Résumé
Climatic and anthropogenic forcing on the long-term changes in the community of rotifers were investigated during the period 1969–98 in Lake Geneva. The results showed a cascade of links between large-scale climate indices and phosphorus concentrations, acting as main factors, and the long-term changes in the pelagic ecosystem in Lake Geneva. Accordingly, we identified substantial changes in the structure of the rotifer community related to changes of the food web at the study site during the last three decades. Furthermore, we identified an abrupt shift in ∼1987 in the size structure of the rotifer community, which was closely linked with both increase in water temperature and decrease in phosphorus. The observed ecological changes in phytoplankton variability and rotifer size structure are discussed in the framework of modification in the trophic state of Lake Geneva. Overall, these results deserve attention by showing that strong environmental changes in pelagic ecosystems affect not only the abundance of pelagic populations but also their size structure, which has implications for ecological interactions and coupling between low and high trophic levels. The size-based approach we have developed appears complementary to the taxonomic one (i.e. keystone species), and therefore it is a useful tool to assess long-term changes in the functioning of the plankton communities.