This work was carried out at the Americana Reservoir, owned by Companhia Paulista de Força e Luz, and was part of a joint R&D project with Faculdade de Ciências Agronômicas - Botucatu - São Paulo - Brazil. Water sampling was collected in February, April, June and October 2004. The following characteristics were analyzed: water temperature, pH, dissolved oxygen, conductivity, total nitrogen, nitrite, nitrate, ammonia, total phosphorus, inorganic phosphate, as well as qualitative and quantitative analysis of phytoplankton and toxicity. The reservoir showed high levels of total phosphorus, ranging from 18 to 509 µg L-1; phosphate, 4 to 463 µg L-1; total nitrogen, 0.99 to 17.25 mg L-1; nitrate, 0.26 to 15.29 mg L-1. For the phytoplankton community, 103 taxa were found during the sampled period, with the greatest richness being found at point P06 and the poorest at the center of the reservoir (P02, P03, P04 and P05). The highest concentration of cyanophyceae was observed on April 2004, 5.375.175 un. L-1. The species presenting higher densities were Microcystis aeruginosa, Anabaena spiroides, Microcystis sp and Pseudoanabaena mucicola, with Anabaena spiroides presenting the highest density of all, 4.178.084 un. L-1. In July and October, the Cryptophyceae class made a large contribution to total density. Despite the great density of Cyanobacteria, toxicity values were below the limit allowed by Ruling number 1.469.
phytoplankton; microcistyn; water quality; reservoir
