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A multi-stable isotope approach to determine mercury sources to St. Louis River fishes
Janssen, S., J. Hoffman, R. Lepak, B. Monson, G. Peterson, G. Hanson, AND D. Krabbenhoft. A multi-stable isotope approach to determine mercury sources to St. Louis River fishes. Minnesota-Dakotas Chapters of the American Fisheries Society, Fargo, ND, February 25 - 27, 2019.
Mercury contamination in sediments is common throughout Great Lakes Areas of Concern. To date, the complexity of mercury cycling and the existence of multiple mercury sources has presented substantial challenges to understanding what effect remediation can have on mercury bioaccumulation in a particular Area of Concern. Here, we demonstrate that legacy mercury is a major contributor to mercury bioaccumulation in the food web of the St. Louis River Area of Concern, but that the relative contribution compared to other sources does vary among species and locations. This is the first such data set from a Great Lakes Area of Concern.
Mercury (Hg) contamination in the Great Lakes region is a prevalent concern due to elevated Hg concentrations in fish in relation to human health and wildlife guidelines. The lower St. Louis River, the largest tributary to Lake Superior, exhibits fish Hg concentrations twice that of the open lake. Despite these highly elevated concentrations, it is difficult to infer the sources of Hg to these fish due to multiple potential Hg inputs (e.g. precipitation, legacy, or upstream runoff). The aim of this study was to utilize Hg stable isotopes along with carbon and nitrogen stable isotopes to elucidate Hg sources to the St Louis River food web including benthic invertebrates, prey and game fish. Invertebrates and prey fish (yellow perch and shiners) from the St Louis River exhibited Hg isotopic fingerprints similar to Hg-contaminated sediments (ä202Hg = -0.6 to -0.4), indicating bioaccumulation of legacy Hg. Game fish (walleye and northern pike) displayed greater variability in isotope values, with some individuals exhibiting highly enriched signatures (ä202Hg = 0.8 to 1.2) compared to prey species, likely due to precipitation. Relationships between ä202Hg and ä13C of walleye revealed that substantial variability was related to foraging in Lake Superior. This indicates that legacy Hg is the prevalent source to smaller fish species in the St Louis River, but larger species can be exposed to different Hg sources based on foraging habits.