Strontium and calcium relationships were measured in populations of Daphnia magna to determine some of the factors which affect Sr accumulation. The Ca and Sr in Daphnia is a function of the size of the Daphnia and the Ca2+ concentration in the water. Although growth and size cannot be considered independently of feeding, the equilibrium concentration of Ca and Sr is not affected by the food chain. Strontium-85 accumulation follows first order reaction kinetics and in populations consisting of mixed sizes, both uptake and release follow first order reaction kinetics. Reaction rates are a function of size in populations and smaller organisms appeared to reach equilibrium before larger organisms. This appears to be due to the fact that Ca and Sr are accumulated mostly in the carapace and hence uptake is a function of the population molting rate. The following model which treats the discrimination factor KOnNr as a constant was compared to the observed data and was found to give reasonable aereement :
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