Patterns in the temporal composition, abundance and diversity of the phytoplankton community of the Paraná river prior to and after the initial filling phase of the Yacyretá reservoir are analyzed. The study site is located 220 km downstream from the Yacyretá reservoir and 30 km downstream from the confluence with the Paraguay river. Because both rivers remain separate and unmixed at the study site, we compared the possible effects of the impoundment on both river banks (left and right banks) in hydrological periods with similar duration and magnitude of the low and high water phases. Physical and chemical conditions measured on the right bank (water from the Paraguay river) were similar at both periods (pre and post-impoundment) whereas conductivity, pH and orthophosphate concentration increased on the left bank (water from the High Paraná river and Yacyretá reservoir) after the impoundment. Changes in phytoplankton density and diversity were observed only in samples collected from water flowing from the reservoir (left bank). The density of Chlorophyceae (Chloromonas acidophila, Chlamydomonas leptobasis, Choricystis minor, Chlorella vulgaris, Scenedesmus ecornis, Monoraphidium minutum, M. contortum and M. pusillum) and Cryptophyceae (Rhodomonas minuta, Cryptomonas marssonii and C. ovata) increased while Cyanophyceae (Cylindrospermopsis raciborskii, Raphidiopsis mediterranea and Planktolyngbya subtilis) and Bacillariophyceae (Aulacoseira granulata and its bioforms) decreased compared to previous studies conducted on the left bank of the Paraná river. Phytoplankton collected from the right bank of the river did not differ in pre and post-impoundment samples because they originate from the Paraguay river, which remains relatively unaffected by human activities.Keywords: temporal and spatial variation, diversity, large rivers, pre and post-impoundment. Abundância e diversidade do fitoplâncton do Rio Paraná (Argentina) 220 km a jusante do Reservatório de Yacyretá ResumoEstudaram-se os padrões temporais de composição, abundância, e diversidade da comunidade fitoplanctônica do Rio Paraná, antes e depois da fase inicial de fechamento do reservatório de Yacyretá (períodos pré e pós-fechamento). A área de estudo está localizada 220 km a jusante do reservatório de Yacyretá e 30 km a jusante da confluência dos Rios Paraná e Paraguai. Considerando que as águas destes rios permanecem sem se misturar no local de estudo, os possíveis efeitos do fechamento da barragem foram comparados em ambas as margens (direita e esquerda), durante as fases de águas altas e baixas de períodos hidrológicos de duração e magnitude semelhantes. As condições físico-químicas medidas na margem direita (águas do Rio Paraguai) foram similares em ambos os períodos (pré e pós-fechamento), observandose aumento nos valores da condutividade, pH e concentração de ortofosfato na margem esquerda (águas do alto Rio Paraná e do reservatório de Yacyretá) no período pós-fechamento. Alterações na densidade e composição da comunidade fitoplanctônica foram ...
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