Changes in stable isotope composition in Lake Michigan trout - a 40 year perspective
Citation:
Lepak, R., J. Hoffman, S. Janssen, D. Krabbenhoft, J. Ogorek, J. DeWild, M. Tate, C. Babiarz, R. Yin, E. Murphy, AND J. Hurley. Changes in stable isotope composition in Lake Michigan trout - a 40 year perspective. International Conference on Mercury as a Global Pollutant, Providence, RI, July 16 - 21, 2017.
Impact/Purpose:
not applicable
Description:
Researchers have frequently sought to use environmental archives of sediment, peat and glacial ice to try and assess historical trends in atmospheric mercury (Hg) deposition to aquatic ecosystems. While this information is valuable in the context of identifying temporal source trends, these types of assessments cannot account for likely changes in bioavailability of Hg sources that are tied to the formation of methylmercury (MeHg) and accumulation in fish tissues. For this study we propose the use of long-term fish archives and Hg stable isotope determination as an improved means to relate temporal changes in fish Hg levels to varying Hg sources in the Great Lakes. For this study we acquired 180 archived fish composites from Lake Michigan over a 40-year time period (1975 to 2014) from the Great Lakes Fish Monitoring and Surveillance Program, which were analyzed for their total Hg content and Hg isotope abundances. The results reveal that Hg sources to Lake Michigan trout (Salvelinus namaycush) have encountered considerable changes as well as a large shift in the food web trophic position as a result of the introduction of several invasive species, especially the recent invasion of dreissenid mussels. Total Hg concentrations span a large range (1,600 to 150 ng g-1) and exhibit large variations from 1975 to 1985. Ä199Hg signatures similarly exhibit large variation (3.2 to 6.9‰) until 1985, followed by less variation through the end of the data record in 2014. ä202Hg showed a relatively consistent value of about 0.4 0.8‰ from 1975 to 1988, followed by a significant (P<0.0001) shift of about 0.7‰ between 1989 and 1996, with relative stability through the end of the record (1.2 - 1.6‰). This ä202Hg shift may be linked to a combination of the contemporaneous implementation of Clear Air Act rules limiting Hg emissions from waste incineration and to changes in carbon cycling resulting from invasive mussels. Utilizing annual Hg stable isotope variability, we propose a dietary convergence in trout isotope values is observable following the onset of dreissenid invasion. We aim to independently confirm this observation using more traditional stable isotopes (C and N) as well as statistical modeling of isotopic shifts. We propose that with the combined use of Hg, C and N stable isotopes, we can begin to identify how Hg in fish have been affected by shifting sources portfolios of Hg and restructuring of food webs over the past 40 years in Lake Michigan.