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.

Summary 1.Field studies indicate that low Ca concentrations limit the distribution and success of Ca-demanding freshwater crustaceans in soft-water localities. A long-lasting depletion of Ca caused by reversed acidification could thus be detrimental to such species. 2. Experimental studies of Gammarus lacustris and Astacus astacus revealed decreased survival and retarded growth at ambient Ca concentrations <5 mg l −1, compared with animals reared at 10 mg l −1 . 3. A significant proportion of the total body Ca content was lost during the moult, but moulting frequencies were not affected by ambient Ca concentration. Complete postmoult calcification of the carapace could not be achieved at ambient Ca concentrations <5 mg l −1 , and individuals reared at such conditions had a lower specific Ca content than individuals reared at sufficient Ca levels. 4. A juvenile bottleneck regarding the susceptibility to Ca deficiency was shown in G. lacustris, as neonates had a relatively higher mass-specific Ca content and a higher Ca threshold for survival than adults. 5. Because of a short postmoult calcification period of less than 2 days, G. lacustris had limited ability to compensate for low water Ca levels by obtaining Ca from food. However, food might be an important source of Ca for A. astacus living at low ambient Ca, because eating could be resumed during the relatively long calcification period (>15 days).

1. Zooplankton differ in their elemental contents. For the essential elements calcium (Ca) and phosphorus (P), Daphnia spp. have particularly high contents, and thus high demands, compared with other cladocera and copepods. 
2. There are recent indications that the growth of zooplankton, notably Daphnia, may be directly constrained by the availability of P and Ca. 
3. Major pools of P and Ca are associated with the carapace, and the moulting process represents a constant drain of these elements, as well as carbon (C), from the animals. 
4. The demands for P and Ca for moulting may pose constraints on the carbon accumulation in the animals, and thus the extensive needs for these elements may translate into a total loss of C transported in the pelagic food web. 
5. While the costs of moulting are substantial for the zooplankton, the export of minerals bound in the moult will play only a minor role for the epilimnetic element budgets in most lakes.

The cladocerans Daphnia magna and D. tenebrosa were exposed to daily 6‐h ultraviolet (UV) radiation along a gradient of ambient Ca in the medium (0.5, 1.0, 5.0, and 10 mg Ca L−1) at nearly constant conductivity. Integrated irradiance over 300–400 nm was 35.95 W m−2, corresponding to maximum outdoor intensities during midsummer. D. magna was most susceptible to UV radiation, but the UV susceptibility for both species increased significantly with decreased Ca content. Ca uptake (45Ca) was apparently not influenced by short‐term (3 h) UV exposure; thus, although lipid peroxidation and membrane damage are likely mechanisms for impaired ionic uptake, the overall role of low Ca may be reduced stress tolerance. While general conclusions on other species and taxa are premature, the data indicate that water hardness could be a major determinant to UV susceptibility among Ca‐demanding zooplankton species like Daphnia.

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