To understand the impact reduced mercury (Hg) loading and invasive species have had on methylmercury bioaccumulation in predator fish of Lake Michigan, we reconstructed bioaccumulation trends from a fish archive (1978 to 2012). By measuring fish Hg stable isotope ratios, we related temporal changes in Hg concentrations to varying Hg sources. Additionally, dietary tracers were necessary to identify food web influences. Through combined Hg, C, and N stable isotopic analyses, we were able to differentiate between a shift in Hg sources to fish and periods when energetic transitions (from dreissenid mussels) led to the assimilation of contrasting Hg pools (2000 to present). In the late 1980s, lake trout δ202Hg increased (0.4‰) from regulatory reductions in regional Hg emissions. After 2000, C and N isotopes ratios revealed altered food web pathways, resulting in a benthic energetic shift and changes to
Hg bioaccumulation. Continued increases in δ202Hg indicate fish are responding to several United States mercury emission mitigation strategies that were initiated circa 1990 and continued through
the 2011 promulgation of theMercury and Air Toxics Standards rule.
Unlike archives of sediments, this fish archive tracks Hg sources
susceptible to bioaccumulation in Great Lakes fisheries. Analysis reveals
that trends in fish Hg concentrations can be substantially affected
by shifts in trophic structure and dietary preferences initiated
by invasive species in the Great Lakes. This does not diminish the
benefits of declining emissions over this period, as fish Hg concentrations
would have been higher without these actions.