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Emissions regulations and food web shifts alter mercury signatures of top predator fish
Citation:
Lepak, R., J. Hoffman, S. Janssen, D. Krabbenhoft, J. Ogorek, D. Engstrom, E. Murphy, AND J. Hurley. Emissions regulations and food web shifts alter mercury signatures of top predator fish. State of Lake Superior Conference, Houghton, MI, October 09 - 12, 2018.
Impact/Purpose:
Every five years, the Cooperative Science and Monitoring Initiative (CSMI) provides a whole-lake perspective on the state of the lake food web. For ongoing fish health studies, the CSMI augments ongoing, long-term monitoring of fish contaminant levels, particularly to demonstrate new contaminant monitoring approaches. In this study, archived tissue samples were analyzed using a mix of stable isotope ratios to investigate how both mercury sources and food web changes have altered mercury sources and bioaccumulation in Lake Michigan fishes. The observed shifts in stable isotope ratios suggest fish are continuing to respond to changes in US mercury mitigation strategies, and highlights the importance of regional source reduction in affecting Hg concentration and isotopic distribution in Great Lakes fisheries.
Description:
Environmental geochemists frequently use archives of sediment, peat, and glacial ice to reconstruct historical trends in atmospheric mercury (Hg) deposition to ecosystems. We used a long-term 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, food web tracers (carbon [C] and nitrogen [N] stable isotope ratios) identified food web influences on total Hg concentration variability (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 diet transitions led to the assimilation of contrasting Hg pools (2000 to present). The 1988-1992 ä202Hg shift, confirmed by two sediment cores, was likely caused by reductions in regional Hg emissions. In contrast, the post-2000 isotope transitions were likely the result of altered food web pathways, which revealed a benthic diet shift following dreissenid mussel invasion. The continued recent shifts in ä202Hg suggest fish are continuing to respond to changes in US mitigation strategies, and highlights the importance of regional source reduction in affecting Hg concentration and isotopic distribution in Great Lakes fisheries.