Mercury and methylmercury in Spring Lake, Minnesota: A mass balance approach comparing redox transformations, methylmercury photodegradation, sediment loading, and watershed processes

Processes of total mercury (HgT) and methylmercury (MeHg) cycling are described for Spring Lake, a small bog lake in northern Minnesota, within the Marcell Experimental Forest north of Grand Rapids. I quantified redox transformations, photodegradation of MeHg, internal and external loading, and modeled Hg cycling using the software StellaRTM. Measurements on lake sediment cores and sediment porewater allowed for separate treatment of lake and sediment environments. Recent sediment accumulation of Hg T was 21.4 mug m-2 yr-1 (1990--2000), two orders of magnitude greater than accumulation of MeHg (0.20 mug m-2 yr-1). Porewater profiles showed a small diffusive flux of MeHg from sediment to water (5 ng m-2 mo-1). 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. Losses of HgT from evasion of Hg0 (lake to atmosphere) during the daytime in 2001 and 2002 averaged 1.1 ng m -2 h-1 and were 21% of the output of Hg T from Spring Lake. The loss from evasion of Hg0, 261 mg (March--November), was much smaller than the HgT loss from burial (859 mg). Atmospheric deposition is the main input of HgT to the lake, but MeHg is supplied by a combination of atmospheric, near-shore wetland, sediment porewater, and biotic sources (methylation). Bog and upland runoff were minor components to the total inputs for both HgT and MeHg. For MeHg, wet deposition is only 8.2% of the total MeHg input, and other external inputs (runoff, porewater) provide an additional 6.8%, indicating that internal production of MeHg is occurring. The residence times of MeHg and HgT in the lake were 47 and 60 d, respectively, during the open-water season. For Hg0, the rate of photoreduction (producing Hg0) was twice that of photooxidation (loss of Hg0) and the residence time in the photic zone was short (hours). Although in-lake methylation provided the majority of inputs of MeHg to the lake (85%; determined by mass balance), the combination of photodegradation and degradation on seston and sediment result in net demethylion at Spring Lake.