Blooms of Cylindrospermopsis raciborskii have become more and more frequent in Brazilian reservoirs because of its high competitiveness in tropical eutrophic systems. This species is able to produce very aggressive toxins which cause problems to public health and environmental hazards. Our objective was to follow up variations of C. raciborskii density and its relation with the environmental parameters. The studied lake is located in the south of São Paulo city (23º39' S and 46º37' W). It is an eutrophic system with frequent blooms of C. raciborskii. Samples were collected weekly in the deepest part of the lake, from September/97 to September/98. Temperature profile, mixing zone, euphotic zone, pH, conductivity, turbidity, dissolved oxygen, alkalinity, nutrients, chlorophyll a, C. raciborskii density and total phytoplanktonic density were analysed. The ordination of abiotic and biotic data was carried out using Principal Component Analysis (PCA) and Canonical Correspondence Analysis (CCA). During November and December/97 the highest values of C. raciborskii densities were observed. From 12/10 to 12/17, the species contributed with more than 50% to total density. In that period the water column was thermally stratified, temperature and turbidity were high and water transparency decreased. The autumn/winter of 1998 was characterized by the absence of the studied species, by unstratified water column, low water level and temperature, high values of transparency (the highest values observed in the lake) and a great increase in biomass of the macrophyte Eichhornia crassipes (Mart.) Solms. All these relations are presently discussed.
Cyanobacteria; Cylindrospermopsis raciborskii; eutrophic reservoir; weekly variation
