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

Abstract

Applying the classic geochemical technique of stable carbon isotope ratios (δ 13C), 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 ([MeHg]) in hydroelectric reservoirs as compared to natural lakes. Comparing natural and flooded environments, the δ 13C and [MeHg] values for FPM and zooplankton differ significantly. Using a mixing model to calculate the contribution of terrestrial carbon to FPM, the differences in δ 13C 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 [MeHg] in FPM and the percentage of terrestrial carbon ( r 2=0.87), we propose that the higher [MeHg] in the zooplankton of flooded sites as compared to lakes are the result of proportionally higher levels of autochthonous material (algae/bacteria; i.e. potential sources/methylators of Hg) in the FPM of reservoirs.