This paper presents the first study of the benthic invertebrate assemblages of the upper section of the Paraguay River, a major tributary to the Pantanal wetland in Brazil. Thirty-eight sites were sampled along a 200 km section below the city of Cáceres in November 2000. Sixty-nine species and morphospecies were identified, which were dominated by Oligochaeta and Chironomidae. Mean density of benthic invertebrates varied between 72 and 10 354 m-2 in the meandering sector of the river, 3611-49 629 m-2 in the straight-transitional sectors, 682-5962 m-2 in the floodplain lakes, and 1704-2208 m-2 in floodplain channels. Highest densities were attained in sand-gravel sediments dominated by the psammophilous oligochaete Narapa bonettoi. The Shannon diversity index ranged from 0.75 to 2.08 and was highest in floodplain lakes. Statistical analysis (UPGMA and CCA) revealed that benthic assemblages in the floodplain habitats were clearly distinct from the riverine habitats. In the river channel, the habitats were distinguished by grain size while the floodplain habitats were mostly determined by current and silt-clay concentration (floodplain channels) or by organic matter concentration (floodplain lakes). Conservation efforts in the Upper Paraguay area should aim to maintain the flood pulse as a permanent source of spatial and temporal habitat heterogeneit

This study synthesizes the results from ten years (1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988) of research in lotic environments of the middle Parana River . We elucidate the important environmental factors that determine differences in quality and structure of the benthos among the main channel, secondary channels and tributaries, and we determine species or descriptive associations of the different types of environment .The results of a principal components analysis reveal a transverse gradient from the main channel through the secondary channels of progressively decreasing size. The gradient shows decreasing depth, current velocity, discharge, sand, oxygen, and density of the benthos, and increasing transparency, pH, conductivity, temperature, silt, clay, sediment, organic matter and species richness and diversity of the benthos .The centre of the river channels was found to be the zone of greatest descriptive and representative value for the characterization of the benthos .In relation to the transverse gradient, the following succession of benthic fauna associations was observed in the center of channels from the main channel to the anabranches : 1 -Narapa bonettoi + Haplotaxis aedeochaeta + Microstomum? sp . 2 -N. bonettoi + H. aedeochaeta + Microstomum? sp . + Parachironomus sp . 3 -Parachironomus sp . + Xenochironomus sp . + Cryptochironomus sp . + Coelotanypus sp . + Limnodrilus hoffmeisteri + Paranadrilus descolei + Campsurus sp . + Pisidium sp .

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Hydraulic and substratum conditions have been identified as two stream features which affect the benthic community composition, abundance and distribution. However, little attention has been given to the influence of hydraulic variables in large river beds. The aim of this study was to analyse the incidence of the near-bottom hydraulic conditions and other environmental variables on the density of Narapa bonettoi (Oligochaeta, Narapidae), a typical and dominant species found in sandy bed rivers of diverse hierarchy of the Paraná River basin. A large amount of existing and available benthic data were used. The highest correlations among N. bonettoi densities and environmental variables were obtained with the hydraulic variables of friction Reynolds number (RÃ) and shear stress (t 0 ); the substratum type (sand, silt and clay) and organic matter content. The results show that N. bonettoi density would be related with the hydraulic variables following a 'bell-shaped' tendency, e.g. with the friction Reynolds number, N. bonettoi would have a preference for transition values of turbulence ($40 < RÃ < $50), which is equivalent to t 0 between $0.6 and $0.8 kg m À2 . Densities show a slight decreasing tendency toward the maximum RÃ values, suggesting that the species does not support a completely developed turbulence (RÃ > $70). Significant but negative correlations were reported in the literature between benthic macroinvertebrates typical of smaller and steeper streams and RÃ values far beyond the threshold 70 value. N. bonettoi shows a clear preference for a completely sandy substratum and for sand particles sizes around 300 mm. Finally, considering the important role played by the hydraulic variables on the benthic community studied herein, the contagious distribution pattern of N. bonettoi, reported by other authors, would be principally linked with the interactions between current and sediment near the bottom rather than with the mobility of these organisms.

-The golden mussel Limnoperna fortunei was introduced into Argentina in 1991 and has since been spreading through the Plata and Guaı´ba basins at a rate of 240 km.year x1 . To assess their ability to invade the Andean tributaries of the Plata Basin (Pilcomayo, Bermejo and Salado del Norte rivers), their current range was assessed and related to the abiotic parameters of sites where they were present and absent. These data were then compared with their known tolerances to identify possible barriers to invasion. Outputs suggested that three environmental parameters are barriers to invasion: salinity and river flow intermittence in different sectors of the rivers Pilcomayo and Salado del Norte, and concentration of suspended sediments in the Bermejo River and in the upper reaches of the Salado del Norte and Pilcomayo rivers. The importance of these findings is discussed in relation to using environmental data to infer the invasion possibilities and the utility of environmental data to better understand invasion patterns and processes.

[1] This study aims to relate the flow structure over mobile dunes recorded on the channel bed of the Paraná River (Argentina) with the spatial distribution of the benthic macroinvertebrates. The following main conclusions have been obtained: (1) the dunes found in the active channel could be considered as hydraulic biotopes at a mesohabitat scale: the bed forms in the thalweg region are subjected to higher shear stresses with low benthic densities; (2) differences in benthic densities were also recorded at within-dunes microhabitat scales: the largest densities were found in the dune troughs where small bed shear stresses occur and minimum densities on the low stoss side of dunes where turbulent agitation near the bottom strongly disturb the bed particles; (3) superimposed dunes on larger dunes may be considered as another microhabitat of still smaller dimensions. Summarizing, multiscale approaches are needed if a comprehensive understanding linking hydrodynamics and morphodynamics processes with benthic ecology is intended.

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