THE ISOLATION OF RIO GRANDE FROM BILLINGS RESERVOIR, SÃO PAULO, BRAZIL: EFFECTS ON THE PHYTOPLANKTON

Abstract

The Rio Grande Reservoir belongs to the Billings System, which detains the highest water volume stored in the metropolitan area of the city of São Paulo. In 1981, a dam was built to isolate it from Billings Reservoir in order to improve the water supply of some cities in São Paulo State. The effects of the isolation on the phytoplankton were evaluated through the relationships with environmental variables. Phytoplankton and environmental variables were studied from March 1985 to March 1986 at Stations 1, 2 and 3, located respectively at 1, 6 and 10 km from the dam, in the depths correspondents to 100; 25; 1 and 0% of transparency to the Secchi Disc. Among the 45 identified genera, 44% were green algae, 27% flagellated, 16% blue-green and 13% diatoms. Previous studies showed dominance of Cyanophyceae/Cyanobacteria and Chlorophyceae, whereas, after the isolation, Chlorophyceae and Bacillariophyceae were dominant, indicating water quality improvement. However, in August-1985 (winter) a bloom of Asterionella formosa (dominance = 96%) inversely correlated with the water Si content (Rs = í  0.51 ***), reported at 1% of transparency depth in Station 1, was registered just when a smell of benzene hexachloride caused problems in the drinking water. Other bloom, of Staurastrum species (> 80%) was registered in January-1986 (summer) at 25% of transparency, in Stations 2 and 3. The results indicate that the improvement attained was still fragile and could have been easily reversed, allowing problems to the supply, triggered even by climatic events.