Comparison of two freshwater turtle species as monitors of environmental contamination

Two species of turtles that occupy different ecological niches were compared for their usefulness as monitors of contamination in freshwater ecosystems. Trachemys scripta (Agassiz) (yellow-bellied slider) and Chelydra serpentina (Linnaeus) (common snapping turtle) were selected for com parison based on species abundance and differences in food habits and sediment contact. A review of the literature on contaminants in turtles and results of preliminary surveys conducted at the field sites, which are included in this study, were used to direct and focus this research project.

White Oak Lake, a settling basin for low-level radioactive and nonradioactive contaminants, and an uncontaminated reference site were used as study sites in the investigation of turtles as indicators of chemical contamination. Turtles were analyzed for concentrations of strontium-90, cesium-137, cobalt-60, and mercury in specific target tissues, and for single-stranded DNA breaks, a non-specific indicator of possible exposure to genotoxic agents in the environment. Significantly higher concentrations of ⁹⁰Sr, ¹³⁷Cs, ⁶⁰Co, and mercury were detected in turtles from White Oak Lake than in turtles from the reference site. In addition, turtles from White Oak Lake contained a significantly greater amount of DNA damage than those from the reference site. Although this suggests greater exposure of White Oak Lake turtles to genotoxic agents, further studies are needed to establish the cause of the enhanced amount of singlestranded breaks.

Interspecific comparisons of the turtles from White Oak Lake indicated that diet may play a significant role in the exposure of turtles to certain contaminants. No difference was detected between the concentrations of ⁹⁰Sr, ¹³⁷Cs, ⁶⁰Co between the two species. Mercury concentrations in C. serpentina were significantly higher than in T. scripta. No difference was detected in the amount of DNA damage between T. scripta and C. serpentina. Although both species were effective monitors of contaminants in White Oak Lake, C. serpentina (the carnivorous species) may be of greater value in the detection of contaminants with high biomagnification potential such as methylmercury. In addition, less variability detected in the C. serpentina data than in the T. scripta data suggests the former species as having greater uniformity of exposure to the contaminants in White Oak Lake, thereby favoring the use of C. serpentina as a bioindicator species.