Lower Water Tables, Not Increased Temperature, Increase Methylmercury Production in Northern Peatlands Under Climate Change.

Northern peatlands are the world’s largest terrestrial carbon (C) store, actively sequestering C due to slow decomposition rates under high water table, low temperature and low nutrient conditions.  They are also important modifiers of downstream water quality, with both dissolved organic carbon and methylmercury being notable and intensely studied solutes of interest.  As a result of future climate change which is anticipated to be the most extreme in northern high latitudes, changes in peatland temperature and moisture regimes are expected to directly alter decomposition rates, above and below ground floral and faunal diversity and activity, and  subsequent downstream water quality.

In a large-scale mesocosm experiment, we subjected 100 intact, vegetated peat monoliths to a range of elevated temperatures, increased atmospheric CO₂, and two water table levels over 18 months in a factorial design to determine the individual and synergistic effects of climate change factors on decomposition, plant and faunal biodiversity, C cycling and water chemistry, including DOC quantity and quality, total mercury (THg) and methylmercury (MeHg).  With respect to MeHg, we hypothesized that under elevated temperatures (as high as +8˚C above ambient), 2x CO₂ and higher water tables, a shift in plant community toward graminoids (sedges) and more persistent anaerobic conditions would promote more microbial methylation of inorganic Hg, reflected in a higher fraction of THg as MeHg in peat porewaters.  Our results are to the contrary, and show that despite significant effects of elevated temperature on plant community composition, decomposition, and DOC concentrations, MeHg concentrations were only related (inversely) to water table position.  Under all experimental treatments, MeHg concentrations were unchanged under high water table conditions, maintaining a relatively constant 4-7% of THg as MeHg.  Under low water table treatments, the mean %MeHg was significantly and consistently 2-3x higher than high water table treatments, reaching a maxima of in the first late summer of the experiment.   At this time, the mean %MeHg was 20% and 7% under low and high water tables, respectively, with individual drier mesocosms having %MeHg as high as 56%.   These results suggest that climate change-induced water table lowering in northern peatlands could be the primary driver of increased methylation of Hg, not warmer soil conditions, as might be expected.