Impact of wildfire and clear-cutting in the boreal forest on methyl mercury in zooplankton

We compared the effects of forest logging and wildfire on methyl mercury (MeHg) concentrations in bulk zooplankton collected in 38 oligotrophic lakes located in the boreal forest. Mean MeHg concentrations were slightly but significantly higher (P < 0.01) in zooplankton collected in lakes with recently logged watersheds (135 ng.g dry weight(-1)) than in lakes with recently burned (97 ng.g dry weight(-1)) or undisturbed watersheds (112 ng.g dry weight(-1)). MeHg levels in zooplankton collected in burned and reference lakes were not significantly different. Multiple regression including all the lakes showed that four variables (dissolved organic carbon, pH, dissolved oxygen, and immature zooplankton biomass) explained 43% of the variability in MeHg levels in bulk zooplankton. Regressions calculated for each group of lakes explained less (reference lakes) or more of the variability in MeHg (logged and burned lakes) than the general model did. Dissolved organic carbon was the only variable common to all models, which indicates the importance of watershed processes in the transfer of humic matter bound Hg to lakes. However, variables related to methylation rate, such as pH, sulfate concentrations, and average dissolved oxygen in lake water, also explained part of the variation in MeHg concentrations in zooplankton.