An aggregation of turtles in a Florida spring yields insights into effects of grazing on vegetation

An aggregation of Suwannee Cooters (Pseudemys concinna suwanniensis) prompted an investigation of their effects on vegetation in Blue Spring, a 2nd-magnitude spring in Gilchrist County, Florida. We counted turtles and documented % cover and wet biomass of vegetation in September and October 2013. The maximum mean density of turtles (1566 ± 319 turtles/ha) was recorded near the spring vent during the 1st survey. Mean density among all reaches of the spring run decreased from 421 ± 133 to 145 ± 58 turtles/ha during the 30-d period between surveys. Percent cover and wet biomass of Hydrilla verticillata, Sagittaria kurziana, and Vallisneria americana decreased significantly between surveys, which indicated that turtles reduced the quantity of vegetation in the spring. Relatively little vegetation was lost downstream as turtles grazed, so removal rates were converted to grazing rates. Mean grazing rates on H. verticillata, S. kurziana, and V. americana were estimated to be 850, 275, and 78 g wet mass (WM) turtle−1 d−1, respectively. These grazing rates translated to 68, 25, and 5 g dry mass (DM) turtle−1 d−1 for H. verticillata, S. kurziana, and V. americana, respectively. Per kg of turtle, grazing rates were 17, 6, and 1 g DM kg−1 d−1, respectively. These results are the first estimates of P. c. suwanniensis grazing rates derived from field data and the first evidence that P. c. suwanniensis grazed more heavily on the invasive H. verticillata than on native vegetation. These findings highlight the role that turtles play in freshwater food webs, and they underscore the importance of submersed vascular plants as a food source for Suwannee cooters. Managers should consider the implication of reduced food for Suwannee cooters and other herbivorous turtles in Florida’s springs as rooted macrophytes are replaced by potentially less palatable, filamentous macroalgae, such as Lyngbya sp.