Eight years of monitoring aquatic Oligochaeta from the Baía and Ivinhema Rivers.

The aim of this study was to analyze the factors that influence spatial and temporal variations of the Oligochaeta assemblage in the Baía and Ivinhema Rivers, located in the Upper Paraná River floodplain (Mato Grosso do Sul State, Brazil). Samples of Oligochaeta were collected between February 2000 and November 2007. A total of 27 Oligochaeta species were identified. A Principal Component Analysis (PCA) of the physical and chemical variables was used to summarize the total variation in the data and to identify major environmental gradients. Detrended Correspondence Analysis (DCA) was conducted to verify possible spatial and temporal gradients in the Oligochaeta species distribution. The highest Oligochaeta species densities and richness values were recorded during limnophases. The intensity and amplitude of the potamophase influenced the density, richness and composition of Oligochaeta since many species were transported by high water current velocities and/or died due to the low oxygen levels that are characteristic of this phase. L. hoffmeisteri, P. descolei and A. pigueti were recorded in the potamophase and in the limnophase and we concluded that they are adapted to different conditions of rivers (lotic and lentic) due to the presence of these species during the entire study period in both rivers. Nevertheless, these species were dominant in the potamophase periods of these rivers as they have body adaptations to survive in these conditions.