Human-induced hydrologic changes have altered De Leon Springs, a second-magnitude spring in central Florida, for nearly two centuries. The spring pool, originally dammed in the 1830s to power a sugar mill, currently serves as a popular public swimming area. The St. Johns River Water Management District (SJRWMD) is mandated by Florida Statute to determine the minimum flow regime needed to prevent significant harm to ecosystem integrity or ecosystem services (e.g., recreational opportunities) at De Leon Springs. Florida manatees currently use the spring run as a winter warm-water refuge. Manatees require water temperatures of at least 20 °C to avoid cold stress. A three-dimensional hydrodynamic model of temperature patterns in the spring run was created using the environmental fluid dynamics code (EFDC). This model was used to evaluate the effects of spring flow on manatee warm-water habitat availability in the spring run. Field surveys during Dec. 2015-Feb. 2016 were used to verify model temperature simulations and document manatee use of the spring run. Conclusions from the model, spring flow records from 1965-2015, rainfall records, and consumptive water use estimates were used to recommend a long-term mean flow for De Leon Springs.
Results/Conclusions
During the field survey period, temperatures in the spring pool remained between 23-24 °C while temperatures in the spring run (the area accessible by manatees) ranged from 14-24 °C. Spring run temperatures dropped below 20 °C during five separate “cold snaps”, each lasting 1-5 days. Manatees tended to be more numerous when water temperatures were higher. No manatees were observed below 19 °C, suggesting manatees sought refuge elsewhere on the coldest days. Preliminary EFDC model results for winter months indicated that declines in spring flow would result in declines in manatee warm-water refugia. Because the long-term recovery of the Florida manatee, a federally threatened/endangered species, is dependent upon warm-water refugia, SJRWMD’s preliminary recommendation for the minimum flow regime is to maintain a long-term mean flow of 23.7 cubic feet per second, calculated by adjusting 1965-2015 flow records as though current levels of consumptive water use had occurred throughout the period. If adopted into state law, this minimum flow regime will be used to set long-term limits on local and regional consumptive water use. The process used to determine this minimum flow regime can be applied to other Florida springs used by manatees.