Hydrobiologia 367: 139–152, 1998. © 1998 Kluwer Academic Publishers. Printed in Belgium. 139 Survival strategies of some species of the phytoplankton community in the Barra Bonita Reservoir (São Paulo, Brazil) A. C. A. Dos Santos & M. C. Calijuri Center for Water Resources and Applied Ecology, Schoolof Engineering at São Carlos, University of São Paulo, C.P. 359, CP 13560-970, São Carlos, S.P., Brazil Received 21 May 1997; in revised form 15 February 1998; accepted 2 March 1998 Key words: phytoplankton, survival strategies, microcosms, tropical reservoir Abstract The dynamics of the phytoplankton community in the Barra Bonita Reservoir (São Paulo, Brazil) were studied through daily sampling in the field (integrated samples from the euphotic zone) and microcosm experiments, for two short periods: the winter of 1993 (June 30 to July 10) and the summer of 1994 (January 24 to February 2). The goal of the study was to evaluate and compare the variations in the composition of isolated phytoplankton com- munity which occur over short periods of time. Three series were separated into Erlenmeyer flasks for each study period, with samples from the euphotic zone divided into three portions: total, smaller than 64 μm, and smaller than 20 μm. All of the Erlenmeyer flasks were inclubated in situ at the sampling station. The maximum period of incubation was 10 d. Variations of the community in the euphotic zone were characterised by high diversity and a community in a state of non-equilibrium in winter, without the predominance of any species. In the summer, the community presented a low diversity and a state of equilibrium, with the predominance of Microcystis aeruginosa. The microcosm experiments showed that the confinement of the community in the Erlenmeyer flasks, and therefore in isolation from the physical variability of the ecosystem, especially in relation to the mixing patterns, stimulated the return of the community to the initial phases of succession with the predominance of small species and those which grow rapidly (r-selective or C-strategist). Introduction The majority of the research which has considered the biomass and the variation in the composition of phytoplankton species has emphasised that the phys- iological and behavioural attributes of individual or- ganisms or populations influence the variability of the community composition (Sandgren, 1988). Pianka’s (1970) concepts of r- and K-selection have been applied to the phytoplankton community by many authors, including Margalef (1978), Kilham & Kilham (1980), Sommer (1981), Harris (1986), Carney & Goldman (1988), and Arauzo & Cobelas (1994). These studies discuss the alternation of r- and K-selection in the community assembles, propos- ing that the succession of species in the phytoplank- ton community is a predictable replacement of the r species with the K species, from an unstable and mixed environment to a stratified and stable environ- ment. Reynolds (1988a) updated the concepts of survival strategies for phytoplankton, relating them to ideas proposed by Grime (1979). According to Reynolds (op. cit.), phytoplankton can be divided into three groups of organisms with distinct but not necessarily mutual exclusive strategies: the C-strategists (pio- neers), comprise species adapted to rapid reproduction and a superior ability to dominate the environment, as soon as the conditions become sustainable, par- tial exploiting environments saturated with light and nutrients; the R-strategist (runderaes), which are pre- dominant in environments with great vertical mixing and are specialised to tolerate turbulent transporta- tion and light gradients; and finally, the acquisitive