Résumé
Species composition and seasonal succession of some planktonic components were studied through monthly samplings during 2 yr (1997 and 1998) in Ingazeira reservoir, Pernambuco, Brazil. Linked to the severe drought in this region (1997 El Nino event) was the dominance of the toxic filamentous cyanobacterium Cylindrospermopsis raciborskii in the phytoplankton in 1998 (96 to 100% of total phytoplankton biomass), with small proportions of heterocytes (12% of filaments). A great part of the variability of the particulate organic carbon (R super(2) = 83.9%) was explained by changes in the C. raciborskii carbon biomass. A more significant change in bacterial communities was observed in the post-bloom phase when biomass increased due to the appearance of larger size-classes of cell volume. This bacterial size structure may be the consequence of a strong pressure by bacterivores. Among the zooplanktonic groups, rotifers were numerically more abundant throughout the survey, but microcrustaceans, especially the copepods, contributed the highest proportion of the biomass. Despite the low edibility of C. raciborskii (large trichomes; mean of 97 mu m, n = 204), zooplankton diversity increased during and after the bloom (March to December 1998). Our data suggest that rotifers and copepods were able to cut up and shorten the filaments to edible size for other zooplankton species, especially the small-bodied herbivorous cladocerans. Thus, in the studied ecosystem, the heterotrophic micro-organism community appeared to be able to develop a strategy to cope with a dominant and relatively inedible algal food source.