ENVIRONMENTAL-INFLUENCES ON THE CS-137 KINETICS OF THE YELLOW-BELLIEDTURTLE (TRACHEMYS-SCRIPTA)

Assessments of ecological risk require accurate predictions of contami nant dynamics in natural populations. However, simple deterministic mo dels that assume constant uptake rates and elimination fractions may c ompromise both their ecological realism and their general application to animals with variable metabolisms or diets. In particular, the temp erature-dependent metabolic rates characteristic of ectotherms may lea d to significant differences between observed and predicted contaminan t kinetics. We examined the influence of a seasonally variable thermal environment on predicting the uptake and annual cycling of contaminan ts by ectotherms, using a temperature-dependent model of Cs-137 kineti cs in free-living yellow-bellied turtles, Trachemys scripta. We compar ed predictions from this model with those of deterministic negative ex ponential and flexibly shaped Richards sigmoidal models. Concentration s of Cs-137 in a population of this species in Pond B, a radionuclide- contaminated nuclear reactor cooling reservoir, and Cs-137 uptake by u ncontaminated turtles held captive in Pond B for 4 yr confirmed both t he pattern of uptake and the equilibrium concentrations predicted by t he temperature-dependent model. Almost 90% of the variance in the pred icted time-integrated Cs-137 concentration was explainable by linear r elationships with model parameters. The model was also relatively inse nsitive to uncertainties in the estimates of ambient temperature, sugg esting that adequate estimates of temperature-dependent ingestion and elimination may require relatively few measurements of ambient conditi ons at sites of interest. Analyses of Richards sigmoidal models of Cs- 137 uptake indicated significant differences from a negative exponenti al trajectory in the Ist yr after the turtles' release into Pond B, We also observed significant annual cycling of Cs-137 concentrations, ap parently due to temperature-dependent metabolism and its influence on ingestion and elimination rates, However, equilibrium concentrations o f the radionuclide in the wild population were predictable from negati ve exponential models based on average annual temperature and its effe cts on intake and elimination rates, also suggesting that predicting e ctotherm responses to long-lived contaminants (such as Cs-137) may be possible without complex ecophysiological modeling.