Calcium content of crustacean zooplankton and its potential role in species distribution

SUMMARY 1. The occurrence and species composition of crustacean zooplankton in Norwegian lakes was related to ambient Ca concentrations, pH, total phosphorus and chlorophyll a, and to the presence/absence of other zooplankters and a planktivorous fish (roach: Rutilus rutilus). 2. Ambient Ca concentrations appeared to influence the distribution of Daphnia species: Daphnia longispina often occurred in Ca‐rich lakes with low fish‐predation pressure while the smaller Daphnia cristata often occurred in opposite conditions. 3. Body Ca contents were measured in zooplankters from a wide range of localities, to examine Ca requirements and thus the potential for Ca‐limitation of common species. 4. All Daphnia species had relatively high specific Ca contents [ranging from 0.8 to 4.4% Ca dry weight (DW)−1] compared with other cladocerans and also copepods (ranging from 0.1 to 0.4% Ca DW−1). Within the Daphnia genus, the specific Ca content increased with increasing body size of the species, and thus the large‐bodied species had especially high Ca demands. 5. Because of their high Ca demands, species of Daphnia could be competitively disadvantaged in softwater lakes relative to less Ca‐demanding species.