Mercury in a boreal forest stream--role of historical mercury pollution, TOC, temperature, and water discharge.

Over a one-year study period (2003), we monitored total Hg (HgT) and methyl Hg (MeHg) at two sites in a Swedish forest stream located above (Site(ref)) and below a stretch of Hg-contaminated sediments (SiteHg). We also monitored HgT, MeHg, and ancillary water chemistry in peat water close to the stream and HgT in open field wet deposition. Despite the presence of historical Hg contaminants, direct atmospheric Hg deposition and transfer of Hg from the catchment explained more than half of the annual HgT load at SiteHg. The concentrations of both HgT and MeHg were sensitive to changes in water discharge (Q) and water temperature (T) at both sites, suggesting that the stream HgT and MeHg load can change dramatically in response to changing weather conditions. The 2003 data together with data from 1996 disclosed intersite differences and temporal variation in the relationships between HgT, MeHg, and TOC (total organic carbon), reflecting variable sources of HgT, MeHg, and TOC and temporal changes in factors affecting Hg speciation.