In this study, the seasonal and interannual fluctuations of desmids species richness was investigated for three years in an Amazon black water floodplain lake in the Negro River basin, Amazon State. The studied environment showed a great annual fluctuation of water column depth and high temperature values with little seasonal fluctuations. In general, pH and electrical conductivity values were low. One hundred five desmids species were identified distributed among families Closteriaceae, Gonatozygaceae and Desmidiaceae. Twenty three species are new references to Amazon State and are illustrated in this study. A marked seasonal fluctuation of desmids species was registered in association with the inundation pulse. The increased water level was the period of more species richness; on the other hand, the others periods showed low and similar species number. An interannual fluctuation of desmids species number was also registered, with the highest number in 2003, especially in the increased water level. Staurastrum Meyen and Closterium Nitzch were the genera with the highest species numbers. Based on this occurrence, the majority of species registered was rare or sporadic, registered in less than 10% or between 10 and 30% of total analyzed sample, respectively.
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