Seasonal variation in the diversity and species richness of phytoplankton in a tropical eutrophic reservoir

A one year study was carried out in a tropical eutrophic reservoir (Pampulha Reservoir, Brazil). Species richness and diversity (Shannon-Wiener, H') were measured at two week intervals at one collection site. Indices were calculated on an individual, cell and biomass basis. Because of the relative importance of colonial species in this reservoir, calculations based on biomass were the best descriptors for the studied environment. Climatic changes, especially rainfall, were important to explain the seasonal variation observed, acting as a possible disturbing factor and improving diversity. The calculation of diversity indices was important to demonstrate the instability of this system, which for most of the year does not allow the dominance of a single species, and showed high H' values.