TEMPORAL FLUCTUATIONS OF THE PHYTOPLANKTON COMMUNITY OF THE BAIA RIVER, IN THE UPPER PARANA RIVER FLOODPLAIN, MATO-GROSSO DO SUL, BRAZIL

Subsurface samples were taken monthly (March/1993 through February/l99 4) in the Baia River, a tributary of the right bank of the Parana Rive r (22 degrees 43' S and 53 degrees 17' W). We analyzed temporal change s in the phytoplankton community in relation to density, biomass, rich ness and species diversity, equitability, size Structure, and dominanc e. We related these to regional climate and hydrology, and to the phys ical and chemical variables of the water column. We determined 119 tax a, wich were numerically dominated by the class Chlorophyceae with 37 taxa. The classes Cyanophyceae and Bacillariophyceae contributed the m ost abundant biomass and were responsible for the two peaks that were observed. The high water period was generally characterized by lower p hytoplankton biomass, higher richness and species diversity, and highe r density of nannoplanktonic species such as Monoraphidium tortile in March, and Cryptomonas brasiliensis in January. During low water, howe ver, the highest values of phytoplankton biomass were recorded, with h eterocytous cyanophytes dominating during the phase of greater stabili ty of the water column, and filamentous diatoms during periods of mixi ng turbulence. The fluctuations observed indicate that the hydrosedime ntological regime of the Parand River, together with the climatologica l factors, constitute the main forcing functions acting on the Baia Ri ver phytoplankton.