Modeling the Impacts of Water Withdrawls on the Thermal Regime of the Weeki Wachee River Winter Manatee Habitat

The Southwest Florida Water Management District is required to develop minimum flows and levels. The MFL for Weeki Wachee Springs was developed to protect the West Indian Manatee, which utilize the warm spring waters as a refuge when offshore waters go below tolerable levels. A 3D-hydrodynamic model application was developed using the EFDC to predict offshore water intrusion into the Weeki Wachee. The 2003-2005 model calibration focused on water surface elevation, salinity, and temperature in the river. The model was applied under critical temperature conditions and low and high flow baseline spring conditions, upon which reduced flows were applied. Manatee habitat refuge areas and volumes were defined as waters that met the following criteria: 1) daily average temperatures >20°C over a 3-day period, and 2) minimum manatee passage of 3.8 ft depth available upstream. Incremental flow reductions were applied until the manatee refuge area or volume reduction was >15%, representing the maximum allowable flow reduction. During low offshore temperatures with low spring flow, a 5% spring flow reduction resulted in a 15% reduction in manatee habitat area/volume. The available manatee refuge is capable, however, of supporting over 1363 manatees, well above the present number that utilize Weeki Wachee Springs.