The concentration and distribution of mercury in Lake Michigan

Total and particulate mercury (Hg) concentrations were determined for the water column of Lake Michigan as part of the EPA Lake Michigan Mass Balance Project. Concentrations for total Hg averaged 1.60 +/- 0.25 pM while the particulate was 0.60 +/- 0.18 pM (20-50% of the total). Overall, fluctuations in total Hg both spatially and vertically showed no consistent trends, while particulate Hg may have been affected by calcium carbonate precipitation in late summer and sediment resuspension late in the season. Dissolved methylmercury (MMHg) concentrations ranged from the detection limit (25 fM) to 210 fM and the particulate fraction was from 5 fM (DL) to 45 fM (2% of total Hg). Disserved gaseous Hg (DGHg) concentrations during one cruise averaged 140 +/- 85 fM. Incubation experiments suggest that biotic processes are primarily responsible for elemental Hg production (2.6-6.5% per day) in Lake Michigan. Overall, the cycling and speciation of Hg in Lake Michigan is more akin to that of the open ocean, than smaller lakes in the mid-western USA. (C) 1998 Elsevier Science B.V.