Mercury stable isotopes for understanding sources & biogeochemical pathways in natural ecosystems

Abstract

Mercury (Hg) is a widely distributed, toxic, and bioaccumulative compound that poses serious health threats to wildlife and humans consuming fish. Thus far, studies of Hg sources and biogeochemical pathways have been extensive in aquatic ecosystems. Increasing evidence suggests that there is significant amount of Hg in forest ecosystems, but the input of multiple sources and complex biogeochemical processing of Hg make it difficult to link sources to receptors. For instance, forest ecosystems may receive Hg directly from atmospheric deposition. Another potentially important Hg source to forest ecosystems is through complex ecological processes such as emergent-insect mediated processes, which provide important energy and nutrient subsidies to forest ecosystems. On the flip side, Hg associated and/or accumulated in terrestrial resources can travel to aquatic ecosystems through watershed runoff or movements of organisms. Here, I demonstrate how the measurements of Hg stable isotope ratios in natural samples can be used to distinguish the relative importance of Hg sources, biogeochemical, and transfer processes between streams, freshwater lakes, and forest ecosystems. Hg stable isotopes are increasingly being used to provide insight into the sources, biogeochemical, and exposure pathways of Hg in diverse ecosystem food webs. Many environmental samples and Hg sources have distinct isotope ratios for Hg. Hg stable isotopes undergo fractionation via environmentally relevant processes, which allow precise source tracing and biogeochemical processing of Hg. Recent studies have also confirmed the utility as ecological and biological tracers for exploring processes such as bioaccumulation and trophic transfer. In particular, with the combination of traditional ecological tracers of stable carbon and nitrogen isotopes, which serve as proxies for feeding behaviors and trophic positions, respectively, Hg stable isotopes can aid understanding of various Hg sources, biogeochemical processes, and ecosystem fate in natural environment.