Patterns of biofilm formation from a bare substratum were investigated in two streams from different bioclimatic regions: a Central-European (the Walzbach, SW Germany) and a Mediterranean (the Fuirosos, NE Spain). Algal biomass, diatom community structure and Extracellular Polymeric Substances (EPS) were analyzed during a 60-day colonization period. There was a faster colonization of algae in the Mediterranean stream and a slower and gradual colonization in the Central-European stream. The successional patterns of diatom communities in the Mediterranean stream were altered by flood episodes that were followed by re-colonization episodes (days 14 and 44). Biofilms in this stream had a higher amount of early colonizing diatom species (Ulnaria ulna and Karayevia clevei) and polysaccharide development in the EPS matrix. Differences might be an adaptation to the hydrological changes. The Central European biofilms showed a gradual increase in microbial community complexity, reflecting the higher hydrological stability as well as higher nutrient availability. While the colonization dynamics in the Mediterranean stream reflects hydrological alterations, the succession in the Central European stream is mostly autogenic and results of the biological interactions in the biofilm.