The aim of this study was to show the effects of hydrometric level variation on phytoplankton structure of the Paranapanema River and a marginal lake in its mouth zone into Jurumirim Reservoir (SP). The samplings were collected in two stations in each environment from July 2004 to July 2005. The highest species richness and diversity of phytoplankton species were found in dry season, while them highest density and biovolume were recorded during the high water period The specie Cryptomonas brasiliensis Castro, C. Bicudo & D. Bicudo (R-strategist) was constant along whole the study, being dominant, mainly, in the end of the drought and in the inundation, when the events of disturbances were more frequent. Diatoms predominated in biovolume in relation to other algae, mainly, for Aulacoseira granulata (Ehrenberg) Simonsem and their varieties. The great water volume stored in Jurumirim Reservoir acts as a "plug system" and the flood pulses of the main tributary are relatively weak. Thus, the stable water level is not a disturbance event that results in high increase of diversity in the studied environments, after the inundation.
phytoplankton; hydrologic pulse; temporal variation
