Sediment and porewater profiles and fluxes of mercury and methylmercury in a small seepage lake in northern Minnesota.

Mercury (HgT) and methylmercury (MeHg) were measured at 1-2 cm resolution in sediment porewater and sediment cores from Spring Lake in the Marcell Experimental Forest of northern Minnesota. Recent sediment accumulation of HgT was 21.4 microg m(-2) yr(-1) (1990-2000), 2 orders of magnitude greater than the accumulation of MeHg (0.20 microg m(-2) yr(-1)). The highest solid phase concentrations of MeHg were observed persistently at the sediment surface and declined sharply with depth. Porewater profiles showed a small diffusive flux of MeHg from sediment to water (5 ng m(-2) month(-1)). Springtime porewater concentrations of MeHg were relatively low (approximately 0.5 ng L(-1)) and increased by late summer to early fall (1.5-2.2 ng L(-1)), showing distinct peaks roughly correlated with maxima in sulfate reducing activity at 5 and 15 cm. Advective transport carrying MeHg deeper into the sediment was evident in summer and fall. The percent of HgT present as MeHg was highest in the water column above the sediment (10%) and decreased with sediment depth in both the solid and porewater phases. Sediments at this study site are a net sink for MeHg, although diagenetic processes of demethylation and methylation are evident within the lake-sediment environment.