Calcium influence on phosphorus regulation in Daphnia magna: implications for phosphorus cycling

We examined the influence of environmental calcium on phosphorus biokinetics and regulation in Daphnia magna. When the Ca concentration in culturing media increased from 0.5 to 200 mg l^-1, the specific P content of D. magna feeding on P-sufficient algae decreased significantly from 1.43 to 1.05% of dry weight. There was a significant negative relationship between the specific Ca and P contents (% dry wt) of daphnids. However, measured biokinetic parameters, including the assimilation efficiency (AE) of dietary P, the weight-specific ingestion rate (IR), the uptake of inorganic P from water, and the efflux rate constant (ke) of P, were similar over the wide range of Ca concentrations, with some exceptions at the lowest Ca level (0.5 mg l^-1). Diet was the dominant source of P for daphnids, and only 1 to 2% of P was incorporated from water, based on the biokinetic calculation. With the elevation of the Ca level from 0.5 to 200 mg l^-1, the proportion of P lost through molting by D. magna feeding on P-sufficient diets increased 2.5-fold, whereas the dissolved release decreased 1.9-fold. P released into the dissolved phase is readily re-utilized by primary producers, while P contained within the shed molts sinks out of the epilimnion and becomes at least temporarily unavailable to primary producers; thus, the effects of Ca on molting and dissolved release have significant implications for P cycling in freshwater lakes. Our results suggest that the availability of P to primary producers can be reduced by sedimentation of P-rich daphnid molts in high-Ca lakes, and Ca concentration in the water needs to be considered in studying the stoichiometric regulation of P in freshwater cladocerans. © Inter-Research 2009.