Mercury Bioaccumulation in Florida Green Watersnakes (Nerodia floridana) Inhabiting Former Nuclear Cooling Reservoirs on the Savannah River Site Amelia L. Russell, M. Kyle Brown, Michaela M. Lambert, Tracey D. Tuberville, Melissa A. Pilgrim Division of Natural Sciences and Engineering University of South Carolina Upstate 800 University Way Spartanburg, SC 29303 {amelialr, mkb}@email.uscupstate.edu;
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[email protected] Abstract — Anthropogenic activities have significantly increased the amount of mercury (Hg) cycling globally. Mercury can become bioavailable, accumulate in organisms, biomagnify in food webs, and can negatively impact wildlife health. Mercury contamination on the Savannah River Site (SRS) is a result of atmospheric deposition, coal combustion, and use of contaminated water from the Savannah River in nuclear reactor cooling reservoirs. Florida green watersnakes, Nerodia floridana, are top predators that inhabit the reservoirs and can serve as bioindicators of Hg contamination. We captured 76 snakes from three reservoirs: Pond B, Pond 2, and PAR Pond. We took tail clip samples from captured snakes and quantified total mercury (THg) concentrations. Snake THg concentrations were not significantly different among sites. We determined there was a significant relationship between snout-vent length and THg in N. floridana. Our work demonstrated that N. floridana can serve as bioindicators of Hg contamination in aquatic systems.
Keywords — Mercury, Ecotoxicology, Watersnake, Savannah River Site, Bioindicator
Introduction Anthropogenic activities have significantly increased the amount of mercury (Hg) cycling globally. Increased national and international recognition of Hg as an environmental concern has resulted in more effective regulations and remediation processes for reducing Hg pollution (EPA 2011). However, legacy Hg from anthropogenic sources such as mining and burning fossil fuels will persist for generations to come (Amos et al. 2013). Mercury can become bioavailable, accumulate in organisms adversely altering their neurological and reproductive systems (Edwards et al. 2014; Green et al. 2010), and biomagnify up trophic levels (Drewett et al. 2013). The U.S. Department of Energy’s Savannah River Site (SRS) is a former nuclear production site that was constructed in the 1950’s and that encompasses nearly 200,000 acres. The SRS is classified as a Superfund Site by the Environmental Protection Agency (EPA 2016). It is an ideal setting for field-based studies determining the levels and effects of contaminants on wildlife (Burger et al. 2006; Tuberville et al. 2011), including Hg. Previous research has shown Hg is a contaminant of concern on the SRS (Burger et al. 2001; Edwards et al. 2014; Kvartek et al. 1994; Tuberville et al. 2011) and is a result of atmospheric deposition from nearby and on-site industrial emissions, coal combustion, and use of contaminated Savannah River water for site operations. Florida green watersnakes (FGW), Nerodia floridana, are primarily piscivorous and top predators in aquatic ecosystems. They are typically found in open, lentic aquatic environments with floating vegetation and grasses (Gibbons & Dorcus 2004), which on the SRS includes former nuclear cooling reservoirs. They are also relatively long-lived reptiles – a trait they share with other species of watersnakes that are known to bioaccumulate contaminants such as Hg as a result of long-term exposure (Drewett et al. 2013). The primary objectives for our study were to (1) assess site level differences of Hg concentrations in FGW among three wetlands and (2) determine if Hg levels are correlated with body size of FGW.
Materials & Methods FIELD SAMPLING OF WATERSNAKES During 10 – 30 June 2016, we collected FGW from three former cooling reservoirs on the SRS: Pond B, Pond 2 and PAR Pond. We set a trap line of 20 arrays at each wetland. An array consisted of 4 minnow traps and 1 funnel trap; thus, we set a total of 100 traps per wetland. We checked traps daily and brought captured snakes into the lab for processing.
LABORATORY PROCESSING OF WATERSNAKES
We determined the sex and measured the mass, snout-vent length (SVL; length from snout tip to cloaca), and tail length (length from cloaca to tail tip) of each snake. We took approximately 1 cm tail clips for Hg analysis. We obtained wet weights for each tail clip and stored tail clips in a -60 °C freezer until an ample number of samples were ready for Hg analysis. Once we had an adequate number of tail tips to complete a mercury run (mercury analyzer held 40 samples), we oven-dried tail clips for a minimum 24 hours at 40 °C. We recorded the dry weight of each tail tip. We used a DMA-80 Tri Cell Direct Mercury Analyzer (Milestone, Shelton, CT, USA) to determine total mercury (THg) concentrations in tail tips, which are reported on a dry mass basis.
Results We captured a total of 76 snakes, ranging in 29 – 82 cm SVL. Analysis of variance (ANOVA) indicated significant among-site variation in average watersnake size (Figure 1; ANOVA: F (2,73) = 14.75; p < 0.001). Post-hoc test revealed that average SVL was significantly different at PAR Pond, Pond B, and Pond 2. Therefore, on average watersnakes from PAR Pond were the largest and Pond 2 watersnakes were the smallest. Total mercury concentrations of captured snakes ranged from 0.167 ppm to 2.10 ppm (dry weight). Because we determined that snake size differed significantly among sites, we used an analysis of covariance (ANCOVA) with size as a covariate. The ANCOVA indicated that average THg was not significantly different among sites after controlling for the effect of body size (Figure 2; ANCOVA: F (2,73) = 2.120, p = 0.127). Because THg did not vary among sampling sites, tail clip samples from all sites were combined to determine the overall effect of body size on THg in Florida green watersnakes. Regression analysis indicated there was a significant positive relationship between size and THg concentration (Figure 3; Regression: R2 = 0.36, t = 6.49, d.f. = 75, p < 0.001).
Conclusions We detected Hg in snakes from all three sites, supporting that mercury is a pervasive contaminant in aquatic systems. However, average THg did not vary significantly among sites. As predicted, Hg levels increased as SVL increased. Snakes are gape limited predators and their diet changes as they grow larger. Larger watersnakes are likely exposed to higher diet Hg. In addition, larger, mature adults have presumably experienced longer-term exposure to contaminants than have the smaller-bodied juveniles. Our work complements that of the ongoing Long-Lived Reptile Project on the SRS. Mercury levels from FGW tail clips in our study fall between those of Yellow-bellied slider turtles, Trachemys scripta scripta, and American alligators, Alligator mississippiensis (Tuberville et al 2011). Sliders feed at a lower trophic level than our snakes (Clark & Gibbons 1969), while alligators feed at a higher trophic level than the snakes (Delany & Abercrombie 1986; Weldon & McNease 1991). Watersnake Hg concentrations relative to sympatric reptiles at other trophic levels corresponds to expected patterns if biomagnification of Hg was occurring in the aquatic systems we sampled. Our work demonstrates that FGW can serve as bioindicators of Hg contamination in aquatic systems.
Acknowledgements We thank Kimberly Price, Kurt Buhlmann, & Kirsten Work for trapping assistance, David Haskins for assistance with trapping & snake processing, & entire Larry Bryan Lab for sample donations. Perry Bovan from Radiological Control for ensuring a safe working environment in the field & Angela Lindell for assistance in the Mercury Laboratory. In addition, Wendy Bogard & Ian Scollon for being a constant reminder to never settle for less than your dreams. Funding was provided by the National Science Foundation, Area Completion Projects of Savannah River Nuclear Solutions, & work was supported by the Department of Energy under Award Number DE-FC09-07SR22506 to the University of Georgia Research Foundation. Animals were collected under South Carolina Department of Natural Resources Permit #022016, & all animal procedures were conducted in accordance with Animal Use Permit A2016 02-066-Y1A0 and approved by the University of Georgia IACUC.
References Amos, H.M., Jacob, D.J., Streets, D.G., & Sunderland, E.M. 2013. Legacy impacts of all-time anthropogenic emissions on the global mercury cycle. Global Biogeochemical Cycles. 27: 410-421. Burger, J., Gaines, K.F., Boring, C.S., Stephens, Jr. W.L., Snodgrass, J., & Gochfeld, M. 2001. Mercury and selenium in fish from the Savannah River: Species, trophic level, and locational differences. Environmental Research 87A: 108-118. Burger, J., Murray, S., Gaines, K.F., Novak, J.M., Punshon, T., Dixon, C., & Gochfeld, M. 2006. Element levels in snakes in South Carolina: Differences between a control site and exposed site on the Savannah River Site. Environmental Monitoring and Assessment. 112: 35-52. Clark, D.B., & Gibbons, J.W. 1969. Dietary shift in the turtle Pseudemys scripta (Schoepff) from youth to maturity. American Society of Ichthyologists and Herpetologists. 1969(4): 704-706. Delany, M.F., & Abercrombie, C.L. 1986. American alligator food habits in Northcentral Florida. The Journal of Wildlife Management. 50(2): 348-353. Edwards, P.G., Gaines, K.F., Bryan, Jr. A.L., Novak, J.M., & Blas SA. 2014. Trophic dynamic of U, Ni, Hg, & other contaminants of potential concern on the Department of Energy’s Savannah River Site. Environmental Monitoring Asses. 186: 481-500 Environmental Protection Agency. 2011. Regulatory impact analysis for the final mercury and air toxic standards (Report No. EPA-152/R-11-011). Research Triangle Park, NC. 27 January 2017. Web. http://www.epa.gov/ttnecas1/regdata/RIAs/matsriafinal Environmental Protection Agency. 2016. EPA superfund program: Savannah River Site (USDOE), Aiken, SC. 9 October 2016. Web. https://cumulis.epa.gov/supercpad/cursites/csitinfo.cfm?id=0403485 Drewett, D.V.V., Wilson, J.D., Cristol, D.A., Chin, S.Y., & Hopkins, W.A. 2013. Inter- and intraspecific variation in mercury bioaccumulation by snakes inhabiting a contaminated river floodplain. Environmental Toxicology and Chemistry. 32(5): 1178-1186. Green, A.D., Buhlmann, K.A., Hagen, C., Romanek, C., & Gibbons, J.W. 2010. Mercury Contamination in Turtles & Implications for Human Health. Journal of Environmental Health. 72(10): 1-22. Gibbons, J.W., & Dorcus, M.E. North American watersnakes: A natural history. Norman: University of Oklahoma Press. 117-124. Kvartek, E.J., Carlton, W.H., Denham, M., Eldridge, L., & Newman, M.C. 1994. Assessment of mercury in the Savannah River Environment (Report No. WSRC-TR-94-0218). Westinghouse Savannah River Company, Aiken, SC. 1-76. Tuberville, T.D., Metts, B.S., & Scott, D.E. 2010-2011. Reptiles as long-lived receptors for ecological risk assessment on the Savannah River Site, South Carolina. University of Georgia’s Savannah River Ecology Laboratory, Aiken, South Carolina. Weldon, P.J., & McNease, L. 1991. Does the American alligator discriminate between venomous and nonvenomous snake prey? Herpetologica. 47(4): 403-406.
Figure 1: Among-site differences in average snout-vent-length (SVL) of Florida green watersnakes, Nerodia floridana (ANOVA: F (2,73) = 14.75, p < 0.001). Letters above bars (a, b, & c) represent statistically significant differences (p < 0.05) among sites. Data are presented as means ± SE.
Figure 2: Average total mercury (THg) in Florida green watersnakes, Nerodia floridana, did not differ significantly among-sites (ANCOVA: F (2,73) = 2.120, p = 0.127). Data are presented as means ± SE.
Figure 3: Effect of body size on total mercury in Florida green watersnakes, Nerodia floridana, (all sites combined; Regression: r = 0.60, t = 6.49, d.f. = 75, p < 0.001).
