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Emissions regulations and food web shifts alter mercury contamination signatures of lake trout in Lake Michigan
Citation:
Lepak, R., J. Hoffman, S. Jannsen, J. Ogorek, J. DeWild, M. Tate, C. Babiarz, D. Krabbenhoft, R. Yin, E. Murphy, D. Engstrom, AND J. Hurley. Emissions regulations and food web shifts alter mercury contamination signatures of lake trout in Lake Michigan. Twin Ports Early Careers Researchers, Duluth, MN, March 28, 2019.
Impact/Purpose:
Elevated mercury in fish poses risks to fish-consuming wildlife and humans. Tracing sources of mercury to fish with stable isotope ratios leads to improved source-receptor understanding and natural resource management. This work uses fish and sediment archives to trace the response of recent domestic mercury mitigation actions. These findings reveal the importance of domestic mercury sources relative to global mercury to the Great Lakes and implies methylmercury concentrations in fish cannot be predicted by emission inventories alone.
Description:
Environmental geochemists frequently use archives of sediment, peat, and glacial ice to reconstruct historical trends in atmospheric mercury (Hg) deposition to ecosystems. These archives primarily record the deposition of inorganic Hg and fail to adequately reconstruct the history of bioaccumulated methylmercury. Here, we used a fish archive, measuring Great Lakes whole fish Hg stable isotope ratios, to relate temporal changes in Great Lakes Hg concentrations to varying Hg sources. In addition, dietary tracers (carbon [C] and nitrogen [N] stable isotope ratios) identified food web influences on total Hg concentration which ranged 180 to 810 ng g-1. By utilizing Hg, C, and N stable isotopes, we show a significant shift in Hg sources to fish (1988 1992), and periods when energetic transitions (driven by dreissenid mussels) led to the assimilation of contrasting Hg pools (2000 to present). The 1988-1992 lake trout ä202Hg increase (0.4‰), confirmed by two Lake Michigan sediment cores, was likely caused by reductions in regional Hg emissions due to implemented regulations. In contrast, the post-2000 transition in the composition of many isotopes was likely the result of altered food web pathways, indicated by C and N stable isotope ratios, which revealed a benthic energetic shift following dreissenid mussel invasion. Further, the increasing ä202Hg following 2000 suggest fish are continuing to respond to changes in US mercury emission mitigation strategies (e.g. Mercury and Air Toxics Standards) and highlights the importance of regional source reduction in affecting Hg concentration and isotopic distribution in Great Lakes fisheries.