|dc.description.abstract||Run-of-river (RoR) hydroelectric dams are an increasingly common alternate energy source on mountain streams. Despite reductions in size and greenhouse gas emissions compared to conventional impoundments, RoR dams may have ecotoxicological impacts through disruption of the natural flow regime. The American Dipper (Cinclus mexicanus) is a high trophic-level river bird that occupies mountain streams year-round and is a well-described indicator of stream health; thus, it is an ideal species to study potential impacts of RoR hydropower on river food webs. From August 2014 to November 2015, I conducted seasonal river bird surveys at 14 streams in coastal British Columbia, Canada and sampled food webs at 13 of these streams (7 regulated and 6 unregulated). Regulated streams create stable habitats that consequently supported significantly higher dipper densities (β=0.78, SE=0.36, p=0.030), a higher proportion of after hatch year (AHY) dippers, and consistent occupancy during breeding and non-breeding seasons compared with free-flowing streams.
Analyses of stable isotope ratios in dipper whole blood revealed strong model support for differences in blood isospace between regulated and unregulated stream types, likely driven by the significantly lower invertebrate δ34S below RoR dams (p=0.010) and 34S-depleted blood at regulated streams (β=-2.42, SE=0.95, p=0.029). Given that the bacteria responsible for 34S-depleted food webs are also the primary methylators of inorganic mercury (Hg) into its toxic and bioavailable form, methylmercury (MeHg), I further investigated Hg levels in American Dippers and their prey. Despite the observation of distinct dipper isospace between stream types, there was no model support for differences in mean dipper blood (417.6 ± 74.1 S.E.) ng/g ww at regulated streams, 340.7 ± 42.7 S.E. ng/g ww at unregulated streams) or feather (1564 .6 ± 367.2 S.E. ng/g dw regulated, 1149.0 ± 152.1 S.E. ng/g dw unregulated) Hg concentrations between stream types.
One recently regulated stream (Douglas Creek, Harrison Watershed), however, supported dippers with MeHg concentrations of toxicity concern (up to 8459.5 ng/g dw in feathers and 1824.6 ng/g ww in whole blood). With a negligible salmon subsidy at these streams and the absence of a known anthropogenic Hg point source, the elevated Hg concentrations recorded in dippers at this regulated stream could be explained by a combination of a) elevated atmospheric deposition of Hg in densely forested, temperate mountain streams, b) Hg-methylation by sulfate- reducing bacteria under anaerobic headpond conditions, and c) increased availability of high-protein prey.
Slow-flowing, stabilized stream reaches regulated by RoR dams potentially offer an opportunity for dippers to exploit a consistent food resource closer to their high elevation breeding territory, enabling them to take on a year-round “resident strategy” that is more typical of low elevation river habitats. The enhanced microbial activity and MeHg production that can occur in RoR headponds, however, suggests that even small dams with minimal storage can create pond-like habitats that may act as ecological traps for river bird specialists. Although at this point there appear to be no effects of river regulation on body condition, future work is needed to determine if long-term chronic MeHg toxicity can impair productivity or survival of predatory river birds.||