(Autodepuration of Rio Grande í Rio Grande í SP Brazil). Major ions and chlorophyll a)

Abstract

This paper is part of a research on the depuration process of Rio Grande Reservoir (23°46'S and 46°38'W, altitude 746.5 m), aiming the determination of the physiographic and anthropic influences in this process. The results of monthly samplings of the water column (October 1982 to September 1983) at 3 sampling stations showed the cationic and anionic proportions Na > Ca > Mg > K and Cl > HC0₃ > S0₄ (meq/L), which were also previously found in 1979. Changes in the cation concentrations led to small changes in the M/D, D/D and M/M relations (M: monovalent, D: divalent). The chlorophyll a content showed a considerable reduction, for the median (4.91 µg/L) was approximately 10 times lower than that obtained in 1979. The median values for each station were quite similar. The reduction of the chlorophyll a content could be a result of the effect of the intensive treatment with algicides which the reservoir periodically receives. The salinity suffered a dilution along the major axis of the reservoir (from the spring to the dam), showing changes if compared to previous studies (1979).