Total mercury and methylmercury fluxes via emerging insects in recently flooded hydroelectric reservoirs and a natural lake

Total mercury (total Hg) concentrations in emerging aquatic insects ranged from 140 to 1500 ng g−1 dry wt. in two hydroelectric reservoirs of northern Québec compared with 50–160 ng g−1 dry wt. in a natural lake. Methylmercury (MeHg) concentrations were somewhat lower, ranging from 35 to 800 ng Hg g−1 dry wt. in reservoirs and from 29 to 90 ng g−1 dry wt. in the natural lake. Contamination of insect taxa of reservoirs was on average 2–3 times higher than their counterparts in the natural lake. There was no difference between total Hg and MeHg concentrations of insects sampled from flooded forest soils and flooded peatland, although total Hg and MeHg concentrations differed between flooded peatland and flooded forest soils themselves. Insect biomasses were approx. two times higher in the reservoirs than in the natural lake (580–2200 mg m2 year−1 dry wt., 950 mg m2 year−1 dry wt., respectively); chironomids dominated in the reservoirs and trichopterans dominated in the natural lake. Similarly, total MeHg fluxes via emerging insects were approx. 2–4 times higher in reservoirs than that of the natural lake (55–224 ng MeHg m2 year−1 dry wt., 74 ng MeHg m2 year−1 dry wt., respectively). Our results show the importance of the insect community in the transfer of MeHg from flooded soils and flooded peatlands to fish, and that this pathway probably makes a significant contribution to the rapid rise of Hg levels in the fish community after flooding.