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A historically unprecedented aggregation of Suwannee Cooters (Pseudemys concinna suwanniensis) prompted an investigation of turtle grazing habits in Blue Spring, a second magnitude spring in Gilchrist County, Florida. Soon after the onset of the aggregation, we counted turtles and sampled vegetation during two separate sampling events (September and October 2013). In September 2013, visual surveys indicated a peak density of 497 turtles ha-1, almost 28x higher than the highest ever recorded at Gilchrist Blue Spring. By the second sampling event (October 2013), turtle density was reduced to 167 turtles ha-1, a density 9x higher than the historical maximum. Mean wet weight (g) of the invasive macrophyte Hydrilla verticillata in the spring run significantly declined (p0.4). Calculated grazing rates ranged between 766.49 and 2178.65 g wet weight H. verticillata turtle-1 day-1 for the high and low densities of turtles, respectively. Per capita grazing pressure was estimated at 58-165 g dry weight H. verticillata turtle-1 d-1 with a mean value of 112 g dry weight H. verticillata turtle-1 d-1. This is the first documented case of P. c. suwanniensis selectively grazing invasive H. verticillata as well as the first estimation of P. c. suwanniensis grazing rates in the wild. Findings point to the important role of turtles in freshwater food webs and also highlight the potential importance of submersed vascular plants in springs as a food source for Suwannee Cooters. Many springs have recently shifted from systems dominated by rooted vascular plants to an algal-dominated state, potentially reducing foraging opportunities for Suwannee Cooters and other herbivorous turtles. |