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

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.

Mercury bioaccumulation in the food web of Three Gorges Reservoir (China): Tempo-spatial patterns and effect of reservoir management

“…Inundated soil has been identified as the primary source of Hg to new reservoirs (Bodaly et al, 1984) although other sources are possible (Cox et al, 1979). Poor organic content in these areas may weaken microbial activity and hence, reduce methylmercury production.…”

Mercury and Arsenic Bioaccumulation and Eutrophication in Baiyangdian Lake, China

“…Bioaccumulation in fish and shellfish represent an exposure route for these atmospheric contaminants such as Hg to humans [Munthe et al 2007]. Moreover, reservoir formation for agriculture or hydroelectric power can produce a significant increase in mercury concentrations in fish inhabiting the reservoirs [Bodaly et al, 1984;Jackson, 1991;Jin et al, 1999;Kidd et al, 1999]. In China, fish consumption is known to be the most important exposure route for Hg and yet very little research has been conducted on Hg trophic transfer in aquatic food webs and virtually none on freshwater food webs [Jiang et al, 2006;Li et al, 2006].…”

Mercury accumulation in bats near hydroelectric reservoirs in Peninsular Malaysia

“…forms of mercury are naturally present in the substrate, but can be transformed by sulphatereducing and iron-reducing bacteria to methylmercury (MeHg) (Chan et al 2003;Poulain and Barkay 2013). Significant amounts of mercury can be introduced into aquatic food webs during the flooding of forests (Barbosa et al 2003), such as during the construction of hydroelectric dams (Bodaly et al 1984;Stokes and Wren 1987;Ikingura and Akagi 2003). When a reservoir is created, submerged vegetation and organic material start to slowly decompose (Rodgers et al 1995), leading to a rise in the dissolution rate of organic carbon, increased release of mercury bound to organic material and higher net mercury methylation rates (Chan et al 2003).…”