Temporal and spatial patterns of contaminants in Lake Erie watersnakes (Nerodia sipedon insularum) before and after the round goby (Apollonia melanostomus) invasion

Temporal and spatial trends in contaminant concentrations were assessed in Lake Erie watersnakes, a threatened (USA)/endangered (Canada) species restricted to western Lake Erie. Temporal changes in plasma contaminant levels were determined in 1990 and 2003, and spatial patterns in 2003 at 12 sites, throughout the species' range. During this period, the watersnakes' diet changed from fish (75%) and amphibians (25%) that avoid zebra mussels, to round gobies (95%) that feed extensively on zebra mussels. Temporal trends indicate that watersnakes on Pelee and North Bass Islands showed a marginal increase in hexachlorobenzene levels, and a significant decline in dieldrin, oxychlordane, and heptachlor epoxide, likely reflecting declines in aerial deposition and clearing of local vineyards. The contaminants with the greatest burdens, sum PCBs and p,p′-DDE, remained stable in the snakes, consistent with trends in other local biota, suggesting that although the dietary switch to round gobies meant consumption of a more contaminated diet, their diet remained at the same trophic position. We suggest that the watersnakes' PCB and p,p′-DDE temporal patterns reflect the lack of change in sediment concentrations with minimal influence from their dietary switch. Similar to top avian predators, PCBs, p,p′-DDE, and technical chlordane, are most prevalent in watersnakes; this ranking remains unchanged. In 2003, the watersnakes demonstrated significant spatial differences in concentrations of p,p′-DDE, dieldrin, technical chlordane and its metabolites. Their 2003 concentrations of p,p′-DDE, and to a lesser extent PCBs, exceed the recommended interim no-observable effects levels on watersnake embryonic survival. Further investigations are required to determine if these higher levels of PCBs, p,p′-DDE, and technical chlordane, affect reproductive and physiological parameters of the Lake Erie watersnake. Until concentrations of sediment contaminants decline in western Lake Erie, these endangered/threatened watersnakes will continue to be exposed to higher concentrations of persistent organic pollutants.