METHYLMERCURY IN A BENTHIC FOOD-WEB OF 2 HYDROELECTRIC RESERVOIRS ANDA NATURAL LAKE OF NORTHERN QUEBEC (CANADA)

Total mercury (HE) concentrations in benthic insects from a 14 years o ld hydroelectric reservoir (La Grande 2) were 2 to 3 times higher than those from a reference lake and, in some groups, up to 7 times. The d ifference was even more pronounced for methylmercury (MeHg) concentrat ions, with a mean of 4 fold and a maximum of 12 fold between systems. The enrichment factors (dw) of insects, relative to the substrate in w hich they reside was around 3 for total Hg and 6 to 22 for MeHg. On th e basis of their diet, we have classified the insects into four differ ent trophic levels: detritivores, grazers, grazers-predators and preda tors. In insects collected in the reservoirs, the MeHg:Hg ratio was 20 -25% in dipterans-ephemeropterans (detritivores) and 30-40% in trichop terans (grazers), but 60-85% in heteropterans-coleopterans (grazers-pr edators) and 95% in odonates (predators). The pattern was similar in t he lake with slightly lower values. In both systems, the proportion of MeHg increases in direct relation to our defined trophic levels. Give n that insects are an important food source for many fish, our results suggest that they are a strong vector of MeHg to these fish in hydroe lectric reservoirs. The bioavailability of MeHg to insect larvae appea rs to depend on the nature and composition of the substrate in which t hey reside.