Primary Research Paper Epilithic diatoms as indicators of water quality in the Gravataı´ river, Rio Grande do Sul, Brazil S.E. Salomoni 1, *, O. Rocha 2 , V.L. Callegaro 3 & E.A. Lobo 4 1 Program of Ecology and Natural Resources, Federal University of Sa ˜o Carlos, Via Washington Luis km 235, PO Box 676, CEP: 13.565-905, Sa ˜o Carlos, SP, Brazil 2 Department of Ecology and Evolutionary Biology, Federal University of Sa ˜o Carlos, Via Washington Luis km 235, PO Box 676, CEP: 13.565-905, Sa ˜o Carlos, SP, Brazil 3 Museum of Natural Sciences, Zoobotanical Foundation of Rio Grande do Sul, Av. Salvador Franc ¸ a, 1427, PO Box 1188, CEP: 90.690-000, Porto Alegre, RS, Brazil 4 Laboratory of Limnology, University of Santa Cruz do Sul, PO Box 236, CEP 96.815-900, Santa Cruz do Sul, RS, Brazil (*Author for correspondence: E-mail: saiosalomoni@hotmail.com) Received 6 October 2004; in revised form 4 August 2005; accepted 27 August 2005 Key words: epilithic diatoms, bioindicators, organic pollution, eutrophication, Gravataı´ River, RS Abstract The potential use of epilithic diatoms as indicators of organic pollution was evaluated in Gravataı´ River, RS, (latitude 29°45¢–30°12¢ S; longitude 50°27¢–51°12¢ W). The river suffers agricultural impacts in its upper course and urban and industrial organic pollution in its lower course. Epilithic diatoms were sampled eight times from September 2000 to August 2002, at six sites. Species were identified and densities and relative abundances of populations were determined. Simultaneously, physical, chemical and microbio- logical variables were measured (water temperature, conductivity, turbidity, pH, dissolved oxygen, bio- chemical oxygen demand (BOD 5 ), chemical oxygen demand, ammonium, organic nitrogen, total nitrogen, ortho-phosphate, total phosphate, chloride and faecal coliforms). In order to interpret the environmental and biological variables, discriminant analysis and the TWINSPAN methods (Two-Way Indicator Species Analysis) were applied. The results indicated that the concentrations of ortho-phosphate, ammonium, total organic nitrogen, BOD 5 and faecal coliforms characterized a pollution gradient along the river, where changes in the abundance or species composition were observed. Species were classified into three groups: Group A, including species more tolerant to heavy organic pollution and eutrophication, represented by Luticola goeppertiana, L. mutica, Eolimna subminuscula, Nitzschia palea and Sellaphora pupula; Group B, comprised of tolerant and widely distributed species such as Eunotia bilunaris, Frustulia crassinervia, F. saxonica, Navicula cryptocephala, N. cryptotenella, Nitzschia palea var. tenuirostris, Surirella angusta, Pinnularia microstauron and Ulnaria ulna and Group C, with less pollution tolerant species represented by Eunotia sp. and Gomphonema parvulum. Introduction High population densities and the multiplicity of industrial and agricultural activities expose most hydrographic basins close to large urban centers to heavy and rising environmental impacts, especially to the pollution by domestic and industrial waste residues. The consequences of the impact depend on a complex combination of factors, such as the volume and load of effluents, and the discharge of receiving water body. Usual approaches to water quality evaluation are divided in two main categories. One based on physical and chemical methods, and another Hydrobiologia (2006) 559:233–246 Ó Springer 2006 DOI 10.1007/s10750-005-9012-3