Limnological studies in the Prado Reservoir have shown variations in physicochemical variables at different depths. Ducharme (1975) found that physicochemical measurements at the surface, 1, 3-8, and 45 m showed an evident thermocline between 3 and 8 m (mean T — 27.3 °C). SODEIC (1993) also evaluated various physicochemical variables at different depths, but only two stations were in the same areas as our data were taken, making them quantitatively incomparable. In the present study, only physicochemical variables at the surface (from 0 to lm) were analyzed.