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Aim: We investigated the spatial and temporal patterns of abundance and diversity of planktonic ciliate community, in the last undammed stretch of the Upper Paraná River, Brazil. Methods In order to reach this result, seven field campaigns were performed over two years. Plankton samples were collected from 10 transects through this stretch of the river (230 Km), near the banks and on the center, as well on seven of its tributaries. Results 118 ciliate species were identified, among which the peritrichs were the most abundant while the order Hymenostomatida was the most specious group. We recorded a remarkable increase in abundance and species richness along the river, especially in the rainy period. Moreover, in this period we found an increase in the beta-diversity along the river, which consists in a remarkable distinction among the low, middle and high stretch of the river. In this way, continuous changes in the community structure of planktonic ciliates were evidenced, highlighting the importance of the precipitation and tributaries in the maintenance of the highest regional diversity in the studied area. Conclusion Our results strongly suggest the requirement for conservation actions with the purpose to maintain those tributaries undammed, in order to avoid biotic homogenization processes and the consequent reduction of aquatic biodiversity in this important neotropical ecosystem. Resumo Objetivo: Investigaram-se no presente estudo os padrões espaciais e temporais da abundância e diversidade de espécies da comunidade de ciliados planctônicos, no último trecho livre de barramentos da planície de inundação do alto rio Paraná, Brasil. Métodos Sete amostragens foram realizadas no período de dois anos. Amostras de ciliados foram coletadas ao longo de 10 transectos deste rio (230 Km), nas margens e no centro, bem como em sete de seus tributários. Resultados Cento e dezoito espécies foram identificadas, dentre as quais, a ordem Peritrichia foi a mais abundante e Hymenostomatida a mais especiosa. Foi evidenciado um aumento na abundância e riqueza de espécies ao longo do rio, especialmente no período chuvoso. Além disso, neste período foi registrado um incremento da diversidade beta ao longo do rio, consistindo em uma grande diferença entre os trechos a montante, intermediário e jusante da área de estudo. Assim, a gradativa alteração na estrutura da comunidade de ciliados evidenciou uma grande influência das chuvas e dos tributários na manutenção da alta diversidade regional no trecho estudado. Conclusão Os resultados evidenciam a necessidade de ações de conservação com o intuito de preservar os tributários livres de barramentos, a fim de evitar processos de homogeneização biótica e a consequente redução da biodiversidade aquática neste importante ecossistema neotropical.

Aim : We investigated the spatial and temporal patterns of abundance and diversity of planktonic ciliate community, in the last undammed stretch of the Upper Parana River, Brazil. Methods : In order to reach this result, seven field campaigns were performed over two years. Plankton samples were collected from 10 transects through this stretch of the river (230 Km), near the banks and on the center, as well on seven of its tributaries. Results : 95 ciliates species were identified, among which the peritrichs were the most abundant while the order Hymenostomatida was the most specious group. We recorded a remarkable increase in abundance and species richness along the river, especially in the rainy period. Moreover, in this period we found an increase in the beta-diversity along the river, which consists in a remarkable distinction among the low, middle and high stretch of the river. In this way, continuous changes in the community structure of planktonic ciliates were evidenced, highlighting the importance of the precipitation and tributaries in the maintenance of the highest regional diversity in the studied area. Conclusion : Our results strongly suggest the requirement for conservation actions with the purpose to maintain those tributaries undammed, in order to avoid biotic homogenization processes and the consequent reduction of aquatic biodiversity in this important tropical ecosystem.