Euglenoid morphospecies replacement along a hydraulic gradient of the Lower Paraná Basin (Argentina)

Abstract

Summary1. This study deals with the distribution of euglenoid morphospecies along the length of the Lower Paraná River floodplain. Six sites located in the Paraná River, Estevez stream and Montiel shallow lake were surveyed monthly from May 1995 to April 1996.2. Eighty‐eight infrageneric taxa were registered though the total density percentage of euglenoids seldom exceeded 15% of total phytoplankton. The increasing species richness towards the lake is mostly the result of a high contribution of species of Trachelomonas and naked infrageneric taxa of Euglena, Lepocinclis and Phacus, while the distribution of Strombomonas species along the transect is fairly constant.3. The richness of euglenoid morphospecies follows a pattern opposite to that found for the entire phytoplankton community. The distribution of euglenoids differs among the three habitats defined in this study: river, stream, shallow lake. These differences depend on the hydraulic condition of each habitat, as revealed by canonical correspondence analysis.4. The degree of association with each type of aquatic habitat varied among the species and differed between water phases. Strombomonas fluviatilis was equally constant along the length of the ecotone, while S. girardiana and S. scabra were particularly associated with the Paraná River and P. longicauda, L. truncata, T. hispida and T. similis with the lake.5. The flood pulse establishes a clear pattern both in species richness and abundance of euglenoids. The number of species recorded during high water was more than twice that recorded during low water.