Regulation of Rotifers by Predatory Calanoid Copepods (Subgenus Hesperodiaptomus) in Lakes of the Canadian Rocky Mountains

Abstract

The abundance of planktonic rotifers in alpine lakes is regulated by predation from Diaptomus (subgenus Hesperodiaptomus) arcticus (a large calanoid copepod) regardless of nutrient conditions. We manipulated both predators and nutrient levels in large in situ enclosure experiments (2250 L). Hesperodiaptomus arcticus suppressed densities of the soft-bodied rotifer Polyarthra dolichoptera significantly (two to three orders of magnitude) and the loricate rotifer Keratella quadrata less so (up to one order of magnitude) relative to populations in predator-free enclosures. Keratella quadrata's armoured lorica may limit predation by Hesperodiaptomus. Nutrient addition resulted in higher phytoplankton standing crops and rotifer densities but did not permit rotifers to exceed predator consumption. In 69 lakes from the Canadian Rocky Mountains, the presence of Keratella or Polyarthra was negatively correlated with Hesperodiaptomus density. A maximum-likelihood logistic regression model predicts that as Hesperodiaptomus densities increase the probability of occurrence for both rotifer genera decreases, with Polyarthra declining more rapidly than Keratella.