Declines in lakewater calcium (Ca) concentrations are occurring in many softwater lakes of the Canadian Shield, and likely elsewhere, and there is growing interest regarding the potential impacts that reduced Ca availability may have on aquatic ecosystems. Here, we test the hypotheses that the Ca limitations of Daphnia pulex (reduced growth/survival when Ca <1.5 mg L-1) identified in laboratory analyses and paleolimnological case studies can be observed among cladoceran assemblages from surface sediments along a narrow Ca gradient (1-3 mg L-1), and also whether the impacts of low Ca concentrations can be distinguished from those of low lakewater pH. Significant differences among daphniid communities were observed across the 1.5 mg L-1 Ca threshold; however, these differences did not extend to the broader cladoceran community and considerable variability in Ca tolerance was observed within the D. pulex species complex. Of the measured environmental variables, only pH and lake depth explained a significant amount of variation among the entire cladoceran community, although when enlarging the dataset to extend the Ca and pH ranges, Ca replaced pH as a significant explanatory variable. Our data adds support to our previous observations that, within the cladoceran community, daphniids are the most responsive to Ca decline, however responses differ within daphniid species complexes and Ca thresholds under natural conditions may be higher for some species than their laboratory-identified values. © 2011 Springer Basel AG.