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

Hutchinson showed that there are several characteristic patterns of seasonal changes in phytoplankton assemblages (Hutchinson, 1967). It is now widely recognized that succession patterns vary little from year to year and are usually similar in lakes, having analogous climatic conditions, morphometric characteristics and trophic status (Reynolds, 1984; Harris, 1986). The recurrent character of this ordered succession of seasonal events was shown to be widespread in the 1980s by a workshop of the PEG [Plankton Ecology Group of the International Society of Limnology; (Sommer et al., 1986)]. It showed that the dichotomies recorded between the seasonal changes in phytoplankton in eutrophic and oligotrophic lakes depend upon the physical environment and biotic interactions, as well as on stresses resetting the progress and/or direction of the succession. The phytoplankton biomass of ecosystems receiving large inputs of turbulent kinetic energy, such as large lakes or oceanic basins, is strongly influenced by meteorological events (Mitchell and Holm-Hansen, 1991; Mitchell et al., 1991). The seasonal successions may be a better predictor of long-term environmental changes in these systems than the more usual descriptors of biomass and productivity indices (Moline and Prézelin, 1996). An initial study of plankton successions in Lake Geneva, based on a biannual cycle (1984–1985), showed that successions in Lake Geneva were globally well described by the PEG model, despite year to year variations and some minor differences (Gawler et al., 1988). The present paper re-examines the reproducibility of plankton annual successions in this lake over a much longer time (25 years). Lake Geneva has been monitored since 1957 as part of the international programme of water protection lead by the Commission Internationale