The use of stable carbon isotopes to evaluate the importance of fine suspended particulate matter in the transfer of methylmercury to biota in boreal flooded environments

Applying the classic geochemical technique of stable carbon isotope ratios d13C., we confirmed the existence of a trophic link between fine particulate matter FPM. and zooplankton in freshwater ecosystems, and examined possible reasons for the elevated MeHg concentrations wMeHgx. in hydroelectric reservoirs as compared to natural lakes. Comparing natural and flooded environments, the d13C and wMeHgx values for FPM and zooplankton differ significantly. Using a mixing model to calculate the contribution of terrestrial carbon to FPM, the differences in d13C between natural and flooded sites are explained by an increasingly important autochthonous component in reservoirs. The stable isotopic evidence presented here strongly suggests that, despite the much greater abundance of detrital vascular-plant carbon, microalgae are important in supporting aquatic food webs in the oligotrophic flooded systems studied. Due to a significant inverse relationship between wMeHgx in FPM and the percentage of terrestrial carbon r2s0.87., we propose that the higher wMeHgx in the zooplankton of flooded sites as compared to lakes are the result of proportionally higher levels of autochthonous material algaerbacteria; i.e. potential sourcesrmethyl- ators of Hg. in the FPM of reservoirs.