Riverine mixing zones as regions of enhanced methylmercury bioaccumulation in Lake Superior

Riverine mixing zones along the southern shore of Lake Superior were sampled in Spring and Fall over a 2 year period (2000-2001) at the following sites: St. Louis River (STL), Fish Creek (FSC), and Tahquamenon River (TAQ). Each nearshore area was sampled along a transect moving from the river mouth, through the mixing zone, toward the offshore waters. Samples for unfiltered, filtered and particulate Hg$_{\rm T}$ and MeHg, SPM, DOC and chl-$a$ were collected from each transect site. The TAQ was re-sampled in Spring 2002 and size-specific seston was also collected from the water column at the mouth and offshore sites. Concentrations of SPM, DOC, unfiltered, filtered and particulate Hg$_{\rm T}$ and MeHg generally decreased between the river mouth and the offshore site of each mixing zone. Decreases in concentrations were presumably the result of dilution as concentrated river water mixed with pristine lake water, or fallout of larger particles with distance. Chl-a appeared to be unaffected by dilution and remained either constant or increased along each transect. Despite the observed decreases in MeHg species in the water column, the amount of MeHg retained on particles always increased along the same transect with a maximum reached in the offshore site. Large increases in the log K$_{\rm d}$ for MeHg, and the biological enrichment in the SPM pool in the offshore waters, suggest that MeHg is strongly bioaccumulated in these riverine mixing zones

Methyl mercury in Lake Superior: Offshore processes and bioaccumulation

The effects of watershed type exert a strong influence on the speciation of mercury and the delivery of mercury to Lake Superior nearshore waters. As a consequence, tributary mixing zones are important locations for enhanced bioaccumulation in Lake Superior. Methyl Hg (MeHg) bioaccumulation, however, is also observed in regions of the lake that are remote from tributary influences. Three cruises aboard the USEPA vessel R/VLake Guardian on Lake Superior revealed that offshore concentrations of total mercury (HgT) were low, similar to Lake Michigan and oceanic waters (0.21 – 1.0 ng L$^{-1}$ HgT). During August 2000, MeHg averaged 3.0 – 12.6 pg L$^{-}$, at least an order of magnitude lower than most tributaries during typical flow régimes. Despite these differences, initial comparisons of phytoplankton revealed only a two to threefold enrichment of MeHg in tributary mixing zones versus offshore regions. MeHg inputs to the open waters of the lake are dependent on three processes: mixing from nearshore zones, direct atmospheric inputs and MeHg diffusion from sediments. Direct sedimentary methylation rates are extremely low and modeling efforts suggest that photodegradation would eliminate tributaryderived MeHg. Therefore, we conclude that atmospheric sources strongly influence MeHg uptake in offshore zones. A detailed profile at a deep-lake station in August 2001 suggests enhanced bioaccumulation at a subsurface chlorophyll maximum, in a zone with close contact to atmospheric fluxes

Some like it cold: microbial transformations of mercury in polar regions

The contamination of polar regions with mercury that is transported from lower latitudes as inorganic mercury has resulted in the accumulation of methylmercury (MeHg) in food chains, risking the health of humans and wildlife. While production of MeHg has been documented in polar marine and terrestrial environments, little is known about the responsible transformations and transport pathways and the processes that control them. We posit that as in temperate environments, microbial transformations play a key role in mercury geochemical cycling in polar regions by: (1) methylating mercury by one of four proposed pathways, some not previously described; (2) degrading MeHg by activities of mercury resistant and other bacteria; and (3) carrying out redox transformations that control the supply of the mercuric ion, the substrate of methylation reactions. Recent analyses have identified a high potential for mercury-resistant microbes that express the enzyme mercuric reductase to affect the production of gaseous elemental mercury when and where daylight is limited. The integration of microbially mediated processes in the paradigms that describe mercury geochemical cycling is therefore of high priority especially in light of concerns regarding the effect of global warming and permafrost thawing on input of MeHg to polar regions