The interactive effects of calcium concentration and temperature on the survival and reproduction of Daphnia pulex at high and low food concentrations

We reared Daphnia pulex in a fully defined medium at seven calcium (Ca) concentrations (0.1, 0.5, 1, 1.5, 2, 5, and 10 mg Ca L-1), two mixed algal densities ( high food: 30 mu g chlorophyll a L-1 and low food: 3 mu g Chl a L-1), and four temperatures (20 degrees C, 24 degrees C, 28 degrees C, and 32 degrees C) in a fully factorial design. The minimum Ca concentration required for the survival and reproduction of D. pulex was between 0.1 and 0.5 mg Ca L-1. Daphniids reared at 0.1 mg Ca L-1 lived no longer than 10 d and did not reproduce. Although offspring were produced at 0.5 mg Ca L-1 and above, reproduction was reduced below 1.5 mg Ca L-1 due to delays in maturity and reductions in both the brood size and number of broods produced within the 15 d of the experiment. As a result, the intrinsic rate of natural increase, r, decreased dramatically between 1.5 and 1 mg Ca L-1, and was undefined at 0.1 mg Ca L-1. Higher temperatures and reduced algal biomass enhanced the susceptibility of D. pulex to low Ca concentrations by raising the reproductive threshold to 1.5 mg Ca L-1. Thermal stress at 32 degrees C was so great that daphniids lived no longer than 5 d and did not reproduce. Hence, r was undefined at 32 degrees C. In order to sustain D. pulex populations and potentially other Ca-sensitive daphniids, fresh waters must maintain a Ca concentration of at least 0.5 mg Ca L-1, although concentrations as high as 1.5 mg Ca L-1 may be required for daphniids to withstand the concurrent reductions in algal biomass and rising water temperatures that are now commonplace on the south-central Canadian Shield.