The population dynamics and life cycle of two mollusc species Pisidium amnicum and Valvata piscinalis of the Saône river were studied for nine years. The newborn of the former species appeared in April each year. The latter species produced two cohorts per year until 2003. Spawning in spring led to the birth of the early cohort, which generally appeared in May, while spawning in the summer led to the birth of the late cohort, which appeared from July to November. The increase of the mean annual

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... temperature by 1.9°C from 1998 to 2000 was followed by an increase in the density of mollusc populations, possibly triggering or amplifying inter and intra specific competition for food. However, the 2003 heatwave was responsible for the sudden decline of the P. amnicum population, then the more progressive decline of that of V. piscinalis after having strongly modified its demographic structure and its life cycle. In 2004 and 2005 V. piscinalis only produced one cohort per year. The observations carried out strongly suggest that the insufficiency of available resources was one of the possible causes of their failure to recover in 2004 and 2005. The increase of global warming predicted by different models could result in the gradual disappearance of P. amnicum from the potamon and of V. piscinalis from the freshwater systems most directly exposed to climatic warming, such as the Saône River.