Mercury inputs and outputs at a small lake in northern Minnesota

Storages and cycling of total mercury (HgT), methylmercury (MeHg), and Hg⁰ 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 HgT and MeHg. Atmospheric deposition was the main input of HgT; MeHg was supplied by a combination of atmospheric, near-shore wetland, and biotic (methylation) sources. HgT outputs were dominated by burial (67%), and Hg⁰ evasion accounted for 26% of HgT outputs. The watershed of Spring Lake is small (3.7x lake surface area), and accordingly, bog and upland runoff were minor contributors to both HgT 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 ~3x the lake mass of MeHg (20 mg) annually, and was the dominant sink for MeHg. Residence times of MeHg and HgT 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 Hg²⁺ to Hg⁰ was greater than the reverse reaction (Hg⁰ photooxidation), and the residence time of Hg⁰ in the photic zone was short (hours). Data from this study show active cycling of all the measured species of mercury (HgT, MeHg, and Hg⁰) and the importance of MeHg photolysis and photo-redox processes.