Temporal changes in the phytoplankton community structure in a tropical and eutrophic reservoir (Barra Bonita, SP-Brazil)

A study of the temporal changes in the phytoplankton community structure in the eutrophic Barra Bonita Reservoir (Sao Paulo State, Brazil) was carried out during 30 days (samples were taken daily for 10 days and, thereafter, on every other day) in two distinct seasonal periods (dry and rainy seasons). Monthly sampling occurred in the period July 1993 to June 1994. One hundred and thirty-one taxa were registered. A greater diversity (112 taxa) was observed in winter than in summer (79 taxa). In the monthly analysis, 90 taxa were identified. The highest values for species diversity and richness were observed in July 1993 and the lowest in April 1994. The phytoplankton dynamics in this system are apparently dominated by competitive exclusion and disturbance. Within theseasonal cycle, the phytoplankton community structure was determined by underwater light availability, mixing by wind, precipitation and the consequent loss of Cyanophyceae biomass in the upper waters by spillage, and nutrient (principally phosphorus) inputs from anthropogenic sources. The environmental variability in this polymictic system may be favouring a phytoplankton community in equilibrium, with dominance alterations of Bacillariophyceae biomass (Aulacoseira granulata), R-strategists, and Cyanophyceae biomass (Microcystis aeruginosa), S-strategists. Short periods (i.e. 10 days) in non-equilibrium conditions with increased species diversity and coexistence can perhaps be explained by the intermediate disturbance hypothesis.