In fresh waters,calcium(Ca)varies dramatically in concentrations and plays an important role in determining the toxicity of metals,such as Cd.Here,we present several of our studies on the protective effects of Ca against the toxicity of Cd in a freshwater zooplankter Daphnia magna.The radiotracer technique was used to quantify the toxicokinetcis of Cd at different Ca levels,which we consider as a useful tool in better understanding the protective effects of Ca.(1)Ca reduced Cd bioaccumulation in D.magna not only by inhibiting aqueous uptake but also inhibiting dietary assimilation.With increasing ambient Ca level from 0.5 to 200 mg/L,the aqueous uptake rate constant of Cd decreased by nine times; and the dietary assimilation efficiency of Cd decreased from 62 to 19%.In contrast,Ca levels did not significantly affect the efflux rates of Cd.(2)Ca was found to influence Cd toxicity in D.magna simply via affecting bioaccumulation,and not by altering intrinsic sensitivity of the organisms.We conducted toxicity tests in parallel with uptake kinetics experiments of Cd in waters of different Ca concentrations and pH levels using acclimated D.magna.Both the acute toxicity and uptake of Cd were reduced by higher Ca concentration.Strikingly constant median effective influx rates(EJ50,1.3-1.6 μg/g/h)of Cd were observed when the concentration of Ca varied from 0.5 to 200 mg/L,indicating that acclimation to different Ca levels did not affect the intrinsic sensitivity of D.magna to Cd.In contrast,the EJ50 values increased consistently with decreasing acclimation pH level,showing that acclimation to acidic water decreased the intrinsic sensitivity.(3)Cladocerans with different Ca demands showed different sensitivity to Cd toxicity.We investigated the patterns of Cd bioaccumulation and toxicity in four cladocerans with contrasting Ca contents.Ca content and the aqueous Cd uptake rate in the cladocerans were significantly correlated.The high-Ca species had higher Cd uptake rates and were more sensitive to aqueous Cd exposure than the low-Ca species.The large interspecies differences in aqueous Cd toxicity can be explained by the differences in the Cd uptake rates.