Methylmercury distribution and partitioning in stratified Finnish forest lakes

TotHg and MeHg were determined in water profiles, sedimenting seston and surface sediments of four stratified forest lake systems in Finland. The radiochemical method was applied to the methylation assays performedin situ. In the anoxic layers the MeHg concentrations were generally higher than in the epilimnia. Thein situ MeHg production also increased in the hypolimnia, but MeHg increase in water was more closely associated to particles thanin situ MeHg production. Methylmercury partitioning between particles and dissolved (0.2 μm) phase was similar in sediment traps and hypolimnetic water, the trap concentrations being one order of magnitude higher. Dissolved MeHg in the hypolimnia exceeded that in sediment surface. Particulate MeHg concentrations in the traps were higher than at the sediment surface. Methylmercury transport through sedimentation and MeHg production in hypolimnetic water are suggested as primary sources of increased MeHg in hypolimnia of the lakes studied.