Calcium limitation in Daphnia magna

The role of ambient calcium concentrations on survival, moulting, growth an d egg production was assessed in the cladoceran Daphnia magna. A threshold for survival was found in the range 0.1-0.5 mg Ca l(-1), even when ionic st rength of the medium was kept constant. Accumulated length and length speci fic dry weight was retarded at low Ca (0.5-1.0 mg Ca l(-1)) at food concent rations above incipient limiting level. For lower food levels, the effect o f Ca on growth was less clear. The effect of low Ca on growth rate was most manifest during the first days after hatching, reflecting the higher Ca de mands of the early juveniles. Age specific egg production was strongly redu ced at Ca concentrations <10 mg Ca l(-1). This was partly an indirect effec t of reduced growth rates, but could also be an effect of higher energetic costs associated with Ca uptake in a Ca-poor medium. The high Ca demands in D.magna may not be representative of other Daphnia species. Nevertheless, high specific Ca content seems to be a common property of Daphnia spp. and Ca deficiency could be a major determinant of species success and community structure among crustacean zooplankton; it also puts constraints on carbon sequestration in the pelagic food web of softwater lakes.