Bioaccumulation of Silver 110m, Cobalt 60, cesium 137 and manganese 54 by the freshwater algae Scenedesmus obliquus and cyclotella meneghiana and by the suspended matter collected during a summer bloom event.

C. ADAM, J. GARNIER-LAPLACE,
Limnol oceanogr, 48(6), 2303-2313

Laboratory experiments were done to assess Cs137, Mn54, Co60 and Ag 110m uptake by two phytoplankton species, the chlorophyte Scenedesmus obliquus and the small diatom cyclotella meneghiana. Co and Mn were characterized by similar uptake kinetic rates, 23-30 d-1, whatever the algal species, whereas depuration rates were 3-60 d-1. Silver uptake and depuration rates were very high (144-293 d-1). However, Cs accumulation and depuration were very slow, with kinetic constants of 0.6-5 d-1. Mn, Co and Ag were more strongly accumulated by cyclotella meneghiana than Scenedesmus obliquus and vice versa for Cs. To evaluate the extrapolation of the kinetic rates fitted for S. obliqqus and C. menghiana to natural conditions, suspended solids were also collected during a bloom event and contaminated. Radionuclide exchanges between 3 distinct compartments among the suspended solid was modeled : the kinetic rates fitted for S obliquus ans C menghiana were used to represent the chlorophyte and bacillariophyte contamination, whereas kinetic rates describing the third compartment were estimated when possible.A third compartment was evidenced only for Mn and Co , whereas for Ag , the chlorophyte and bacillariophyte were sufficient to describe the particulate phase. For Cs, algae kinetic rates could not be used, so only a single compartment was fitted. these experiments confirm the low affinity of Cs for phytoplankton and the high bioavailability for Ag. In the case of Co and Mn several processes acting simultaneously govern the contamination of natural suspended solids.