The Guarapiranga and Billings reservoirs are main sources of public water supply to millions of people in the city of São Paulo, Brazil. They have been under intense antropic action as a result of domestic, industrial, farm and livestock waste being dumped in the reservoirs. Cyanobacteria develop very well in such an environment, producing blooms that are most often toxic. Cylindrospermopsis raciborskii(Woloszynska) Seenayya & Subba Raju is a toxic species which is rapidly spreading all over the world and is abundant in the studied reservoirs. The goal of the study was to follow the year-round variation of the C. raciborskii population density and to correlate it with selected environmental factors. Samples were collected monthly on the surface of the water column and studied under a binocular optic microscope, whereas quantitative studies were carried out under an inverted microscope, according to the Utermöhl method. Among the phytoplankton community, organisms of the Cyanobacteria Class were represented by the greatest population density (cells mL-1). Cylindrospermopsis raciborskii was one of the abundant species in the Billings reservoir, both in the dry and rainy season. The principal environmental factors that influenced C. raciborskii population dynamics were water temperature, high pH values and low euphotic zone values
bloom; Cylindrospermopsis raciborskii; eutrophic reservoirs; phytoplankton
