Phytoplankton taxonomic composition and temporal changes in a tropical reservoir

Phytoplankton assemblages of the Barra Bonita Reservoir (Sao Paulo State, Brazil) were studied monthly between April 2002 and January 2004. Samples were taken at six depths (0 m, 1 m, 3 m, 5 m, 10 m and bottom) at a single station located 3 km upstream of the reservoir dam. The vertical distribution of environmental and biological variables was homogeneous (p > 0.05) during most of the period, possibly due to wind action and the rainfall regimes. Significant variability was found only on the temporal scale (p < 0.05). Generally, the highest values of species richness and diversity were recorded during the dry period. Taxonomic analysis of the phytoplankton assemblages revealed 139 distributed over nine groups: Chlorophyceae (44), Bacillariophyceae (40), Cyanophyceae (28), Conjugatophyceae (10), Euglenophyceae (8), Xanthophyceae (5), Chrysophyceae (2), Dinophyceae (1) and Cryptophyceae (1). The relative abundance of Cyanophyceae (colonies of Microcystis aeruginosa and free cells of Microcystis sp.) and Bacillariophyceae (Aulacoseira granulata filaments) was notably greater than that of other groups during dry and rainy seasons. The dynamics of phytoplankton assemblages in Barra Bonita Reservoir was mainly driven by physical factors, particularly vertical mixing and residence time. The coexistence of R strategists (diatoms) and S strategists (cyanobacteria) recorded in the first year, and the continuous dominance of cyanobacteria in the second year revealed a pattern of inter-annual variability mainly driven by changes in z mix /z eu ratios and continued eutrophication.