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

Ashforth, Dawn, and Norman D. Yan

Limnol. Oceanogr., 53(2), 2008, 420-432 | DOI: 10.4319/lo.2008.53.2.0420

ABSTRACT: 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 mg chlorophyll a L-1 and low food: 3 mg Chl a L-1), and four temperatures (20ºC, 24ºC, 28ºC, and 32ºC) 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 32ºC was so great that daphniids lived no longer than 5 d and did not reproduce. Hence, r was undefined at 32ºC. 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.

Article Links

Please Note