Declines in submerged aquatic vegetation (SAV) and potential implications for wintering manatees (Trichechus manatus latirostris)
Florida’s estuarine and coastal waters are increasingly affected by human development and other activities on the adjacent landscape. Kings Bay, located along the west coast of peninsular Florida, is no exception. Over the last several decades, Kings Bay has been subjected to numerous alterations including shoreline development, urbanization of the watershed, reduced freshwater inputs and a general decline in the abundance of submerged aquatic vegetation (SAV). These environmental changes are of particular concern as Kings Bay is the primary winter refuge for endangered Florida manatees (Trichechus manatus latirostris) in the northern Gulf of Mexico. Previous studies of manatees in Kings Bay using natural abundance measurements of stable carbon and nitrogen isotopes have implicated freshwater and estuarine plants as important dietary components during winter. Thus, we examined the relationship between changes in freshwater macrophytes and manatee foraging behavior. Telemetry data for six male and four female manatees in the winters of 2007–2013 (provided by the U.S. Geological Survey Sirenia Project) were analyzed to obtain the number of trips to, and proportion of time spent in, the Gulf of Mexico, presumably to feed on more readily available seagrass resources.
Linear mixed-effect models applied to quarterly, triplicate samples of freshwater SAV percentage cover and biomass at 71 stations throughout Kings Bay revealed a nearly 35% decrease in percentage cover and 36% decrease in biomass from 2005 to 2013. The average SAV percentage cover in winter (Dec – Feb) decreased from roughly 38% in 2005 to just under 12% in 2013. While wintering males spent significantly more time in the Gulf than females (median ~57% vs ~40%), there were no significant temporal trends in the number of trips taken or time spent in the Gulf. Using a basic bioenergetic model and a simplifying assumption that all time in the Gulf involved feeding, seven of the ten animals spent sufficient time in the Gulf to meet their energetic needs (> 8 hours/day). Results indicate that Kings Bay provides an important thermal refuge, but coastal seagrasses have served as a key source of forage in recent winters. As coastal areas continue to experience broad habitat degradation, long-term management strategies for species like the Florida manatee could benefit from considering multiple habitats that support metabolic needs during critical periods, such as winter.