Periphytic diatom communities were sampled from glass substrates immersed along a gradient of organic and metallic pollution. We investigated the influence of nutrients and a combination of nutrients and metals on biofilms and diatom communities settling on the glass over three weeks. Biofilm was characterized through organic biomass, chlorophyll a concentrations and metal content; structure of diatom assemblages was assessed by studying densities, mean biovolumes and taxonomic composition. Exposure to organic pollutants resulted in an increase of biomass (dry weight, chlorophyll a concentrations and diatom densities) and diatom community structure was similar to that at an unpolluted site relative to nutrient concentrations. Cyclotella meneghiniana was dominant and the species Nitzschia palea, Navicula gregaria and Melosira varians were well-represented. Downstream of the metal-contamination source, biofilm biomass, as well as chlorophyll a concentrations, decreased as cadmium and zinc content got higher (up to 60µgCd/g dry weight and 1400µgZn/g dry weight). Concurrently, the size distribution of diatoms, changing from larger to smaller individuals, reflected changes in the taxonomic composition of the assemblages where Eolimna minima was found in high proportions. Statistically significant amounts of abnormal frustules were also enumerated in the metal-polluted environment (p