| Summary: | Este trabalho trata do estudo da flora de Desmodesmus An, Friedl & Hegew. e Scenedesmus Meyen, encontrada em lagoas isoladas, lagoas interligadas, banhados, açudes e ambientes lóticos da Planície Costeira do Rio Grande do Sul. A composição taxonômica, e as relações entre a riqueza e a densidade com as variáveis temperatura e pH foram avaliadas. O estudo baseou-se em 41 amostragens efetuadas no outono e na primavera de 2003. Um total de 14 espécies e duas variedades de Desmodesmus e duas espécies e uma variedade de Scenedesmus foram identificadas. Além disso, quatro combinações novas (Desmodesmus granulatus (W.& G. S. West.) Hentschke & Torgan, Desmodesmus heteracanthus (Guerr.) Hentschke & Torgan, Desmodesmus microspina (Chod.) Hentschke & Torgan e Desmodesmus spinoso-aculeolatus (Chod.) Hentschke & Torgan) são propostas. A variação da riqueza e da densidade estiveram influenciadas pelo pH (r2 = 0,53 e r2= 0,97). A média de densidade (ind. mL-1) de Desmodesmus e Scenedesmus na primavera diferiu significativamente da média obtida no outono (p = 0,04) e, e a média de riqueza na zona litoral diferiu significativamente da média obtida na zona pelágica (p = 0,02). === This paper is about the Desmodesmus An, Friedl & Hegew. and Scenedesmus Meyen flora, found at isolated ponds, interlinked ponds, swamps, dams and lotic environments, at the Coastal Plain of Rio Grande do Sul. The taxonomic composition and how richness and density relate to the variables temperature and pH were evaluated. The study was based on 41 samplings collected in autumn and spring of 2003. A total of 14 species and two varieties of Desmodesmus, and two species and a variety of Scenedesmus were identified. Four new combinations (Desmodesmus granulatus (W.& G. S. West. ) Hentschke & Torgan, Desmodesmus heteracanthus (Guerr.) Hentschke & Torgan, Desmodesmus microspina (Chod.) Hentschke & Torgan and Desmodesmus spinosoaculeolatus (Chod.) Hentschke & Torgan) were proposed. The statistical analysis revealed that the variation of richness and density were influenced by pH (r2 = 0,53 e r2= 0,97). Comparing autumn and spring, it is observed that the density average (ind. mL-1) of Desmodesmus and Scenedesmus in spring differs significantly of the average obtained in the autumn (p = 0,04) and, comparing the richness averages, that the average of the litoral zones differs significantly of the average obtained in the pelagic zones (p = 0,02).
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