Increases in Fish Mercury Levels in Lakes Flooded by the Churchill River Diversion, Northern Manitoba

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Abstract
Reservoir creation has often been inferred as a cause of elevated fish mercury concentrations. Increases in fish muscle mercury levels, occurring coincidentally with flooding, are documented for three lakes affected by the Churchill River diversion for which pre- and post-impoundment data were available. For example, northern pike (Esox lucius) and walleye (Stizostedion vitreum vitreum) muscle mercury levels from Southern Indian Lake, which was increased in surface area by 21% by flooding in 1976, increased from baseline values of 0.2–0.3 μg∙g−1 prior to flooding to 0.5–1.0 μg∙g−1 in 1978–82. Muscle mercury levels from predatory species (northern pike and walleye) from all 10 lakes tested in the Churchill, Rat, and Burntwood valleys flooded by the Churchill diversion are near to or exceed the current export marketing limit of 1.0 μg∙g−1. Because mercury levels in fish from nearby unflooded lakes have not shown recent increases, atmospheric fallout of the metal does not appear to be the cause of the problem. Also, there are no known industrial sources of mercury in the area and no agricultural activity. Postimpoundment mercury levels in predatory fish appeared to be related to the flooded terrestrial area compared with preimpoundment lake area. They were highest (1.15–2.90 μg∙g−1) in Rat and Notigi lakes, which were increased in surface area by 282%, lower (0.60–1.53 μg∙g−1) in lakes immediately below Notigi Reservoir, increased in surface area by 31–37%, and lowest (0.45–1.03 μg∙g−1) in Southern Indian and Wuskwatim lakes, increased in surface area by 13–21%. Fish mercury levels responded quickly to impoundment, increasing noticeably within 2–3yr. Declines in mercury concentrations had not, in general, taken place within 5–8yr of impoundment, with the exception of lake whitefish (Coregonus clupeaformis) from Southern Indian Lake. It is hypothesized that observed fish mercury level increases were due to the bacterial methylation of naturally occurring mercury found in flooded soils.