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Mercury bioaccumulation assessment for the St. Louis River Area of Concern
Citation:
Hanson, G., Greg Peterson, S. Janssen, J. DeWild, J. Ogorek, R. Lepak, M. Tate, N. Johnson, A. White, B. Monson, D. Krabbenhoft, AND J. Hoffman. Mercury bioaccumulation assessment for the St. Louis River Area of Concern. St. Louis River Estuary Summit, Superior, WI, March 13 - 14, 2018.
Impact/Purpose:
Sediment remediation sites commonly contain high levels of mercury. Associating related exposure, bioaccumulation, and effect often is confounded by atmospheric and other regional sources of mercury. We applied a new tracer of mercury source (mercury isotopic composition) to assess mercury related sources with respect to bioaccumulation in a Great Lakes Area of Concern, and demonstrate its utility for identifying where legacy mercury from contaminated sediments is the dominant source of bioavailable of mercury.
Description:
Both Minnesota and Wisconsin have posted fish consumption advisories within the St. Louis River Area of Concern (SLR AOC), in part because fish have elevated mercury concentrations. To assess mercury concentrations in fish tissue within the SLR AOC relative to reference conditions, our project goals were to evaluate relative methylation potential and food web bioaccumulation between the SLR AOC and the Bad River (reference), and to assess the contribution of legacy-related mercury contributions (versus other sources) to the SLR AOC food web using mercury stable isotope ratios. During summer 2017, research teams collected water, sediment, and biota from six SLR AOC and four Bad River “zones” to be analyzed for total (THg) and methyl (MeHg) mercury concentrations; a subset of samples are being analyzed for mercury stable isotope ratios. To characterize trophic relationships, biota samples will also be analyzed for carbon and nitrogen stable isotope ratios. Food web collections included algae, zooplankton, benthic invertebrates, riparian invertebrates, prey fish, piscivorous fish, and nesting birds. Based on preliminary data, Odonate mercury concentrations demonstrated similar patterns between SLR AOC and the Bad River. Based on Hg stable isotope ratios, after corrections for photochemical demethylation, both Odonates and lower estuary fish strongly resemble legacy sediments. We conclude that the combined information of mercury concentration and stable isotope composition can serve as powerful diagnostics for mercury assessment.