Suspended sediments were recovered in 1988 and 1989 by continuous flow centrifugation of water samples at the mouths of the Rhone, Venoge, Dranse and Aubonne rivers. ln addition the effluent of the sewage treatment plant {STP) at Lausanne-Vidy was sampled in each of the two years (fig. 4.1 ). The performance of the system using two centrifuges in parallel (a Westphalia and an Alfa-Laval) has been assessed (Santiago & al. 1989). Both centrifuges show good agreement for particle size and trace metal concentrations in suspended solids, providing the sediment trapped in the brass core of the Alfa-Laval is discarded.

Results indicate that primary production can be promoted in the short term by the suspended sediments, and that the bioassay is particulate specific and well adapted to determine the bioavailable proportion of the particulate phosphorus. Conditions of incubation (temperature, illumination, turbidity effect, length) and methodological procedures (filtration, DMSO extraction, liquid scintillation counting) have been assessed (figs. 4.5, 4.6, tabs. 4.1, 4.2). Addition of soluble nutrients, either in filtered river water or as a pure solution of orthophosphate, did not enhance growth before 15 hours of incubation (fig. 4.7). This lag is believed to be necessary to allow algal adaptation.