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We studied fluxes of methylmercury (MeHg) through a Precambrian Shield lake using a mass balance approach. The primary goal of the study was to determine the importance of various sources of MeHg to the water column of the lake. The relative importance of all sources was: in-lake production ϾϾϾ inflow from a brown-water lake with riparian wetlands ϾϾϾ wet deposition Ͼ inflow from an upstream oligotrophic lake Ͼ direct inflow from uplands surrounding the lake. MeHg accumulated in the hypolimnion of Lake 240 when oxygen was present. Watercolumn sinks for MeHg included photodegradation of MeHg, which was about 3.5 times greater than the loss of MeHg through outflow. At present, there are few studies available on mass balance fluxes of MeHg in lakes, and this is the first study that includes losses of MeHg by photodegradation. The inclusion of photodegradation in this study results in a clear demonstration that in-lake production of MeHg is very important. In drainage lakes, the relative importance of in-lake production versus inflow of MeHg from wetlands will vary according to the extent of wetlands in the drainage basin, as well as the volume of precipitation, which produces runoff and transports MeHg from wetlands to downstream lakes.

Three parameters must be known to use the thin boundary-layer model (or other bulk transfer models) for CO, flux between water and air: the concentration of dissolved CO*, CO,,,,,, the concentration of CO, in the air immediately above the water, COZ(atm), and the wind velocity, which is used to determine the appropriate transfer coefficient. These parameters change hourly and from day to day in a nonlinear fashion, so the frequency of measurements is an important factor in determining the accuracy of flux estimates for any period. To achieve a high frequency measurement, we developed a self-contained, solar-powered, in situ sampling system that continuously measures and records CO,,,,,, COZ(atm), and windspeed. Unique to this technique is an underwater in situ equilibration chamber (ISEC). The ISEC was tested in a shallow wetland pond in which changes in both CO,,,,, and COZ(ntm) were large. The data obtained showed that large errors may result from extrapolating flux calculations made from short-term data (e.g. daily) to longer time periods.The most commonly used method for estimating gas flux across the air-water interface is the thin boundarylayer (TBL) model, which requires measurements of windspeed and of the concentrations of the gas in air and water. Gas concentration data are usually acquired by periodic sampling, and the frequency of sampling is usually limited by accessibility to the site and the time available for analyses.When measurements are made for the purpose of calculating CO2 flux in lakes where diel changes in water and air arc small, limitations on sampling frequency may not be serious. However, there are two common situations in which very frequent measurements in both air and water may be necessary: eutrophic lakes and shallow ponds-both of which have strong diel changes due to high rates of photosynthesis and respiration. Even where diel changes are not so strong (as in oligotrophic lakes), other considerations, such as large changes in windspeed, may make sampling frequency important.Because CO2 flux is proportional to the dzjkwzce bcAcknowledgments

Between 1962 and 1969, 10 tonnes of mercury were discharged from a chlor-alkali plant in Dryden, Ontario, to the English–Wabigoon River. Present-day fish mercury concentrations are amongst the highest recorded in Canada. In 2017, the Grassy Narrows Science Team found no evidence of ongoing discharges from the plant site to the river water, even though large quantities of mercury remain at the site. Instead, our data suggest that ongoing erosion of high mercury particles by the river, as it meanders through contaminated floodplains, is responsible for present-day transport of mercury to Clay Lake and to Ball Lake, located 154 km downstream. In Clay Lake, surface sediment total mercury concentrations and inflow water concentrations are still about 15 times above background (86 km downstream), and in Ball Lake mercury concentrations in sediments appeared to be still increasing. The remobilization of legacy inorganic mercury from riverbank erosion between Dryden and Clay Lake stimulates methyl mercury production there, in Clay Lake, and in Ball Lake. The large quantities of methyl mercury produced between Dryden and Clay Lake are mostly dissolved in water and are swept downstream, elevating concentrations in water and biota throughout the system. Several options for remediating the ongoing contamination are discussed.

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