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We summarize the information on the phytoplankton of the Paraná River in the Argentine stretch, comparing the density, biomass, functional groups and diversity studies of the mainstem and fl oodplain since the 70's. We described the Yacyretá Reservoir and its impact downriver since it started operating in 1994. The spatio-temporal changes of the phytoplankton are associated to the hydrosedimentological regime. Regular variations in the hydrological phases and shorter permanent fl uctuations are essential for phytoplankton maintenance, since they allow the lateral exchange with zones of higher algal accumulation in the main channel and with the fl oodplain. During ordinary hydrological cycles, the mainstem shows a typical potamoplankton, dominated by Aulacoseira granulata and, alternatively, with sub-dominance of Chlorococcales, Cryptophyceae at the Middle and Lower Stretch, Cyanobacteria abound at Yacyretá Reservoir and downriver at the High and the beginning of the Middle Stretch. The disruption of Yacyretá Reservoir produces profound changes in the High Paraná phytoplankton evidenced in increased density and replacement of dominant species; Cryptophyceae replaced the Bacillariophyceae. Since 2001, water transparency increased and phosphorus concentration decreased, as well as the density and diversity of phytoplankton. Since 2004, an increase in Cyanobacteria was observed with bloom of Microcystis aeruginosa that extended 220 km downstream of the dam. Anomalous hydrological cycles, related to the ENSO phenomenon produced changes in phytoplankton density and structure. During El Niño, the system maintains a relative homogeneous phytoplankton assemblage, although mean density values are signifi cantly lower than those registered in ordinary hydrological periods. La Niña provokes a more drastic change in the community, due to the disruption in the exchange with the alluvial valley. The river-fl oodplain connectivity is essential for the maintenance of phytoplankton diversity.

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Cyanobacteria constitute the main toxin producers in inland water ecosystems and have extensive global distribution. The presence of hepatotoxins in aquatic environments is hazardous to human and animal health; even though the presence and identification of hepatotoxic microcystins in rivers and reservoirs of the world have been confirmed by several studies in the last few years. Herein, we studied the abundance and toxicity of Microcystis aeruginosa in the Argentine section of the Paraná River at the beginning of the Middle Paraná (Corrientes Hydrometer), near Corrientes city (27º28´ S -58º51´ W) and approximately 220 km downstream of the Yacyretá dam (High Paraná). The Paraná River basin, with a drainage area of 3