MODELING THE ELEMENTAL MERCURY CYCLE IN PALLETTE LAKE, WISCONSIN, USA

The spatial and temporal distribution of elemental Hg (Hg-o) and reactive Hg (Hg-R) has been studied on Pallette Lake, Wisconsin during May - August, 1993 and May, 1994. In general, Hg-o concentrations near the lake surface greatly exceeded saturation with respect to atmospheric Hg-o indicating a flux out (-) of the lake. Evasional losses were estimated using a thin film model and averaged -101 pmol m(-2) d(-1) during July and August, 1993. A large portion of atmospherically deposited Hg is re-emitted. Thus, in-lake Hg-o production and evasion to the atmosphere will significantly reduce the amount of Hg which is transported to the sediments, the principal site of methylation. Laboratory experiments were conducted to ascertain the rate of Hg-o formation from Hg(II). Reduction was significantly lower in heat sterilized lakewater suggesting Hg-o production was biologically mediated. The temporal distribution of epilimnetic Hg-o, as measured at the lake center, was influenced by Hg-o evasion, Hg-o production and advective transport of water parcels of differing Hg content. Spatial gradients in Hg-o and Hg-R were identified and a transport model was employed to estimate the advective flux of Hg-o. The importance of atmospheric deposition and sediment-water interaction as sources of Hg-R to epilimnetic waters were examined. Porewater concentrations of Hg-o and Hg-R were determined on several occasions. During May, 1994, the depletion of lakewater Hg-R following a input pulse due to rain was observed and the estimated removal rate (16-20% d(-1)) agrees well with reduction rates obtained in the laboratory (23% d(-1)).