Total Mercury and Methylmercury Contents of Insects from Boreal Lakes: Ecological, Spatial and Temporal Patterns

Abstract

Abstract Mercury (Hg) and methylmercury (MeHg) concentrations in insects from 19 lakes in Quebec (Canada) and Sweden ranged from 550 ng Hgg−1 dry weight (dw). The mean proportion of MeHg to total Hg concentrations depended on the feeding behaviour of the animals, increasing from 35-50% in detritivores-grazers (dipterans, ephemeropterans, trichopterans) to 70–95% in predators (heteropterans, coleopterans, odonates). These differences were attributed to the biomagnification of MeHg in the food web since the MeHg/total Hg ratio in the organisms was not related to their body size. A large part of the overall variability of total Hg (r2=0.57, p=0.0001) and MeHg (r2=0.79, p=0.0001) concentrations in insects was explained by taxonomic differences, probably related to feeding behaviour, and the differences between lakes. MeHg concentrations in chironomids and in odonates were significantly correlated with sediment MeHg contents (r=0.78, p=0.005; r=0.62, p=0.001, respectively). However, our results suggest that animal feeding behaviour and the quality of ingested food are more important in determining MeHg accumulation in insects than either MeHg contents of sediment or atmospheric Hg deposition rates. Our data indicate that the bioavailability of Hg to the lower trophic levels of the food web is determined by abiotic factors and controls contamination of predators at the top of the food chain. Total Hg concentrations in insects increased from spring to fall, from 60–80 ng Hgg−1 dw to 125–175 ng Hgg−1 dw in the detritivore-grazer taxa, and from 88–120 ng Hgg−1 dw to 180–200 ng Hgg−1 dw in the predator taxa. MeHg/total Hg ratios showed little variability from spring to fall and the increase of Hg and MeHg concentrations in insects seemed to be related to enhanced methylation resulting from the rise of water temperature and to the variations in the nature of the food ingested.