313 - Spatial and Seasonal Patterns of Mercury Concentrations and (De)Methylation in Boreal Soils and Stream Sediment

Forest ecosystems store a large amount of mercury (Hg) and knowing the concentrations and distributions of Hg, as well as the biogeochemical processes in relation to methylmercury (MeHg) production in forest soils and sediment, is important. A large-scale investigation was therefore conducted in 17 undisturbed, central Canadian boreal forested watersheds to characterize the spatial (upland and riparian/wetland soils, and stream sediment) and seasonal patterns of total Hg (THg) and MeHg concentrations, as well as Hg methylation and MeHg demethylation potentials (Kmeth and Kdemeth). Upland soil had the highest THg concentrations, followed by riparian/wetland soils and stream sediment. Stream sediment showed considerable MeHg production and accumulation, manifested by the largest Kmeth values, MeHg concentrations and %-MeHg compared to soils. Significantly greater THg and MeHg concentrations, %-MeHg and Kmeth in stream sediment were observed in watersheds within physiographic regions dominated by exposed bedrock and shallow surface soils compared to those characterized by deeper glaciolacustrine sandy plain soils. Kdemeth was variable across the watersheds, but discernible spatial patterns were not observed. Kmeth exhibited the most seasonal variation compared to other variables and was substantially higher in summer, particularly in stream sediment. This study suggests that even compared to wetland soils, forested stream sediment plays a disproportionately important role in Hg methylation within this region, which is an important finding in relation to exposure of aquatic biota to MeHg. Broadly, this large and spatiotemporally diverse dataset is an important baseline for understanding Hg biogeochemistry in Canadian boreal forests, particularly with respect to future possible impacts from natural and anthropogenic perturbations, which are common to the region.

Authors: Haiyong Huang, Wai Ying Lam, Vaughn Mangal, Carl Mitchell