Inventaire des mollusques d’une rivière franc-comtoise l’Ognon, déclin des populations de bivalves autochtones (Unionidae et Sphaeriidae) entre 1977 et 2007

The mollusc fauna of the Ognon river, one of the main tributaries of the Sa ne, surveyed for the first time in 1977, was re-sampled in 2007 to assess long-term changes. Overall, the total species richness remained stable during the period (40 vs 39 species) although there was considerable species turnover: 8 disappearances offset by 7 appearances including that of the invasive Corbicula fluminea which accounted for 30.4% of the total number of individuals in 2007. However, we observed radical changes in the structure of mollusc communities between years which mainly occurred in downstream area. The chronology of the changes observed strongly suggest that competition for food following the proliferation of C. fluminea and the 2003 heatwave may be, responsible for the dramatic decline of native bivalves (Unionidae and Sphaeriidae, respectively). Given the magnitude of these changes, a return to a state prior to the disturbance of the malacological structure is hardly conceivable. However, the evolution towards a new relatively stable state is an alternative hypothesis.