Sources and Transfers of Methylmercury in Adjacent River and Forest Food Webs

Nearly all ecosystems are contaminated with highly toxic methylmercury (MeHg), but the specific sources and pathways leading to the uptake of MeHg within and among food webs are not well understood. In this study, we report stable mercury (Hg) isotope compositions in food webs in a river and an adjacent forest in northern California and demonstrate the utility of Hg isotopes for studying MeHg sources and cross-habitat transfers. We observed large differences in both delta Hg-202 (mass-dependent fractionation) and Delta Hg-199 (mass-independent fractionation) within both food webs. The majority of isotopic variation within each food web could be accounted for by differing proportions of inorganic Hg [Hg(II)] and MeHg along food chains. We estimated mean isotope values of Hg(II) and MeHg in each habitat and found a large difference in delta Hg-202 between Hg(II) and MeHg (similar to 2.7 parts per thousand) in the forest but not in the river (similar to 0.25 parts per thousand). This is consistent with in situ Hg(II) methylation in the study river but suggests Hg(II) methylation may not be important in the forest. In fact, the similarity in delta Hg-202 between MeHg in forest food webs and Hg(II) in precipitation suggests that MeHg in forest food webs may be derived from atmospheric sources (e.g., rainfall, fog). Utilizing contrasting delta Hg-202 values between MeHg in river food webs (-1.0 parts per thousand) and MeHg in forest food webs (+0.7 parts per thousand), we estimate with a two-source mixing model that similar to 55% of MeHg in two riparian spiders is derived from riverine sources while similar to 45% of MeHg originates from terrestrial sources. Thus, stable Hg isotopes can provide new information on subtle differences in sources of MeHg and trace MeHg transfers within and among food webs in natural ecosystems.