COMPARATIVE ECOTOXICITY OF SUSPENDED SEDIMENT IN THE LOWER RHONE RIVER USING ALGAL FRACTIONATION, MICROTOX(R) AND DAPHNIA-MAGNA BIOASSAYS

The toxicity associated with suspended sediments from the Rhone River (Switzerland-France) was determined with three acute bioassays. Large volume water samples were centrifuged for recovery of suspended solids in November 1989, one sample was taken as a control upstream from Lak e Geneva and the 9 remainder downstream from Geneva to the Mediterrane an Sea, with a single sample of the major tributary the Saone at Lyon. Heavy metals (Hg, Cd, Cr, Cu, Ni, Pb, Zn) and organic contaminants (O Cs, PCBs, PAHs) bound to sediment were analysed and extracted by elutr iation with filtered lake water and by organic solvent (dichloromethan e). Sediment water elutriates were tested with algal fractionation bio assays (AFB) using Lake Geneva ambient phytoplankton, with Daphnia mag na and Microtox(R) acute toxicity tests, whereas organic extracts were utilized in the latter two bioassays to evaluate the potential sedime nt toxicity. The bulk analyses of the sediment together with elutriate metal concentrations indicated the highest contamination of sediment downstream of Lyon. Medium contamination appeared for the stations dow nstream of Geneva, in the Saone River and at the Rhone River mouth. Th e station upstream of Lyon had low concentrations, comparable to the v alues in the Upper Rhone. Organic contaminants are mainly observed dow nstream of Lyon and their concentrations decline onwards to the sea. T he bioassays Microtox(R) on organic extracts and AFB on the elutriates show a similar toxicity trend, but differ in that Microtox was more s ensitive to organics whereas the algal test responded predominantly to metals. This difference is believed to be due to the different extrac tion procedures used, rather than to the tests themselves. Daphnia mag na was the least sensitive and appeared to give a broader band respons e to the observed contaminants in the sediment. The bioassay results w hen integrated confirm that the biotoxicity trends relate well to the composition of the sediment, a factor which emphasizes the need for ba ttery testing in ecotoxicological assessment.