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 eutrophication and has provided a uniform set of phytoplankton data. This study aimed to define the mean annual pattern of the seasonal successions of phytoplankton species, and to determine whether inter-annual distortions in this seasonal structure occur. We analysed the 25 annual patterns using the STATIS multi-table method. The phytoplankton successions in the first part of the year fitted well with the pattern predicted by the Plankton Ecology Group (PEG) model. But the temporal evolution in the summer phytoplankton community differed from the PEG model, and was subject to between-years differences. We identified three homogeneous periods for the annual 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 second period, and this autumn community developed earlier during the third period. This study reflects the good ability of the phytoplankton community of Lake Geneva to resist both inter-annual fluctuations and brutal shifts in the annual functioning of the system. We believe these shifts resulted from a gradual change in environmental parameters, including the deepening of the phosphorus-depleted layer and a change in its timing as a result of the long-term meteorological changes.