Mercury emission from a temperate lake during autumn turnover.

Lakes in temperate regions stratify during summer and winter months, creating distinct layers of water differentiated by their physical and chemical characteristics. When lakes mix in autumn and spring, mercury cycling may be affected by the chemical changes that occur during mixing. Sampling was conducted in Lake Lacawac, Eastern Pennsylvania, USA, throughout the autumn of 2007 to characterize changes in emission of gaseous elemental mercury (Hg(0)) from the lake surface and dissolved mercury profiles in the water column during mixing. Water chemistry and weather parameters were also measured, including dissolved organic carbon (DOC), iron, and solar radiation which have been shown to interact with mercury species. Results indicate that emission of Hg(0) from the lake to the atmosphere during turnover was controlled both by solar radiation and by surface water mercury concentration. As autumn turnover progressed through the months of October and November, higher mercury concentration water from the hypolimnion mixed with epilimnetic water, increasing mercury concentration in epilimnetic waters. Dissolved absorbance was significantly correlated with mercury concentrations and with iron, but DOC concentrations were essentially constant throughout the study period and did not exhibit a relationship with either dissolved mercury concentrations or emission rates. Positive correlations between dissolved mercury and iron and manganese also suggest a role for these elements in mercury transport within the lake, but iron and manganese did not demonstrate a relationship with emission rates. This research indicates that consideration of seasonal processes in lakes is important when evaluating mercury cycling in aquatic systems.

Mercury emission to the atmosphere from experimental manipulation of DOC and UVR in mesoscale field chambers in a freshwater lake.

Mesocosm experiments in an optically transparent lake allow the manipulation of both dissolved organic carbon (DOC) and incident ultraviolet radiation (UVR) in order to study mercury reduction and emission processes. In the absence of UVR and the presence of visible light, mercury emission is very low (approximately0.3 ng/m2/h). When UVR is permitted in the mesocosm chambers, mercury emission increases, with emission rates ranging from 0.3 to 2.5 ng/m2/h. At concentrations between 1.5 and 2.5 mg/L DOC, mercury emission does not appear to depend on either the concentration or the optical properties of the DOC. In particular, the addition of 1.0 mg/L DOC from a nearby wetland to a photobleached mesocosm did not increase the emission of mercury. The similarities between mercury emission from highly photobleached 1.5 mg/L DOC and from terrestrially enriched 2.5 mg/L DOC suggest that the moieties responsible for mercury reduction are far in excess of that needed for mercury reduction. Using the measured flux rate of mercury from the water surface, we calculated a dissolved gaseous mercury (DGM) concentration thatwould need to be present to drive the emissive flux. The buildup of DGM was used to approximate a kinetic rate constant for the net mercury reduction in this system of approximately 0.17 h(-1), which is consistent with existing published values.