Salish Sea Ecosystem Conference

Using American dippers Cinclus mexicanus to investigate the influence of run-of-the-river dams on mercury exposure and food webs in mountain streams around the Salish Sea

Run-of-river (RoR) dams are an increasingly common alternate energy source on mountain streams, and many of the streams and rivers draining into the Salish Sea on the British Columbia side had or a scheduled to have ROR developments. Despite reductions in size and greenhouse gas emissions compared to conventional impoundments, RoR hydro 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 impacts of RoR hydropower on river food webs. During 2014 and 2015, we conducted seasonal river bird surveys and sampled food webs at 7 regulated and 7 unregulated streams in coastal British Columbia, Canada. Regulated streams supported higher densities and consistent occupancy of dippers during breeding and non-breeding seasons, suggesting year-round residency occurs at high elevation streams stabilized by RoR-regulation. Analyses of dipper whole blood revealed significantly different isospace of birds between stream types (p=0.032), driven by significantly lower invertebrate δ34S below RoR dams (p=0.01) and consequently 34S-depleted blood at regulated streams. The bacteria responsible for 34S-depleted food webs also methylate inorganic mercury into its toxic and bioavailable form, methylmercury (MeHg). Despite our observation of distinct dipper isospace between stream types, there was no model support for differences in mean mercury concentrations in dipper blood (417.55 ± 74.07ng/g ww at regulated streams, 340.73 ± 42.73ng/g ww at unregulated streams) or feather (1564 .55 ± 367.18 ng/g dw regulated, 1149.01 ± 152.10ng/g dw unregulated). There was, however, one recently regulated stream where dippers had MeHg levels of potential toxicity, and mercury levels in dippers at this stream were significantly higher than unregulated streams. With a negligible salmon contribution to the dipper diet and the absence of a known anthropogenic Hg point source, the elevated mercury levels recorded in American dippers at this regulated stream were best explained by demonstrated methylmercury production within the headpond and slight diet shifts. There are likely to be other such streams draining into the Salish Sea system.