Seasonality of macroalgae in three tropical drainage basins in São Paulo State, southeastern Brazil

Abstract

Three drainage basins belonging to different drainage systems, but in close proximity, were evaluated to compare the seasonal dynamics of macroalgal communities in the northwest region of Sao Paulo State, southeastern Brazil and to evaluate the most influential stream environmental variables. Monthly samplings were carried out from September 1992 through September 1993 in three sampling sites along the main river of each basin. 10 m length cross segments were evaluated for species percent cover and richness, on both the population and community levels. Selected stream variables were measured: specific conductance, temperature, turbidity, mean cross-sectional area of the reach, current velocity, pH, and oxygen saturation (%). Pearson Product-Moment correlation coefficient and multiple regression analysis were applied to evaluate interactions and contributions of the variables on seasonal variation of communities and populations. Results suggested that several factors influenced community dynamics. Significant correlations of species percent cover to oxygen saturation, cross-sectional area, pH, current velocity, specific conductance, turbidity, and precipitation were found, whereas richness was correlated to oxygen saturation, temperature, turbidity, pH, and current velocity. Multiple regression analysis revealed that precipitation and turbidity were the most influential parameters accounting for percent cover and richness variations. Seasonal changes of the cyanophyte Phormidium splendidum, the chlorophyte Stigeoclonium helveticum, the rhodophytes Chantransia stage, and Batrachospermum delicatulum were correlated with different variables. The seasonal community patterns were essentially similar in the three drainage basins, with high values of percentage cover and species number occurring typically during the dry season. However, local characteristics can modify the trend of seasonality. Interactions among some parameters (e.g. turbidity, oxygen saturation, temperature, and current velocity) are important, but precipitation seems to be the source of much of the environmental changes and the key factor responsible for seasonality of macroalgal communities in the basins investigated and, presumably, in tropical lotic systems.