Direct and indirect effects of predation by a calanoid copepod (subgenus: Hesperodiaptomus) and of nytrients in a fishless alpine lake

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Abstract
The calanoid copepod Diaptomus (subgenus: Hesperodiaptomus) arcticus is a keystone predator in fishless alpine lakes of the Canadian Rockies. We quantified the effects of predation by D. arcticus on other copepods, rotifers, and algae in large mesocosm experiments (2250 L) using two levels of predator (present and absent) and two levels of nutrient addition (ambient and 4×). Standing stocks of algal taxa were assessed by high performance liquid chromatography of phytoplankton pigments. Diaptomus arcticus suppressed the biomass of rotifers and cyclopoid nauplii at both nutrient levels. The indirect effect of D. arcticus on algal biomass was minimal under ambient nutrient conditions, possibly owing to high rates of nutrient recycling by grazers. Biomass of algae, cyclopoid nauplii, and rotifers responded positively to nutrient additions. Nutrient addition increased algal standing crop 2- to 4-fold and changed dominance from diatoms and chrysophytes to blue-green algae. Diaptomus arcticus accelerated these changes, possibly by eliminating grazing by rotifers. These results suggest that in the absence of increased nutrients D. arcticus directly limits the biomass of herbivorous zooplankton but the indirect effect on phytoplankton is minimal.