Associations of diatom species were identified, in the Arroio Sampaio Basin, Rio Grande do Sul, Brazil, based on monthly samplings over a year along Arroio Sampaio and its main tributaries, using polyamide thread as an artificial substrate. The species groupings showed four different environments: medium-lower course of Arroio Sampaio; and lower course of Arroio Teresinha; upper course of Arroio Sampaio; and lower course of Arroio Duvidosa. Among the physical and chemical variables measured, water pollution, particularly organic contamination and eutrophication, measured from BOD, and total phosphate concentration, respectively, appeared to be one of the most important environmental factors determining the composition and structure of species associations in the area studied.

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The responses of epilithic bacterial and algal communities to sewage-treatment-works (STW) effluent were studied in three streams in North Yorkshire, England, using both conventional microbiological techniques and the techniques of molecular genomics. Cod Beck, Thornton Beck and the River Wiske, were visited in May-June 2000 and January 2001 and the epilithic communities on submerged stones were sampled to determine chlorophyll-a, leucine assimilation, bacterial abundance, identity and abundance of microalgae, and epilithon dry and organic weights. Additionally, DNA from the epilithon of stones, collected in March-April and November 2000, was extracted, and samples were amplified using universal primers appropriate, respectively, for bacteria [Muyzer et al., 1993. Applied and Environmental Microbiology 59: 695-700] and microscopic phototrophs [Nu¨bel et al., 1997. Applied and Environmental Microbiology 63: 3327-3332] followed by DGGE (Denaturing Gradient Gel Electrophoresis). Differences in algal community composition and abundance were greater between streams than between sites upstream and downstream of the STW outfalls. DGGE banding profiles of bacterial communities revealed grouping according to stream, and not the formation of communities characteristic of downstream sites; seasonal variation was also evident. Thus the discharge of STW effluent to the streams did not bring about the development of communities that are characteristically associated with organic pollution. Changes in composition and structure of bacterial and micro-algal epilithic communities were detected, but the relatively high quality effluents had only mild effects, not altering the environments sufficiently to override the natural differences between the three streams.

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