Sources and fates of mercury and methylmercury in Wisconsin lakes

The mercury cycle in seven northern Wisconsin seepage lakes was characterized by high atmospheric influx, removal by sedimentation and evasive efflux, and by in-lake transformation of Hg to biologically sequestered methyl Hg species. Direct depositional Hg loading from lhe atmosphere to lakes was roughly lO ug/m2/year, with rain and snow Lhe principal delivery vectors. Annual atmospheric Hg deposition exceeded estimated fish bioaccumulation by a factor of roughly 10. The atmospheric Hg inffux was roughly balanced by losses to sediments and the return of volatile Hg° to the atmosphere. The relative importance of sedimentation and gaseous evasion as Hg loss terms varied from lake to with sedimentationl/evasion ratios ranging from 9:1 to 1:1 in the seven lakes studied. Residence times for Hg varied from roughly 125 to 300 days in these lakes. Methyl-Hg in these lakes also had an atmospheric source, estimated to he roughly 1% of the total Hg inputs. Although the direct atmospheric deposition and sediment accumulation of methy-Hg was miuch lower than annual rates of methy-Hg bioaccumalation...

Modeling the biogeochemical cycle of mercury in lakes: the mercury cycling model (MCM) and its application to the MTL study lakes

As a part of the Mercury in Temperate Lakes (MTL) study in Wisconsin, U.S., we have developed a mechanistic model of the biogeochemical cycle of mercury in lakes. The Mercury Cycling Model (MCM) is a deterministic simulation model that incorporates the major processes that transport mercury across lake boundaries-atmospheric deposition, gas exchange, inflow and outflow of water, and burial in sediments; chemically transform it—reduction, methylation, and demethylation; and lead to its accumulation in aquatic biota—uptake, depuration, and trophic levei transfer. In this chapter, we discuss the theory of mercury biogeochemical cycling, apply a simplified, steady-state version of the MCM lo analyze field data from lhe MTL study, and examine mechanistic issues using lhe dynamic MCM model. Theories of mercury biogeochemistiy are based on knowledge of lhe aqueous speciation of mercury and the mechanisms of its reactions—considerable gaps in...