MERCURY SPECIATION IN LAKE BAIKAL

Research on mercury (Hg) distribution and speciation was carried out i n Lake Baikal, a large, strong-oligotrophic freshwater reservoir in Si beria, Russia, during June 1992 and march 1993. In summer, total Hg in the water column ranged from 0.14 to 0.77 ng Hg/L, with the highest c oncentrations observed in the central basin of the lake in surface wat er samples. Labile inorganic Hg was found to be 7 to 20% of the total Hg content. Highest total Hg concentrations were found in river waters : up to 2 ng Hg/L. Labile methylmercury (MeHg) concentrations ranged from 2 to 38 pg Hg/L in the water column, with the higher concentratio ns in the central part of the lake, and showing a slight increase in n ear bottom waters. Labile MeHg makes up 1 to 15% of the total I-Ig con tent in the water column, with larger fractions in deep waters. The sl ight increase of the MeHg gradient with depth corresponds with the O-2 minimum region. Highest MeHg concentrations were observed in river wa ters (up to 145 pg Hg/L) and in some bays of the lake (up to 160 pg Hg /L). In these high temperature- and phytoplankton-rich water masses, t he MeHg-fraction increased up to 35% of total Hg. Labile MeHg concentr ations in water samples taken in winter in the southern basin (under t he ice cover), showed slightly higher concentrations than in summer, p ossibly due to an early spring bloom. In rainwater, total Hg ranged fr om 3 to 20 ng Hg/L and MeHg from 0.1 to 0.25 ng Hg/L. In snow, a large fraction of total Hg is bound to particulate matter; concentrations o f total Hg ranged from 8 to 60 ng Hg/L and labile MeHg from 0.1 to 0.2 5 ng Hg/L. Atmospheric Mg was found to be 0.73 to 2.31 ng/m(3) as gase ous Hg and 0.005 to 0.02 ng/m(3) in its particulate form. Spatial dist ribution patterns of atmospheric Hg show slightly higher to 0.02 ng/m concentrations over the central part of the lake and the Selenga river delta. In winter, atmospheric Hg values 3 (measured in the southern r egion), ranged from 1.2 to 6.1 ng/m(3) as total gaseous Hg and 0.02 to 0.09 ng/m(3) as total particulate Hg, and are higher than in summer, probably influenced by coal burning and traffic by the local populatio n. MeHg contents in fish ranged from 20 ng Hg/g dry weight in small Co ttocomephorus to 300 ng Hg/g dry weight in pike and trout species, whi ch were caught in organic-rich waters.