Concentration of Mercury Species in Relationship to Other Site-Specific Factors in the Surface Waters of Northern Wisconsin Lakes

To investigate relationships between mercury speciation and site-specific factors in temperate freshwaters, we measured the concentration of seven Hg species along with 18 environmental variables in the surface waters of 23 northern Wisconsin lakes during spring and fall. The lakes spanned relatively wide gradients in $Hg (0.15-4.8 ng liter^-1)$ and methylmercury $(MeHg: 0.04-2.2 ng liter^-1)$. Over the range of measured variables, Hg and MeHg were most strongly correlated with each other $(r^2 = 0.83-0.88)$ and with dissolved organic C (DOC) $(r^2 = 0.64-0.92)$. Multiple regression models containing DOC and a $(DOC \times pH)$ interaction term accounted for 85-90% of the variability in Hg and MeHg between lakes. Observed differences between lakes reflected internal cycling processes and external transport pathways. Internally, high DOC and low pH favored Hg methylation and retention over Hg evasion across the air water interface. Externally, watershed mapping suggested that the cotransport of DOC, Hg, and MeHg from riparian wetland was also a potentially important process. Observed seasonal differences indicated a 30% increase in MeHg across lakes during summer due to internal or external processes. The effects of DOC on bioaccumulation may be towfold and antagonistic. Although waterborne Hg and MeHg increased with DOC, seston-water partition coefficients tended to decrease, indicating disproportionately more Hg in the dissolved phase. These observations are consistent with previous data on bioaccumulation factors for zooplankton and fish.