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Factors Affecting Mercury Stable Isotopic Distribution in Piscivorous Fish of the Laurentian Great Lakes Ryan F. Lepak, Sarah Elizabeth Janssen, Runsheng Yin, David P. Krabbenhoft, Jacob M Ogorek, John F. DeWild, Michael T. Tate, Thomas M. Holsen, and James P. Hurley Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06120 • Publication Date (Web): 15 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018
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Environmental Science & Technology
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Factors Affecting Mercury Stable Isotopic Distribution in Piscivorous Fish of
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the Laurentian Great Lakes
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Ryan F. Lepak1,2, Sarah E. Janssen2, Runsheng Yin1,3, David P. Krabbenhoft2, Jacob M. Ogorek2, John F. DeWild2, Michael T. Tate2, Thomas M. Holsen4 and James P. Hurley*1,5,6
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Environmental Chemistry and Technology Program, University of Wisconsin-Madison, Madison, WI 53706, USA
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State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China
Institute for a Sustainable Environment, Clarkson University, Potsdam, New York 13676, USA 5
Department of Civil and Environmental Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA
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United States Geological Survey, Wisconsin Water Science Center, Middleton, Wisconsin 53562, USA
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University of Wisconsin Aquatic Sciences Center, Madison, WI 53706, USA
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* corresponding author
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James P. Hurley, University of Wisconsin-Madison, Environmental Chemistry and Technology
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Program, Madison, WI 53706;
[email protected]; 608-262-0905.
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Abstract
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Identifying the sources of methylmercury (MeHg) and tracing the transformations of mercury
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(Hg) in the aquatic food web are important component of effective strategies for managing
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current and legacy Hg sources. In our previous work, we measured stable isotopes of Hg (δ202Hg,
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∆199Hg, and ∆200Hg) in the Laurentian Great Lakes and estimated source contributions of Hg to
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bottom sediment. Here, we identify isotopically distinct Hg signatures for Great Lakes trout
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(Salvelinus namaycush) and walleye (Sander vitreus), driven by both food web and water quality
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characteristics. Fish contain high values for odd-isotope mass independent fractionation (MIF)
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with averages ranging from 2.50 (Western Erie) to 6.18‰ (Superior) in ∆199Hg. The large range
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in odd-MIF reflects variability in the depth of the euphotic zone, where Hg is most likely
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incorporated into the food web. Even-isotope MIF (∆200Hg), a potential tracer for Hg from
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precipitation, appears both disconnected from lake sedimentary sources and comparable in fish
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among the five lakes. We suggest that similar to the open ocean, water-column methylation also
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occurs in the Great Lakes possibly transforming recently deposited atmospheric Hg deposition.
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We conclude that the degree of photochemical processing of Hg is controlled by phytoplankton
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uptake rather than by dissolved organic carbon quantity among lakes.
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INTRODUCTION Despite reduction in contaminant point-source inputs since the 1970’s, fish consumption
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advisories remain in the Laurentian Great Lakes. In an ongoing effort to address beneficial use
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impairments in the Great Lakes Areas of Concern (AOC), water resource managers continue to
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develop tools that address water quality, the range of trophic status, the disparate sources of
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contaminants, and the magnitude of watershed inputs versus those from direct atmospheric
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deposition.1-3 Variations in land use may lead to differing rates of contaminant loading, which in
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turn can influence contaminant cycling in lakes. In addition, estuaries and many AOC’s are
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typically zones of elevated primary production, thus creating an entry point for watershed-
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derived and locally-discharged contaminants to the aquatic food web. The Great Lakes span a
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wide range in trophic status, from hypereutrophic in Western Lake Erie, to oligotrophic in
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offshore regions of Lakes Superior, Michigan, and Huron.1 The drivers for trophic status are
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similar to drivers for contaminant levels in lakes. These include land use, nutrient-loading, and
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agricultural and urban influences.
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Sources of mercury (Hg) to Great Lakes include local industrial discharge, watershed
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runoff, and direct atmospheric deposition,4 but the reactivity of the mercury derived from these
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sources susceptible to bioaccumulation is dependent on microbially-mediated conversion of
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inorganic Hg to methylmercury (MeHg).5 In the Great Lakes, microbial Hg methylation is
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typically assumed to occur in anoxic sediment, wetlands, and in nearshore regions such as the
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growth and depositional zones of Cladophora.5–8 The identification of sources of MeHg to biota
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is crucial to understanding why pelagic fish in the Great Lakes are elevated in MeHg. The
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offshore regions of the Great Lakes exhibit some of the lowest aqueous MeHg concentrations
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reported, often rivaling the open oceans.6,9–11
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The ability to quantify the reactivity of Hg from specific sources susceptible to
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methylation is a key to contaminant management. Analysis of natural isotopic variations of Hg
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stable isotopes have been useful for this purpose.12 Measuring subtle changes in natural isotopic
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Hg ratios using high resolution multicollector inductively coupled plasma mass spectrometry
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(MC-ICPMS) allows researchers to discriminate among Hg sources and better understand the
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physical, chemical, biological, and photochemical processes affecting Hg cycling.12
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Processes driven by kinetics, or as the result of equilibrium exchange, are delineated
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through an analysis of isotope-specific mass dependent fractionation (MDF).13–18 Typically
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δ202Hg is used to denote MDF and MDF is a useful indicator for differentiating Hg sources.19–21
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In addition, unlike many other heavy-metal stable isotopes in natural ecosystems, Hg is
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susceptible to mass independent fractionation (MIF), often the result of photochemical
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processing.22 When exposed to sunlight, odd isotopes of Hg undergo magnetic isotope effects
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(MIE)23 and to a lesser extent, nuclear volume effects (NVE).24 These effects lead to an
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enrichment of odd Hg isotopes in the reactant pool (i.e.: natural waters) and a depletion of odd-
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numbered Hg isotopes in the product pool (i.e.: evasive gaseous Hg°). Odd-MIF is denoted here
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as ∆199Hg. It should be noted that MIF forming processes also impact MDF signatures, often in a
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semi-quantitative manner. Although not fully delineated, even-MIF has been observed in
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environmental matrices and linked to atmospherically-sourced Hg in the gaseous phase,25
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precipitation,26,27 sediment,4,28 surface waters,28,29 and fish.30 While the mechanistic
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understanding of this process is still developing, research suggests that even-MIF formation is
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the result of reactions in the tropopause involving UV-induced halogen radicals (with •Br being
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of greatest impact).26,31,32 Upon deposition to a terrestrial or aquatic ecosystems, even-MIF is
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presumed conservative, however, it is susceptible to dilution. Together, these three isotopic
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fingerprinting techniques (MDF, odd-MIF and even-MIF) can serve as a three dimensional
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roadmap to better understand Hg sources and processing of Hg in the natural environment.4,19,21
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In this study, we investigate Hg stable isotopic signatures in piscivorous fish from the
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Laurentian Great Lakes to better understand the sources and processing-pathways for Hg in the
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ecosystem. To date, Hg isotope research has focused primarily on sedimentary inorganic Hg,
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which is then susceptible to methylation, as the primary source of MeHg to fish.12 Studies have
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also suggested the magnitude of odd-MIF is correlative with aqueous dissolved organic carbon
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(DOC) concentration and quality due to both DOC - Hg associations promoting photoreactions,
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and DOC as a factor inhibiting light attenuation.33–36 Our study compares isotopic signatures of
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MeHg, measured as HgT, in higher trophic level fish among Great Lakes, to signatures from
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bottom sediments. We also investigate the role of water quality on the isotopic Hg composition
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of these key fish species used for monitoring ecosystem health. Based on preliminary results
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from our previous work,4 we hypothesized that sources of MeHg to the food web would be
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decoupled from sediment. Here, we compare MDF and MIF in piscivorous fish and compare
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water quality characteristics among the lakes in an effort to understand both the sources and the
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processes that influence isotopic signatures for these bioindicators.
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MATERIALS AND METHODS
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Fish collection and HgT analysis
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Fish were obtained from each of the Great Lakes (Michigan, Huron, Superior, Ontario,
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and Erie) from 2004 to 2013 following protocols in the Great Lakes Fish Monitoring and
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Surveillance Program (GLFMSP).37 Two annually-alternating locations per lake included an
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even-year shallow (nearshore) and an odd-year deep (offshore) location. Composited samples
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representing five equally proportioned (600-700 mm) lake trout (Salvelinus namaycush) were
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collected from all lakes except Erie, where walleye (Sander vitreus, 400-500 mm) represented
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indicator fish species, due to insufficient lake trout harvests. Detailed site locations (Figure S1)
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are described elsewhere (GLFMSP 2012).37 Upon collection, fish were frozen (
