Mercury is a concern for fish, wildlife, and human health because of its toxicity and tendency to bioaccumulate and biomagnify in food webs. Turtles are potentially excellent model organisms for contaminant studies because of their unique ecological and life-history attributes, which include their wide geographic distribution, variation in habitat types, and range of trophic levels in which they feed. In addition, turtles are long-lived, allowing for long-term exposure to contaminants.  Turtles often reach higher biomasses in an ecosystem compared with endotherms that occupy similar trophic levels. Moreover, their eggs and young are often important prey for other organisms. Yet, compared to birds and mammals, turtles have received little attention in terms of mercury contamination.

Experimental Approach

Turtle life histories, relative trophic positions, and dietary preferences in aquatic systems are likely important determinants of how much mercury is ingested and bioaccumulated. We examined the several turtle species inhabiting two mercury contaminated rivers in Virginia, the South River and the North Fork of the Holston River. A large mercury contamination gradient exists in both river systems, ranging from low concentrations, upstream from the point source to high concentrations downstream. At the South River we sampled red-bellied turtles (Pseudemys rubriventris), painted turtles (Chrysemys picta), stinkpots (Sternotherus odoratus), and snapping turtles (Chelydra serpentina) and at the North Fork of the Holston River we sampled snapping turtles, stinkpots, and common map turtles (Graptemys geographica). These aquatic species differ drastically in their feeding ecology, providing an opportunity to assess the influence of trophic niche on mercury accumulation within a single turtle assemblage. Blood (both rivers) and nail (North Fork of the Holston River only) samples were obtained from 100s of turtles along these rivers. Samples were analyzed for total mercury and blood samples were analyzed for stable isotopes of nitrogen and carbon.


We found differences in mercury concentrations in turtles from the contaminated sites at both rivers that were consistent with their known feeding ecology. In the South River, snapping turtles and stinkpots had higher mercury concentrations in their blood than painted turtles, and concentrations in primarily herbivorous red bellied turtles were lower than all other species. At the North Fork of the Holston River, mercury concentrations in nails and blood were consistently higher for snapping turtles and stinkpots than map turtles. These trends were supported by the isotope data, which suggested that individual turtles were feeding at more than one trophic level. Interestingly, stinkpots appear to have exposure that rivals that of snapping turtles at some sites. We suggest that the benthic feeding habits of stinkpots place them at risk of exposure to mercury that is comparable to large predatory snapping turtles.

We also determined that both blood and nail tissue are useful indices of mercury exposure in turtles, but that nails may be superior because they provide a signal of mercury exposure that is integrated over time. Because nondestructive tissue sampling is often preferred over lethal sampling, especially in long-lived vertebrates (e.g., turtles) and species of conservation concern, we believe both of these techniques show great promise for ecological monitoring of turtles.


The blood mercury concentrations for the turtles in our studies are some of the highest reported in reptiles, necessitating further studies to investigate the potentially adverse effects of these high concentrations. The best approach for determining whether mercury contamination is affecting turtles will be to assess their reproductive status using controlled incubation of eggs collected from females upstream and downstream from the contamination sources, and to relate the females’ blood, nail, and egg mercury concentrations to these reproductive outcomes.

Learn more: Listen to Dr. Bill Hopkins talk about his work with snapping turtles on Pulse of the Planet Radio