Nutrient dynamics and ecological roles of freshwater turtles
Sterrett, Sean Christopher
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Nutrient cycling is necessary for the support of ecosystem processes and services. Animals are influential in the storage and transformation of critical nutrients, yet many populations are impacted by a suite of human related activities, including unsustainable harvest. Turtles are a globally imperiled taxa with half of all known species threatened, yet little is known about their influences on ecosystems. Turtles are unique in morphology, physiology, life history and ecology, which suggest that they impact ecosystems in unconventional ways. This dissertation explores the ecological stoichiometry, nutrient dynamics and ecological roles of freshwater turtles using field-collected and experimental research. Research was conducted on four common North American species in three rivers and one collection of ponds in the southeastern USA. The nutrient content of 33 individuals across four focal species indicated that a turtle's skeleton composed 27.5% of total fresh mass and the shell alone composed 93% of turtle skeletal mass. Due to the high Phosphorus (P) content of bone, whole body nutrient content of turtles is the most extreme of any measured organism (%Nitrogen (N):P =1.04). Because bone has such a slow turnover time, this research suggests that adult turtles are in low demand of P, such that their recycling is proportional to their biomass. These results challenge conventional thinking in ecological stoichiometry on nutrient limitation. Turtle standing crop biomass and nutrients are comparable with estimates of other aquatic taxa, but higher in P per unit of biomass. Further, mass-specific excretion rates of N and P were similar or exceeded estimates of salamanders and fishes. Results from a mesososm experiment suggest that carnivorous juvenile turtles reduce detritivorous macroinvertebrates, thereby reducing invertebrate feeding on nutrient rich leaves. Therefore, juvenile turtles indirectly shift leaf litter nutrient content. These results highlight the complexity of direct and indirect consumer effects on ecosystem processes. Based on our field and experimental results, we would expect juvenile and adult turtles to have contrasting effects on top down and bottom up effects in aquatic ecosystems. Turtles can occur in high abundance in many freshwater ecosystems, making their conservation potentially important in the storage and recycling of nutrients.