Spatial and seasonal patterns of mercury concentrations, methylation and demethylation in central Canadian boreal soils and stream sediment

Abstract

Terrestrial ecosystems store large amounts of mercury (Hg), which may be subject to methylation, mobilization and uptake into downstream aquatic ecosystems. Mercury concentrations, methylation and demethylation potentials are not well characterized simultaneously across different habitats in boreal forest ecosystems, particularly not so in stream sediment, leading to uncertainties about the importance of various habitats as primary production areas of the bioaccumulative neurotoxin methylmercury (MeHg). In this study, we collected soil and sediment samples from 17 undisturbed, central Canadian boreal forested watersheds during spring, summer and fall to robustly characterize the spatial (upland and riparian/wetland soils, and stream sediment) and seasonal patterns of total Hg (THg) and MeHg concentrations. Mercury methylation and MeHg demethylation potentials (Kmeth and Kdemeth) in the soils and sediment were also assessed using enriched stable Hg isotope assays. We found the highest Kmeth and %-MeHg in stream sediment. In both riparian and wetland soils, Hg methylation was lower and less seasonally variable compared to stream sediment, but had comparable MeHg concentrations, suggesting longer-term storage of MeHg produced in these soils. Soil and sediment carbon content, and THg and MeHg concentrations were strong covariates across habitats. Additionally, sediment carbon content was important for delineating between stream sediment with relatively high vs. relatively low Hg methylation potential, which generally separated between different landscape physiographies. Broadly, this large and spatiotemporally diverse dataset is an important baseline for understanding Hg biogeochemistry in boreal forests both in Canada and possibly in many other boreal systems globally. This work is particularly important with respect to future possible impacts from natural and anthropogenic perturbations, which are increasingly straining boreal ecosystems in various parts of the world.