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

摘要

Processes of HgT and MeHg cycling are described for Spring Lake, a small bog lake in northern Minnesota, USA, within the Marcell Experimental Forest. We quantified redox transformations, photodegradation of MeHg, internal and external loading for both HgT and MeHg, and modeled Hg cycling using the software StellaN. Measurements on sediment cores allowed for separate treatment of lake and sediment environments. Atmospheric deposition is the main input of HgT, but MeHg is supplied by a combination of atmospheric, near-shore wetland, and biotic sources (methylation). Photodegradation annually removes about twice the lake burden of MeHg (20 mg), and burial removes slightly more than one lake burden per year. The residence time of MeHg in the lake was 84 d during the ice-free season, compared with only 4.5 days for the residence time of MeHg on settling particles (seston) in the spring. The rate of photoreduction (1.1 h~(-1)) was twice that of photooxidation (0.58 h~(-1)). Losses of Hg_T from evasion of Hg~0 during the daytime in 2001 and 2002 averaged 1.1 ng m~(-2)h~(-1). The evasional loss of Hg~0, 365 mg (Mar. - Nov.), was much smaller than the HgT loss from burial (1435mg).