SREL Reprint #2062

 

 

 

ENVIRONMENTAL INFLUENCES ON THE 137CS KINETICS OF THE YELLOW-BELLIED TURTLE (TRACHEMYS SCRIPTA)1

ERIC L. PETERS
Department of Radiological Health Sciences, Colorado State University, Fort Collins, Colorado 80523 USA


I. LEHR BRISBIN, JR.
Savannah River Ecology Laboratory, P.O. Drawer E, Aiken, South Carolina 29802 USA

 

Abstract.

Assessments of ecological risk require accurate predictions of contaminant dynamics in natural populations. However, simple deterministic models that assume constant uptake rates and elimination fractions may compromise both their ecological realism and their general application to animals with variable metabolisms or diets. In particular, the temperature-dependent metabolic rates characteristic of ectotherms may lead to significant differences between observed and predicted contaminant kinetics. We examined the influence of a seasonally variable thermal environment on predicting the uptake and annual cycling of contaminants by ectotherms, using a temperature-dependent model of 137Cs kinetics in free-living yellow-bellied turtles, Trachemys scripta. We compared predictions from this model with those of deterministic negative exponential and flexibly shaped Richards sigmoidal models. Concentrations of 137CS in a population of this species in Pond B, a radionuclide-contaminated nuclear reactor cooling reservoir, and 137Cs uptake by uncontaminated turtles held captive in Pond B for 4 yr confirmed both the pattern of uptake and the equilibrium concentrations predicted by the temperature-dependent model. Almost 90% of the variance in the predicted time-integrated 137Cs concentration was explainable by linear relationships with model parameters. The model was also relatively insensitive to uncertainties in the estimates of ambient temperature, suggesting that adequate estimates of temperature-dependent ingestion and elimination may require relatively few measurements of ambient conditions at sites of interest. Analyses of Richards sigmoidal models of 137Cs uptake indicated significant differences from a negative exponential trajectory in the I St yr after the turtles' release into Pond B. We also observed significant annual cycling Of 137Cs concentrations, apparently due to temperature-dependent metabolism and its influence on ingestion and elimination rates. However, equilibrium concentrations of the radionuclide in the wild population were predictable from negative exponential models based on average annual temperature and its effects on intake and elimination rates, also suggesting that predicting ectotherm responses to long-lived contaminants (such as 137Cs) may be possible without complex ecophysiological modeling.

Key words: biogeochemistry; biotic concentrations; cesium-137; contaminant modeling; diet; kinetics modeling; model sensitivity analysis; physiological ecology; radioecology; radionuclides; Richards sigmoidal model; stochastic models; Testudines; thermal ecology; Trachemys scripta.

SREL Reprint #2062

Peters, E.L. and I.L. Brisbin, Jr. 1996. Environmental influences on the 137Cs kinetics of the yellow-bellied turtle (Trachemys scripta). Ecological Monographs 66:115-136.

 

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