Use of epilithic diatoms as bioindicators from lotic systems in southern Brazil , with special emphasis on

Abstract

Use of epilithic diatoms as bioindicators from lotic systems in southern brazil, with special emphasis on eutrophication. Epilithic diatom biocenoses have been recommended by researchers from several countries as particularly suitable for water quality evaluation. However, saprobic systems using diatoms have been developed to assess water organic pollution, not taking into account the effects of eutrophication on the biological composition of the communities. Thus, the main aim of this study was to determine the tolerance of diatom species to eutrophication in three streams (Sampaio, Grande and Bonito) of the Mato Leitão countryside area, RS, Brazil, using multivariate analyses techniques. Results of physical, chemical and biological analyses obtained in studies carried on in the area between the years of 1993 to 1998 were used to develop the present study. Multivariate analyses of species composition data were applied in two different approaches. Firstly, species and sampling units were grouped using TWINSPAN (Two-way Indicator Species Analysis). Secondly, Canonical Correspondence Analysis (CCA) was applied in order to uncover the main gradients of changes in species composition, relating these changes to the eutrophication process. From the levels of tolerance to eutrophication determined for each diatom species, they were given an indicative value from 1 to 5, which corresponded, respectively, to very low, low, medium, high and very high tolerance levels, thus allowing the calculation of the Biological Water Quality Index (BWQI). These results are complementary to the saprobic system already proposed for use in Southern Brazilian rivers. Key-words: diatoms, bioindicators, water quality assessment, organic pollution, eutrophication.