Inorganic ionic mercuric (Hg2+) and elemental mercury (Hg0) in the gaseous or liquid state both express some toxicity to living organisms, however, the most important mercury species in the environment is methylmercury (CH3Hg+, or MeHg). In fact, if mercury were not methylated in the environment, there essentially would be no "mercury problem" because MeHg is the only component of the environmental mercury pools that bioaccumulates in food webs. Mercury methylation is a byproduct of microbial sulfate reduction, thus any environmental changes to the net loading sulfur and carbon (the key elements for microbes that respire sulfate), or "availability" of mercury itself could lead to the production of more MeHg in the Yukon basin. In addition, thermal enrichment of northern latitudes could affect MeHg production and mobility without invoking changes to Hg loading rates. Therefore, an understanding of the fundamental processes that lead to the formation and mobilization of MeHg in the environment, and how these processes may be affected by global-scale perturbations such as global warming, is central to our ability to predict whether global warming scenarios may have a net positive or negative effect on mercury in the environment.
Hypotheses
Mercury methylation
Methylmercury is produced in the multitude of intra- and lateral-stream sloughs and wetlands of the Yukon River, and the annual flushing of these sub-ecosystems during spring runoff regulates observed fluxes of MeHg in the River itself.
No new MeHg formation is currently active in the Yukon basin, and measurable fluxes of MeHg at the present are the result of mobilization of relic MeHg formed during warmer, wetter historical times, retained in frozen peat lands, and only recently mobilized by melting permafrost.
Because MeHg formation is microbially mediated, warmer conditions would likely yield higher methylation rates in native soils and peat.
Methylmercury levels in the Yukon Basin are generally low, and are reflective of abiotic methylation in the atmospheric and subsequent deposition of MeHg.
Mercury and methylmercury sources and transport in the Yukon River
Clear water tributaries and sloughs contribute "reactive" Hg and MeHg to the Yukon River, however, the high particulate loads quickly scavenge the Hg and MeHg from aqueous solution and limit the methylation and bioaccumulation processes.
Glacial erosion and melt water mobilize "geologic" particulate mercury from watershed sources (both intact natural mineralized areas and areas contaminated by past or current mining activity) down gradient to the Yukon River, where sedimentation in methylmercury forming environments (sloughs and wetlands).
Annual flooding and flushing of the Yukon Basin wetlands by snow and glacial melt water enriched in particulate mercury stimulates methylation due to the combined influences of inundation and mercury loading.
Although spring floods bring high loads of particulate, inorganic mercury to the Yukon Basin, this mercury pool is largely unreactive and the more reactive and bioavailable mercury in rainfall is what drives MeHg formation.
Gaseous mercury production and fluxes:
Gaseous mercury production and evasion is a result of photo reduction by UV light, and due to the continual sunlight conditions of high latitude sites, mercury depletion of wetland surfaces will results and limits MeHg formation.
Gaseous mercury production and subsequent evasion from the water column is an important process in most ecosystems, however, the high turbidity of the Yukon River prevents (or greatly limits) this process.
Although significant total mercury levels exist in the Yukon River, the majority is particulate and not sensitive to photo reduction by ultraviolet sunlight.
