Variations in Stable Isotope Fractionation of Hg in Food Webs of Arctic Lakes

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; Delta Hg-199) and mass dependent fractionation (MDF; delta Hg-202)were calculated and compared among samples. IRS of Hg in sediment were characterized mainly by MDF and low MIF (Delta Hg-199 -0.37 to 0.74 parts per thousand). However, all biota showed evidence of MIF, most pronounced in zooplankton (Delta Hg-199 up to 3.40 parts per thousand) and char (Delta Hg-199 up to 4.87 parts per thousand). 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 VC measurements, benthic chironomids make up a large proportion of char diet, explaining in part why MIFchar < MIFzooplankton 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 (similar to 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.