| Summary: | The Itaqui reservoir in Paraná state, southern Brazil, is dominated by the floating macrophyte Pistia stratiotes L. and is used for recreation and irrigation. The reservoir's excessive plant cover suggests an extreme trophic state and interferes with multiple uses. The aims of this study were to determine the trophic state of the reservoir water and to document the limnological conditions and the composition of the periphytic diatom community before and after the mechanical removal of macrophytes. As each diatom species has certain autoecological requirements in a given geographic area, another objective of the study was to identify diatoms that can be considered tolerant of the reservoir's trophic state in a substropical environment. Local water samples collected for physical and chemical analyses, including estimates of chlorophyll a, showed the hypereutrophic status of the reservoir before and after macrophyte removal. Environmental conditions exceeded acceptable values for fishing and irrigation, providing a clear example of how the inadequate management of water resources can directly reduce their usefulness. Trimestral sampling was carried out between May 2008 and February 2009. For quantitative analyses, biofilms were scrubbed off glass slides submerged for 30 days at a depth of approximately 40 cm. Diatom samples were cleaned with potassium permanganate and hydrochloric acid and mounted on permanent slides with Naphrax. All individuals found in random transects under three replicates were identified and counted up to a minimum of 600 valves. Thirteen species tolerant of eutrophication were selected. Four species mostly known from low-nutrient sites may be considered tolerant of eutrophic conditions. The composition of the diatom community was influenced by seasonal changes in temperature and rainfall. Canonical Correspondence Analyses confirmed a correlation between higher diatom densities and the increased photic zone following macrophyte removal.
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