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Determination of MeHg sources to fish in the St. Louis River, MN, USA, using Hg stable isotopes
Citation:
Janssen, S., R. Lepak, J. Hoffman, B. Monson, J. Ogorek, J. DeWild, M. Tate, AND D. Krabbenhoft. Determination of MeHg sources to fish in the St. Louis River, MN, USA, using Hg stable isotopes. International Conference on Mercury as a Global Pollutant, Providence, RI, July 16 - 21, 2017.
Impact/Purpose:
not applicable
Description:
Mercury contamination in the Great Lakes region has become a prevalent concern due to elevated methylmercury (MeHg) levels in fish. While atmospheric deposition of Hg is ubiquitous, releases from legacy point-sources give rise to numerous Areas of Concern (AOCs) across the Great Lakes region. One of these AOCs is the lower St. Louis River estuary, which has MeHg concentrations in predatory fish double that of Lake Superior. Despite these highly elevated concentrations, it is difficult to infer the sources of the MeHg to these fish due to different Hg(II) inputs, as well as the complex ecology of estuarine systems. The aim of this study is to utilize Hg stable isotopes to elucidate sources of MeHg to populations of walleye and white suckers in the St. Louis River and Lake Superior region. Walleye from Lake Superior display enhanced Ä199Hg ? 5 ‰ and significant Ä200Hg = 0.08-0.10 ‰ for total Hg (THg) isotopes, likely from atmospheric influence and high levels of photochemical demethylation in the MeHg source. However, white suckers from Superior show depleted Ä199Hg and ä202Hg signatures and no significant Ä200Hg. This indicates that there are two distinct Hg sources for these fish in Lake Superior, presumably benthic and pelagic derived MeHg. In contrast to Lake Superior, the walleye and suckers feeding in the St. Louis River mirror the depleted signature observed in white suckers from the lake, which strongly suggests that Hg sources to these populations are obtained from the estuary and are likely sediment-based. While data supports sediment being the primary source of MeHg to fish in the St Louis River it still remains unclear whether the MeHg being produced is derived from legacy or more recent Hg (II), possibly of terrestrial or wetland origin. Preliminary data shows the presence of Ä199Hg (-0.12 ‰) and Ä200Hg (-0.05 ‰) anomalies as well as negative ä202Hg (-0.90 to -1.7) in upstream sediment THg commonly associated with precipitation and vegetation (litterfall). This signature is likely diluted downstream due to the high concentrations of legacy Hg (II) in the estuary and is not reflected in the bulk analysis of THg isotopes even though there is still a potential for methylation of this pool. In order to fully address the sources of MeHg to the St. Louis food web, further experiments examining the isotopic composition of MeHg pools in sediments and fish tissue will be performed.