Storages and cycling of total mercury (Hg T ), methylmercury (MeHg), and Hg 0 are described for Spring Lake, a small bog lake in the Marcell Experimental Forest in north-central Minnesota. We quantified photoredox transformations, MeHg photolysis, burial to the sediments, and internal and external loadings of Hg T and MeHg. Atmospheric deposition was the main input of Hg T ; MeHg was supplied by a combination of atmospheric, near-shore wetland, and biotic (methylation) sources. Hg T outputs were dominated by burial (67%), and Hg 0 evasion accounted for 26% of Hg T outputs. The watershed of Spring Lake is small (3.7· lake surface area), and accordingly, bog and upland runoff were minor contributors to both Hg T and MeHg inputs. Wet deposition was *9% of total MeHg input, and other external inputs (runoff, sediment porewater) provided only an additional 7%, indicating that internal production of MeHg was occurring in the lake. Photolysis of MeHg, measured in the field and laboratory, removed *3· the lake mass of MeHg (20 mg) annually, and was the dominant sink for MeHg. Residence times of MeHg and Hg T in the lake were 48 and 61 days, respectively, during the open-water season, compared with only 8 days for the residence time of MeHg on settling particles (seston). Photoreduction of Hg 2+ to Hg 0 was greater than the reverse reaction (Hg 0 photooxidation), and the residence time of Hg 0 in the photic zone was short (hours). Data from this study show active cycling of all the measured species of mercury (Hg T , MeHg, and Hg 0 ) and the importance of MeHg photolysis and photo-redox processes.

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.

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