Abstract
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 the Mercury 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.
Highlights
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 the late 1970s, the US Environmental Protection Agency (US EPA) Great Lakes Fish Monitoring and Surveillance Program (GLFMSP) was established to assess ecosystem health using top predator fish, which were archived as sentinels for monitoring chemical contaminants
Each composed of five 400- to 600-mm whole-body lake trout grinds collected during the fall season from 1978 to 2012, averaged 361 ng g−1 HgT dry weight (117 ng g−1 1 SD; n = 132 composites, 660 individual fish) with a maximum HgT of 812-ng g−1 dry weight and a minimum 182 ng g−1 (Fig. 1A)
Summary
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). As domestic Hg mitigation strategies have affected the emission portfolio of Lake Michigan’s airshed during the time covered by the archive (1978 to 2012), we hypothesize that changes in the Hg isotopic composition and MeHg concentration of fish will be evident.
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