Factors Influencing Radiostrontium Accumulation in Daphnia

Abstract

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 :

The effect of calcium concentration on the calcification of Daphnia magna

“…The latter is supported by Hessen et al (in press) who showed that the negative effect of low Ca concentrations on growth and weight gain in D. magna was most pronounced at the juvenile stages and at high food concentrations. Porcella et al (1969) reported that the specific Ca content of D. magna (as % Ca of DW) decrease as the animals grow larger, and this is supported by the present study (Fig. 3).…”

“…The present study suggested that the carapace-specific Ca content decrease with increasing individual size, as also observed by Porcella et al (1969). The main reason for this is probably that the surface : volume ratio decrease with increasing size.…”

“…This is in agreement with our previous observation that the intermolt Ca content was not significantly different from the amount of postmolt Ca accumulation. This sloppiness in the Ca metabolism supports Porcella et al (1969), who found that at least 85% of the total Ca in D. magna was lost during molting.…”

“…Most of the calcification in D. magna was completed during the first 48 h after molting, in line with Porcella et al (1969). The individuals reared in media with low Ca concentrations (0.013 and 0.025 mM) were unable to compensate for the lower Ca 2ϩ uptake rate in early postmolt by prolonging the period of Ca uptake.…”

“…(Lampert and Muck 1985), thus securing survival while not strongly encouraging reproduction. The production of eggs or neonates could constitute a problem in calculating adult calcification, as the specific Ca content of D. magna is higher in neonates than in adults (Porcella et al 1969).…”