991.
The role of ambient calcium concentrations on survival, moulting,growth and egg production was assessed in the cladoceran
Daphniamagna. A threshold for survival was found in the range 0.10.5mg Ca l
1, even when ionic strength of the medium waskept constant. Accumulated length and length specific dry weightwas retarded at low Ca (0.51.0 mg Ca l
1) at foodconcentrations above incipient limiting level. For lower foodlevels, the effect of Ca on growth was less clear. The effectof low Ca on growth rate was most manifest during the firstdays after hatching, reflecting the higher Ca demands of theearly juveniles. Age-specific egg production was strongly reducedat Ca concentrations <10 mg Ca l
1. This was partlyan indirect effect of reduced growth rates, but could also bean effect of higher energetic costs associated with Ca uptakein a Ca-poor medium. The high Ca demands in
D.magna may notbe representative of other
Daphnia species. Nevertheless, highspecific Ca content seems to be a common property of
Daphniaspp. and Ca deficiency could be a major determinant of speciessuccess and community structure among crustacean zooplankton;it also puts constraints on carbon sequestration in the pelagicfood web of softwater lakes.
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