death and the recycling of inorganic nutrients as described in WASP. Secchi depths were calculated from Kd values based on light absorption by water, total chlorophyll (a, b and c), colored disolved organic matter, and the effects of turbidity. The model was calibrated against monthly salinity, total chlorophyll, ammonia, total kjeldahl nitrogen, and dissolved oxygen data collected during 1985 through 1991. The model was calibrated in two stages: (1) by first calibrating the exchange coefficients using the box model and (2) by calibrating selected nutrient cycling parameters using the complete water clarity model. Validation of the model was conducted with a similar data set collected during 1992 through 1994. We assessed the sensitivity of the model to changes in nutrient load by running the model in predictive mode. Scenarios were run with nutrient inputs from gauged rivers and unguaged drainage basins scaled by factors of 50% to 500%. A linear relationship was found between nutrient inputs and average secchi depth, ammonia, and total chlorophyll for the seven year (1985-1991) model runs. A 5-fold increase in gauged river nitrogen input resulted in a 10 percent decrease in secchi depth which resulted in a 30 percent decrease in the maximum depth for seagrass growth in the middle portion of Old Tampa Bay.