METHYLMERCURY DISTRIBUTION AND PARTITIONING IN STRATIFIED FINNISH FOREST LAKES

TotHg and MeHg were determined in water profiles, sedimenting seston a nd surface sediments of four stratified forest lake systems in Finland . The radiochemical method was applied to the methylation assays perfo rmed in situ. In the anoxic layers the MeHg concentrations were genera lly higher than in the epilimnia. The in situ MeHg production also inc reased in the hypolimnia, but MeHg increase in water was more closely associated to particles than in situ MeHg production. Methylmercury pa rtitioning between particles and dissolved (0.2 mu m) phase was simila r in sediment traps and hypolimnetic water, the trap concentrations be ing one order of magnitude higher. Dissolved MeHg in the hypolimnia ex ceeded that in sediment surface. Particulate MeHg concentrations in th e traps were higher than at the sediment surface. Methylmercury transp ort through sedimentation and MeHg production in hypolimnetic water ar e suggested as primary sources of increased MeHg in hypolimnia of the lakes studied.