Climate Change and Mercury Accumulation in Canadian High and Subarctic Lakes

Mercury (Hg) profiles were compared to profiles of climate-indicators including microfossil remains and algal-derived or S2 carbon (C) in dated sediment cores from 14 lakes spanning latitudinal and longitudinal gradients across the Canadian high and Subarctic. Hg fluxes increased post-industrialization (post approximately 1850) in 11 of these lakes (anthropogenic Hg fluxes corrected for sediment particle focusing (ΔHgFF) = 1.5-24.2 μg m-2 y-1). Correction of HgFF for catchment contributions demonstrated that Hg deposition originating from catchment-independent factors, such as atmospheric deposition, increased since industrialization in all 14 lakes while inputs from erosion increased in 11. Several of these lakes also showed post-industrial shifts in algal assemblages consistent with climate-induced changes. 11 lakes showed post-1850’s increases in S2 fluxes (S2FF), suggesting that lake primary productivity has recently increased in the majority of our sites (ΔS2FF = 0.1-3.7 g m-2 y-1). Other studies have interpreted significant relationships between Hg:S2 concentrations in Arctic sediment as support for the algal scavenging hypothesis, which postulates that Hg fluxes to Arctic sediments are largely driven by S2. However, in six of our lakes we observed no Hg:S2 relationship, and in one lake a significant negative Hg:S2 relationship was observed due to increased Hg and decreased S2 C deposition during the post-industrialization period. In six of the seven lakes where a significant positive Hg:S2 relationship was observed, algal assemblages either did not change through time or the timing of the shifts did not correspond to changes in Hg deposition. Our results demonstrate that, although Arctic lakes are experiencing a myriad of changes, including increased Hg and S2 deposition, and/or changing algal assemblages, increased lake primary productivity is not driving changes in Hg fluxes to sediments.