Diatoms are cosmopolitan aquatic organisms and are a major component of periphyton. They are also the basis of the trophic chain, thus they can respond quickly to environmental changes such as water metal contamination. Effects of metals on diatom communities have been extensively studied in field and laboratory experiments, underlying their high potential for metal contamination assessment. Arsenic (As) pollution is a global problem. Water (both surface and groundwater) is the main route of arsenic contamination. We conducted an experiment to investigate the effects of this metal on functional and structural descriptors. Specifically, we analysed changes in photosynthesis-related endpoints and diatom community structure (species relative abundances) and cell size of each species. Thus, by combining ecological and ecotoxicological descriptors, we expected to better characterize toxic impact of arsenic on periphyton. Results showed that arsenic exposure caused inhibition of periphytic algal growth, as well as community adaptation specially in diatoms. We identified 52 diatom taxa of which Achnanthidium minutissimum (Kützing) Czarnecki was the most abundant species, representing almost the 77% of the total abundance of diatoms: 75% in control and 79% in arsenic exposure, showing a tolerence to arsenic. Moreover, arsenic reduced diatom species richness (p=0.051). Diatom average cell biovolume was clearly reduced by arsenic (p=0.003). Individual cell biovolume (or cell size) in some species also changed with arsenic exposure. A general trend in biovolume decrease was observed, but Fragilaria species biovolume increased under arsenic exposure, due to greater cell and/or higher cell numbers in the arsenic treatment. Measured biovolumes were compared with theoretical biovolume data for each species and were poorly correlated, suggesting that the real measurements are recommended to better assess real impacts. In conclusion, both functional and structural algal parameters were affected by arsenic exposure. The strongest effect of arsenic on diatom community is the decrease in real cell biovolume, but arsenic also selected for species like Achnanthidium minutissimum (tolerant to metal exposure) and reduced diatom species richness. Considering how low is the arsenic concentration and exposure time in this experiment compared with reality, the results call into question the limits of arsenic concentration established to environmental quality, which are higher.