Through the analysis of specific composition and diversity of diatom community, in two sampling areas of the Guaíba system (RS, Brasil) in September and October 1975, a meaningful difference in the structure of that community was found. Such difference is probably due to exclusive presence of Achnanthes inflata (Kütz.)Grun. var. elata (Leud. - Fort.)Hust., A. lanceolata(Bréb.)Grun. var. lanceolata, Cocconeis placentula Ehr., Cyclotella meneghiniana Kütz., Cymbella minuta Hilse ex Rabh., and of Aulacosira distans(Ehr.) Sim., Navicula mutica Kütz., Surirella robusta Ehr. var. splendida (Ehr.)Van Heurck, Synedra ulna(Nitz.)Ehr., abundant species which reflect different environmental characteristics because of their different ecological tolerances. A total of 114 taxa have been identified; 57 of them to specific and infraespecific levels, 14 of which mentioned for the first time in the Guaíba system and 4 for the first time in the state.
Algae; Bacillariophyceae; structure of community