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/l994) in the Baia River, a tributary of the right bank of the Parana River (22 degrees 43' S and 53 degrees 17' W). We analyzed temporal changes in the phytoplankton community in relation to density, biomass, richness and species diversity, equitability, size Structure, and dominance. We related these to regional climate and hydrology, and to the physical and chemical variables of the water column. We determined 119 taxa, wich were numerically dominated by the class Chlorophyceae with 37 taxa. The classes Cyanophyceae and Bacillariophyceae contributed the most abundant biomass and were responsible for the two peaks that were observed. The high water period was generally characterized by lower phytoplankton biomass, higher richness and species diversity, and higher density of nannoplanktonic species such as Monoraphidium tortile in March, and Cryptomonas brasiliensis in January. During low water, however, the highest values of phytoplankton biomass were recorded, with heterocytous cyanophytes dominating during the phase of greater stability of the water column, and filamentous diatoms during periods of mixing turbulence. The fluctuations observed indicate that the hydrosedimentological regime of the Parand River, together with the climatological factors, constitute the main forcing functions acting on the Baia River phytoplankton.