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

摘要

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.