Mercury Content of Vegetation across a Subarctic Mire Thaw Gradient

Abstract:

Mercury deposition from natural and anthropogenic sources is known to accumulate in subarctic environments, particularly peatlands, due to their abundant organic matter that effectively sequester mercury and other heavy metals. Given direct links between mercury mobilization and aquatic and terrestrial ecosystem health, it is vital to understand the degree to which thawing peatlands serve as sinks or sources of mercury to the environment. In a peat mire underlain by permafrost in subarctic Sweden (Stordalen Mire, lat. 68°21' N, lon. 19°03' E), the onset of climate-driven permafrost thaw influences regional hydrology and therefore the composition of plant communities. The purpose of this work is to assess mercury content of vegetation and underlying peat across a thaw gradient. The study was conducted on two transects that transition from an unthawed dwarf shrub-dominated hummock community to a fully thawed graminoid-dominated wet depression community. Drained hummock sites at initial stages of the thaw sequence are characterized by a diverse graminoid and shrub community, including Eriophorum, Andromeda, and Empetrum populations underlain by Sphagnum litter. Semi-wet sites are characterized by moist Sphagnum mats with sparse populations of Eriophorum and Empetrum. Wet sites are characterized by tall graminoid Eriophorum underlain by submerged Sphagnum mats. Total mercury abundances in vegetation was analyzed via thermal decomposition. Generally, mercury contents in plant tissues exhibit decreasing concentrations of mercury with increasing thaw. Higher concentration of mercury in vegetation in drained sites versus that in wet sites supports the notion that mercury in the dry mire is exported into the local water and peat column. Changing mercury concentration profiles in peat core afford a means to assess that mercury is mobilized during the thaw but not fully exported from the thawed wetlands. Our results, coupled with earlier findings of mobilization and subsequent partial retention of mercury in peat along other thaw sequences, suggest complex pathways for mercury reallocation during the thawing process.