Intensity and duration of effects of impoundment on mercury levels in fishes of hydroelectric reservoirs in northern Québec (Canada)

At the La Grande Hydroelectric Complex (Québec, Canada), total mercury (THg) levels in fish were monitored from 1978 to 2012 in more than 37 000 fish comprising 5 species: lake whitefish (Coregonus clupeaformis), longnose sucker (Catostomus catostomus), northern pike (Esox lucius), walleye (Sander vitreus), and lake trout (Salvelinus namaycush). In reservoirs, concentrations of all species increased rapidly after impoundment, peaking after 4–11 yr in nonpiscivorous species and after 9–14 yr in piscivorous species, at levels 2–8 times higher than those measured in surrounding natural lakes. In fish of standardized length, maximum levels reached 0.33–0.72 μg g⁻¹ in nonpiscivorous species and 1.65–4.66 μg g⁻¹ in piscivorous species. Depending on the reservoir, the return to levels equivalent (p < 0.05) to those found in fish in surrounding natural lakes was completed after 10–20 yr for all nonpiscivorous species and after 20– 31 yr in most piscivorous species, if no additional flooding occurred. These results tend to confirm the findings of other authors suggesting that the following reservoir characteristics play a major role in determining the intensity and duration of after-impoundment THg increases in fish: flooded area, annual volume of water flowing through the reservoir, filling period, water temperature, and percentage of flooded area located in the drawdown zone.