Calcium stress in Daphnia pulicaria and exposure to predator-derived cues: Making a bad situation worse?

Abstract

Calcium concentrations ([Ca]) in some boreal lakes on the Canadian Shield have been declining over the previous couple of decades. With [Ca] in many lakes below 2 mg Ca/L, there are concerns about possible ecological effects of sustained Ca limitation on zooplankton assemblages. Here we examine whether short- and long-term effects of Ca stress on a keystone herbivore, Daphnia pulicaria, change with the presence of an invertebrate predator, Chaoborus flavicans. We did so by testing whether effects of low Ca on Daphnia pulicaria differ in the presence and absence of kairomones derived from C. flavicans. We compared mass-specific growth rates, body %Ca content, life-history traits, and survival of Daphnia exposed to a range of [Ca] with or without Chaoborus kairomones. We found a mix of individual and interactive effects of kairomones and low Ca supply on Daphnia responses. With longer and chronic exposure to both stressors, the effects of low Ca were intensified by the presence of kairomones, particularly for several reproductive measures (interbrood duration, mean brood size, and number of broods). Given the ubiquity of Chaoborus in softwater boreal lakes, our results show the need to consider their presence when assessing possible effects of low [Ca] on zooplankton assemblages.