Mercury (Hg) is a global atmospheric pollutant that has been deposited into virtually all ecosystems on the Earth. Methylmercury (MeHg), the most toxic form of Hg, can strongly bioaccumulate and biomagnify in food webs, posing a serious threat to natural populations of wildlife. Production and degradation of MeHg have been extensively studied in aquatic ecosystems but have been studied to a much lesser extent in terrestrial ecosystems. Previous studies have consistently shown that only trace amounts of MeHg exist in the atmosphere (precipitation) and forests (foliage, litter and soil), yet high-trophic level organisms in forests (e.g., insectivorous birds) can accumulate dangerously high levels of MeHg in their tissues due to biomagnification. Although it does not have a well-defined source, it has been speculated that MeHg is produced in precipitation, foliage and/or soil through abiotic and/or microbial processes. Microbial and/or photochemical degradation are thought to be the main pathways for breaking down MeHg in forests. However, there is only a cursory understanding of most of these processes in forests, in part due to the lack of suitable research tools available to probe the complex biogeochemistry of Hg in the natural environment. Recent developments in stable Hg isotope measurement techniques have allowed precise determination of the Hg isotopic composition in a wide range of environmental samples, and can serve as a highly sensitive tool to fingerprint sources and understand degradation mechanisms of MeHg in natural ecosystems. Mercury has seven stable isotopes (196Hg, 198Hg, 199Hg, 200Hg, 201Hg, 202Hg, and 204Hg) and all undergo mass-dependent fractionation (MDF, expressed as ?202Hg) through a variety of redox reactions. The odd-mass Hg isotopes (199Hg and 201Hg) also display mass-independent fractionation (MIF, either odd isotope excesses or deficits, expressed as ?199Hg) during photochemical reactions. In five temperate forests across North America, we have observed a large range of both ?202Hg (~3 permil) and ?199Hg (~2 permil) in MeHg accumulated in different forest invertebrates. The ?202Hg values of MeHg in forest biota resemble those of inorganic Hg in the atmospheric gas-phase and in precipitation, but not in foliage and soil, prompting a hypothesis that MeHg in forest food webs originates from the atmosphere but not the forest itself. The research proposed here is aimed at better understanding the origins and degradation mechanisms of MeHg using the stable Hg isotope approach in four semi-remote forest ecosystems (in California, Michigan, New Hampshire, and North Carolina) with contrasting climatic regimes and atmospheric Hg loading. Specifically, this proposed project will isolate and determine the isotopic compositionof MeHg present in trace amount in precipitation, foliage/litter and surface soil in each study forest and determine whether it resembles the isotopic composition of MeHg found in specific forest food webs. Moreover, the isotopic composition of MeHg can allow the estimation of the relative contribution from each of these potential sources to the food web bioaccumulation, and estimation of the extent and pathways of MeHg degradation in natural forests with isotope fractionation factors obtained from previous controlled studies. Through answering these interrelated questions, the proposed work will be able to identify the dominant source(s) and degradation pathway(s) of MeHg in temperate forest ecosystems, and their differences under contrasting forest types, climates and atmospheric Hg loading.