Variations in Stable Isotope Fractionation of Hg in Food Webs of Arctic Lakes NIKOLAUS GANTNER,* ,†,‡ HOLGER HINTELMANN, § WANG ZHENG, § AND DEREK C. MUIR †,‡ Department of Environmental Biology, University of Guelph, Gordon Street, Guelph, ON, N1G 2W1, Canada, Water Science and Technology Directorate, Environment Canada, 867 Lakeshore Drive, Burlington, ON, L7R 4A6, Canada, and Department of Chemistry, Trent University, 1600 West Bank Drive, Peterborough, ON, K9J 7B8, Canada Received June 16, 2009. Revised manuscript received October 3, 2009. Accepted October 12, 2009. Biotic and abiotic fractionation of mercury (Hg) isotopes has recently been shown to occur in aquatic environments. We determined isotope ratios (IRs) of Hg in food webs (zooplankton, chironomids, Arctic char) and sediments of 10 Arctic lakes from four regions and investigated the extent of Hg isotope fractionation. Hg IRs were analyzed by multicollector inductively coupled plasma mass spectrometry (MC-ICP/MS). Hg mass independent fractionation (MIF; Δ 199 Hg) and mass dependent fractionation (MDF; δ 202 Hg) were calculated and compared among samples. IRs of Hg in sediment were characterized mainly by MDF and low MIF ( Δ 199 Hg -0.37 to 0.74‰). However, all biota showed evidence of MIF, most pronounced in zooplankton ( Δ 199 Hg up to 3.40 ‰) and char ( Δ 199 Hg up to 4.87‰). Zooplankton takes up highly fractionated MeHg directly from the water column, while benthic organisms are exposed to sedimentary Hg, which contains less fractionated Hg. As evidenced by δ 13 C measurements, benthic chironomids make up a large proportion of char diet, explaining in part why MIF char < MIF zooplankton in lakes, where both samples were measured. Hg IRs in char varied among regions, while char from lakes from each region showed similar degrees of MIF. A MIF-offset was derived representing the mean MIF difference between sediment and fish, and indicated that fish in two regions retain sediment signatures altered by a consistent offset. Due to its minimal lake- to-catchment area and very high water retention time ( ∼330 years), the meteor impact crater lake (Pingualuk) reflects a “pure” atmospheric Hg signature, which is modified only by aqueous in-lake processes. All other lakes are also affected by terrestrial Hg inputs and sediment processes. Introduction Mercury (Hg) is naturally present as seven stable isotopes ( 196 Hg, 198 Hg, 199 Hg, 200 Hg, 201 Hg, 202 Hg, and 204 Hg), which differ in relative mass by up to 4%. Characterizing the Hg isotope fingerprints (i.e., ratios) in the environment has been proposed as a tool of tracing sources of Hg pollution (atmospheric or point sources) (1, 2). Stable isotope ratios (IRs) of a number of elements (Li, Mg, Ca, Fe, Cu, Pb, Zn, Se, and Mo) are being used more and more as isotopic tracers in the environment (3). Hg isotopes are probably fractionated by industrial processes prior to deposition in the environ- ment, resulting in isotopic ratios that may be specific to a region or even to an industry (4). While Hg IRs could be highly useful in linking Hg found in the environment to a specific source, fractionation effects are likely to change Hg IRs, which may obscure the initial fingerprint. While earlier studies have focused on detecting mass-dependent frac- tionation (MDF), more recently the discovery of mass- independent fractionation (MIF) of Hg in biota has received more attention. MIF is specifically associated with Hg isotopes of odd mass numbers ( 199 Hg and 201 Hg) and may be useful to track Hg transfer through the environment, as MIF is only induced by a few, selected transformation reactions and remains unchanged by simple transport or transfer processes. Environmental fractionation of Hg, aided by biotic and abiotic (e.g., photochemical) processes, and with varying degrees of both MDF and MIF has been experimentally demonstrated in water (1), Hg resistant bacteria (5), and confirmed in sediments (6-8), ore deposits (9, 10), and biota (1, 11-13). Jackson et al. (11) reported biotic MIF in food webs (including fish) of three lakes in Canada (Lakes Cli, Shipiskan, and Ontario) and MIF was documented in several fish species by Bergquist and Blum (1). MDF values range from ∼6‰ by bacteria (δ 202 Hg, experimental) and ∼5.5‰ in ores, to -1‰ to 4‰ in sediments (δ 202 Hg, natural) (8), and MIF values range from 2.5‰ (Δ 199 Hg, photo reduction experiments) (1), up to ∼4‰ (Δ 201 Hg) in various fish species (1, 11). The precision of isotope ratio measurements by MC-ICP/MS allows detection of fractionation of about 0.2‰ or better. The Arctic is an area of great concern because indigenous peoples, particularly the Inuit (of Alaska, Canada, and Greenland) largely depend on a subsistence harvest of apex predators (fish, birds, or marine mammals), which are known to contain high concentrations of Hg (14). The Arctic receives most of its anthropogenic Hg through the atmosphere (15, 16), but inputs into lakes can also occur from terrestrially derived (anthropogenic and natural) Hg sources (17). High Arctic regions (north of 70° N), are known to be influenced by Eurasian sources (18). Anthropogenic Hg deposition in Arctic lakes in Canada based on the analysis of dated sediment cores averaged 2.8 μgm -2 y -1 in good agreement with results from global mercury deposition modeling (19). The relative contribution from atmospheric and terrestrial Hg sources and their pathways into freshwater food webs including Arctic food webs are not fully understood, thus IRs, and in particular MIF, have been proposed as a possible tool to trace sources. However, information on Hg isotopic ratios in Arctic samples is limited to one sediment core from Romulus Lake, Ellesmere Island, Nunavut, Canada (6) and one study on Alaskan seabird eggs (20). Arctic lake food webs are low in biodiversity and the transfer of nutrients to fish is coupled to benthos (21, 22): from sediment (main site of Hg methylation) to benthic invertebrates (chironomids) to adult Arctic char (Salvelinus alpinus L.). Zooplankton (dominated by Copepoda) is part of the char diet, but may not contribute greatly to Hg uptake (22). Cannibalism is known to occur (23), and can effect Hg concentrations in adult char (24). We set out to determine the following: (1) if Hg IRs in sediments and char vary by lake and region; (2) if there is * Corresponding author current address: Water & Climate Impacts Research Centre Environment Canada, University of Victoria, Victoria, British Columbia, Canada V8W 3R4; phone: (250) 363 8947; fax: (250) 363 3586; e-mail: Nikolaus.Gantner@ec.gc.ca. † University of Guelph. ‡ Environment Canada. § Trent University. Environ. Sci. Technol. 2009 43, 9148–9154 9148 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 24, 2009 10.1021/es901771r CCC: $40.75  2009 American Chemical Society Published on Web 11/10/2009