Effects of waterborne uranium on survival, growth, reproduction and physiological processes of the freshwater cladoceran Daphnia magna

Titre de la revue : aquatic toxicology Volume : 86 N° : 3 Pagination : 370-378 Date de publication : 18/02/2008

Acute uranium toxicity (48h-immobilisation test) for Daphnia magna was determined in two different exposure media, differing in pH and alkalinity. LC50 varied strongly between media, from 390±40 µg L-1 U at pH 7 to 7.8±3.2 mg L-1 U at pH 8. According to the Free Ion Activity Model uranium toxicity varies as a function of free uranyl concentration. This assumption was examined by calculating uranium speciation in our water conditions and in those reported in the literature. Predicted changes in free uranyl concentration could not solely explain observed differences in toxicity, which might be due to a competition or a non competitive inhibition of H+ for uranium transport and/or the involvement of other bioavailable chemical species of uranium. Chronic effects of uranium at pH7 on mortality, ingestion and respiration, fecundity and dry mass of females, eggs and neonates were investigated during 21-day exposure experiments. A mortality of 10% was observed at 100 µg L-1 U and EC10 reproduction was 14±7 µg L-1 U. Scope for growth was affected through a reduction in feeding activity and an increase in oxygen consumption at 25 µg L-1 U after 7 days of exposure. This had strong consequences for somatic growth and reproduction, decreasing respectively by 50% and 65% at 50 µg L-1 U after 7 days and at 25 µg L-1 U after 21 days. Uranium bioaccumulation was quantified and associated internal alpha dose rates from 2.1 to 13 µGy h-1 were estimated. Compared to the toxicity of other alpha-emitting radionuclides and stable trace metals, our results confirmed the general assumption that uranium chemical toxicity predominates over its radiotoxicity.