Historical and Contemporary Patterns of Mercury in a Hydroelectric Reservoir and Downstream Fishery: Concentration Decline in Water and Fishes

Abstract

Mercury (Hg) contamination can pose risks to human and animal health as well as commercial fisheries. Reservoir construction in riverine systems produces flooded conditions amenable to Hg(II)-methylating bacteria, which can transform this relatively benign environmental contaminant into the bioaccumulative, environmentally relevant, and neurotoxic methyl-Hg (MeHg). Hg concentrations ([Hg]) in fishes from reservoirs can take decades to decrease to pre-dam levels, but less is known about Hg exported downstream and its dynamics within downstream fish populations. We examined and compared the multidecadal rates of biotic [Hg] decrease and contemporary factors affecting [Hg] in fish collected from a hydroelectric reservoir (Tobin Lake) and a related downstream fishery (Cumberland Lake) along the Saskatchewan River, Canada. Rates of [Hg] decrease were considered in four species-northern pike (Esox lucius), sauger (Sander canadensis), goldeye (Hiodon alosoides), and walleye (S. vitreus)-all of which showed a significant decrease over time (p < 0.001) and are now lower than Health Canada consumption guidelines (0.5 μg/g). Rates of decrease ranged from 0.5 to 3.9 %/year and were similar between sites in the cases of northern pike and sauger. Contemporary factors affecting [Hg] in walleye collected downstream include fish length (p < 0.001), fish age (p < 0.001), and trophic magnification through the food web (p < 0.001), and relationships between [Hg] and trophic level in predatory and prey fish are now similar to those found in non-Hg-inundated systems at a similar latitude. Together, these results suggest connected contamination between the two sites and delineate the timeline during which [Hg] in a variety of fish species decreased to nontoxic levels in both locations.

Energy stores and mercury concentrations in a common minnow (spottail shiner, Notropis hudsonius) associated with a peaking hydroelectric dam

“…Spot measurements of temperature, pH, turbidity and conductivity were taken at study sites upstream and downstream of the dam in both months immediately prior to sampling. Aqueous [Hg] are already known to be minimal (low ng/L; Green et al, 2016) in this system, but Hg analyses were conducted regardless in July and September.…”

“…Mercury concentrations (hereafter [Hg]) in impounded fishes are often elevated in hydroelectric reservoirs across Canada, posing risks to fisheries and consumers (Anderson, 2011; Bodaly et al, 2007; Green et al, 2016). Concentrations peak in sportfish (e.g., northern pike, Esox lucius , and walleye, Sander vitreus ) within 2–8 years post‐impoundment (Bodaly et al, 2007; Willacker et al, 2016) with maximum concentrations increasing proportionally to the area flooded (Bodaly et al, 2007).…”

“…Concentrations peak in sportfish (e.g., northern pike, Esox lucius , and walleye, Sander vitreus ) within 2–8 years post‐impoundment (Bodaly et al, 2007; Willacker et al, 2016) with maximum concentrations increasing proportionally to the area flooded (Bodaly et al, 2007). After peaking, [Hg] in impounded fish can take decades to return to baseline, with rates of decline depending upon the trophic levels and feeding habits of contaminated species (Anderson, 2011; Bodaly et al, 2007; Green et al, 2016).…”

“…Elevated [Hg] have also been observed in fish post‐impoundment as reservoir‐discharged methylmercury (MeHg) is advected to downstream environments, and rates of [Hg] decline may be slower in downstream fish (Anderson, 2011; Green et al, 2016; Schetagne, Doyon, & Fournier, 2000). These elevated biotic [Hg] persist despite minimal [Hg] retention in the water column, suggesting that characteristics of the downstream environment may be partly responsible for the phenomenon (Sandheinrich & Drevnick, 2016; Sousa, Domingos, Poggiale, & Kooijman, 2010; Trudel & Rasmussen, 2006).…”

“…It is possible, therefore, that a delayed decline in [Hg] in downstream fish may be an artefact of reduced growth in fish inhabiting particularly strongly impacted downstream regions. To evaluate this hypothesis, we captured small‐bodied fish (spottail shiner; “spottail”: Notropis hudsonius ) upstream and downstream of a hydropeaking dam with a history of aqueous and biological Hg contamination, observed delays in [Hg] decline from downstream fishes, and negligible evidence of [Hg] persistence in the water column (Green et al, 2016). Spottail are an ideal sentinel species for studying the influences of hydroelectric facilities in North America as there is substantial and extensive overlap between their home‐ranges and areas of hydroelectric development and are also a species with abundant, non‐threatened populations (IUCN, 2013; Page & Burr, 1991; US EIA, 2014; Wells & House, 1974).…”