Estrutura e dinâmica da comunidade de ciliados planctônicos em um grande rio Neotropical : a relevância da sazonalidade e dos tributários na manutenção da biodiversidade.

Abstract

The damming of great rivers produces expressive changes in the natural processes of ecosystems, with a significant amount of variation in the water characteristics, a strong decrease in the flood pulse amplitude, as well a gradual reduction of nutrients load and a continuous increase in the water transparency. The aim of this study was to investigate the spatial and temporal patterns of abundance and species composition of ciliates community, in the last undammed stretch of the Upper Paraná River, in Brazil. In order to reach this result, four field campaigns were performed over a year. 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. 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 higher water 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. As a consequence of the aspects previously presented, continuous changes in species composition evidenced a great influence of precipitation and tributaries in the maintenance of highest regional diversity of the studied stretch. Therefore 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.