Long-term changes in mollusc communities of the Ognon river (France) over a 30-year period

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 number of collected individuals increased by 33.6 % (55.2 % for gastropods and 23.7 % for bivalves) but 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. The use of trend test, similarity index and principal component analyses (PCA) revealed radical changes in mollusc communities between years particularly in the upstream-downstream gradients of species richness, total number of molluscs, and gastropods, bivalves, pulmonates, sphaeriid and unionid mussel densities. The upstream colonisation of several species was observed in the upper part of the river but in the lower part divergences in terms of species richness, density and structure increased in a downstream direction. 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.