The Influence of Sulphate Deposition on the Seasonal Variation of Peat Pore Water Methyl Hg in a Boreal Mire

In this paper we investigate the hypothesis that long-term sulphate (SO4 22) deposition has made peatlands a larger source of methyl mercury (MeHg) to remote boreal lakes. This was done on experimental plots at a boreal, low sedge mire where the effect of long-term addition of SO4 22 on peat pore water MeHg concentrations was observed weekly throughout the snow-free portion of 1999. The additions of SO4 22 started in 1995. The seasonal mean of the pore water MeHg concentrations on the plots with 17 kg ha21 yr21 of sulphur (S) addition (1.360.08 ng L21, SE; n = 44) was significantly (p,0.0001) higher than the mean MeHg concentration on the plots with 3 kg ha21 yr21 of ambient S deposition (0.660.02 ng L21, SE; n = 44). The temporal variation in pore water MeHg concentrations during the snow free season was larger in the S-addition plots, with an amplitude of.2 ng L21 compared to +/20.5 ng L21 in the ambient S deposition plots. The concentrations of pore water MeHg in the S-addition plots were positively correlated (r2 = 0.21; p = 0.001) to the groundwater level, with the lowest concentrations of MeHg during the period with the lowest groundwater levels. The pore water MeHg concentrations were not correlated to total Hg, DOC concentration or pH. The results from this study indicat