Will environmental calcium declines hinder Bythotrephes establishment success in Canadian Shield lakes?

Recently, calcium-rich daphniids have declined on the Canadian Shield in response to falling lake-water calcium concentrations, or [Ca]. Meanwhile the invader Bythotrephes longimanus , a predator that feeds on Daphnia , continues to spread. Our goal was to determine if ongoing calcium declines might directly or indirectly affect Bythotrephes ’ establishment success. To address direct effects, we provide the first quantification of Bythotrephes’ calcium content, which is very low (0.03% as dry mass) compared with other Cladocera. We also examined the effects of differing [Ca] (0.1–2.6 mg·L–1) on Bythotrephes’ performance in the laboratory. For all [Ca], population growth rates remained positive, indicating that Bythotrephes has great tolerance of low [Ca]. Finally, we examined Bythotrephes’ distribution in relation to [Ca] on the Shield where is it relatively new, alongside its distribution in Norway where it is endemic and found that Bythotrephes inhabits very low calcium environments in Norway (minimum = 0.2 mg·L–1). These results suggest that Bythotrephes establishment in Canada is currently not — and in the future will likely not — be limited by falling calcium. Rather, as Bythotrephes is more tolerant of low [Ca] than are its daphniid prey, we propose that both calcium decline and Bythotrephes invasions may contribute to Daphnia decline.