Post-Impoundment Evolution of Fish Mercury Levels at the La Grande Complex, Québec, Canada (from 1978 to 1996)

Abstract

At the La Grande hydroelectric complex, the evolution of fish mercury (Hg) levels was monitored in reservoirs, along river diversion routes, and in rivers with modified flow, from 1978 to 1996. Five fish species were considered: two non-piscivorous, lake whitefish (Coregonus clupeaformis) and longnose sucker (Catostomus catostomus), and three piscivorous, northern pike (Esox lucius), walleye (Stizostedion vitreum) and lake trout (Salvelinus namaycush). Total Hg concentrations were measured by standard cold vapour atomic absorption spectrophotometry, and expressed in mg kg-1, wet weight. In reservoirs, concentrations in all species increased rapidly after impoundment, peaking after 5 to 9 years in non-piscivorous fish, and after 10 to 13 years in piscivorous species, at levels 3 to 7 times those measured in surrounding natural lakes, then significantly and gradually declined. Data from the La Grande complex strongly suggest that concentrations return to natural levels after 10 to 25 years for non-piscivorous species, and after 20 to 30 years for piscivorous ones. This duration of the phenomenon is corroborated by results from other reservoirs in Canada and Finland. Monitoring of Hg levels in fish and studies of drifting organisms also show that Hg is exported downstream from reservoirs, probably mostly by reservoir Hg-rich organic debris as well as by plankton, aquatic insects or small fish. This export may affect Hg levels in fish over long distances downstream from reservoirs in the absence of large deep bodies of water allowing sedimentation of organic debris or biological uptake of Hg-rich organisms originating from reservoirs. Along a series of 4 large reservoirs emptying into one another, the highest Hg levels were systematically measured in fish caught immediately downstream from reservoir outputs, but no cumulative effect in fish Hg levels was observed from the first to the last reservoir. In a river which sustained a reduction of flow of over 90%, fish Hg levels remained within the range of concentrations measured in fish collected from natural surrounding lakes.