Long-term study (1974-1998) of seasonal changes in the phytoplankton in Lake Geneva: a multi-table approach

The monitoring of Lake Geneva began after one decade of eutrophicationand has provided a uniform set of phytoplankton data. This studyaimed to define the mean annual pattern of the seasonal successionsof phytoplankton species, and to determine whether inter-annualdistortions in this seasonal structure occur. We analysed the25 annual patterns using the STATIS multi-table method. Thephytoplankton successions in the first part of the year fittedwell with the pattern predicted by the Plankton Ecology Group(PEG) model. But the temporal evolution in the summer phytoplanktoncommunity differed from the PEG model, and was subject to between-yearsdifferences. We identified three homogeneous periods for theannual patterns: 1974–1985, 1986–1991 (except 1988)and 1992–1996 (including 1988). During the first period,phytoplankton succession followed the reference annual pattern,the typical autumnal community was missing during the secondperiod, and this autumn community developed earlier during thethird period. This study reflects the good ability of the phytoplanktoncommunity of Lake Geneva to resist both inter-annual fluctuationsand brutal shifts in the annual functioning of the system. Webelieve these shifts resulted from a gradual change in environmentalparameters, including the deepening of the phosphorus-depletedlayer and a change in its timing as a result of the long-termmeteorological changes.