The importance of food as radionuclide source for the crustacean Daphnia magna was investigated using a planktonic food chain composed of young pre-adult daphnids and two algal species (Scenedesmus obliquus and Cyclotella meneghiana). Daphnids placed in a tank containing natural 0.45 mum filtered water were fed on algae previously kept during 4 days in natural water contaminated by (110)mAg, Co-60, Cs-137 and Mn-54. After about one week of exposure, daphnids were placed in non-contaminated water on a diet of non-labelled algae, in order to monitor radionuclide release. The results suggest that the Trophic Transfer Factor (TTF) of radionuclides in daphnids was generally greater for the transfer via Scenedesmus than via Cyclotella and that it could be linked to the intracellular fraction of accumulated radionuclides and consequently to their biochemical behaviour. For the radionuclide transfer via Cyclotella meneghiana, the biological periods ranged, for the first compartment, from 7 to 30 min and for the second, from 10 h to 1.8 d. As regards the transfer via the green algae Scenedesmus obliquus the biological half-lives were longer, since Tb-1 characterizing the first compartment, ranged from 11 min to 5.2 h, whereas Tb-2 ranged from 1.2 to 2.1 d. From an operational point of view, this paper underlines the importance of considering the food contamination in the models of radionuclide transfer through trophic chains, in order to widen their applications in different seasons or ecosystems.
