The purpose of this study was to describe the composition and abundance of rotifers in different environments: one lotie (the Ivinheima River) and another lentic (the Patos Lake) located in the floodplain of the Upper Paraná River. The influence of limnological variables on the structure and dynamics of rotifers was also examined. Samples were taken monthly from March 1992 through February 1993, in the open water region of the lake and river and in the channel between them and, in the littoral region of the lake and river. Of the 96 species identified, the best represented families were: Brachionidae (20 species), Lecanidae (18 species), and Trichocercidae (15 species). 55 species were widely distributed, 13 were pantropical, 16 neotropical (8 endemic to South America). Trichocerca gracilis was a new record for this floodplain. Dipleuchlanis propatula propatula, Lecane curvicornis, L. leontina, L. bulla, Plationus patulus patulus, Platyias quadricornis quadricornis, Testudinella patina, and T. mucronata hauriensis were consistently present. More species were recorded during the high water period, probably because of the interconnection among the different environments of the floodplain that allows fauna exchange and an increase in available habitats. Rotifer densities were higher mainly in the lake, during the high water. Using principal components analysis, was identified four variables that influence the seasonal variation in the rotifer densities: water level, dissolved oxygen, chlorophyll-a concentration, and water temperature.
Rotifera; Paraná River; floodplain; neotropics