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To investigate relationshi.Qs 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 ') and methylmercury (MeHg: 0.04-2.2 ng liter-I). Over the range ofmeasured variables, Hg and MeHg were most strongly correlated with each other (r2 = 0.83-0.88) and with dissolved organic C (DOC) (r2 = 0.64-0.92). Multiple regression models containing DOC and a (DOC x 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 airwater 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 twofold 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.

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Rates of Hg methylation and demethylation were measured in anoxic hypolimnetic waters of two pristine Wisconsin lakes using stable isotopes of Hg as tracers. One of the lakes is a clear-water seepage lake situated in sandy terrain with minimal wetland influence. The other is a dark-water lake receiving channelized inputs from a relatively large terrestrial wetland. Methyl mercury (MeHg) accumulated in the anoxic hypolimnia of both lakes during summer stratification, reaching concentrations of 0.8 ng·L–1 in the clear-water lake and 5 ng·L–1 in the dark-water lake. The stable isotopic assays indicated that rate constants of Hg(II) methylation (Km) ranged from 0.01 to 0.04·day–1 in the clear-water lake and from 0.01 to 0.09·day–1 in the dark-water lake, depending on the depth stratum. On average, Km was threefold greater in the dark-water lake. Hypolimnetic demethylation rate constants (Kdm) averaged 0.03·day–1 in the clear-water lake and 0.05·day–1 in the dark-water lake. These methylation rates were sufficient to account for the observed accumulation of MeHg in hypolimnetic water during summer in both lakes. Despite substantial export of MeHg from the wetland to the dark-water lake, our study indicates that in-lake production and decomposition of MeHg dominated the MeHg cycle in both lakes.

Here, we describe and evaluate two low-power wireless sensor networks (WSNs) designed to remotely monitor wetland hydrochemical dynamics over time scales ranging from minutes to decades. Each WSN (one student-built and one commercial) has multiple nodes to monitor water level, precipitation, evapotranspiration, temperature, and major solutes at user-defined time intervals. Both WSNs can be configured to report data in near real time via the internet. Based on deployments in two isolated wetlands, we report highly resolved water budgets, transient reversals of flow path, rates of transpiration from peatlands and the dynamics of chromophoric-dissolved organic matter and bulk ionic solutes (specific conductivity)-all on daily or subdaily time scales. Initial results indicate that direct precipitation and evapotranspiration dominate the hydrologic budget of both study wetlands, despite their relatively flat geomorphology and proximity to elevated uplands. Rates of transpiration from peatland sites were typically greater than evaporation from open waters but were more challenging to integrate spatially. Due to the high specific yield of peat, the hydrologic gradient between peatland and open water varied with precipitation events and intervening periods of dry out. The resultant flow path reversals implied that the flux of solutes across the riparian boundary varied over daily time scales. We conclude that WSNs can be deployed in remote wetland-dominated ecosystems at relatively low cost to assess the hydrochemical impacts of weather, climate, and other perturbations.