Urban ecosystems are submitted to multiple anthropogenic stress impacting aquatic communities. Among those urban stressors, artificial light at night (ALAN) causes many damages to aquatic ecosystems, changing for instance the composition of plant communities and altering species interactions. ALAN can also disrupt the photosynthetic cycles of autotrophic organisms, with possible consequences on cell oxidative stress. The combined impacts of ALAN and other types of urban stressors, in particular chemical contamination which can also generate oxidative impacts in aquatic primary producers, remain unexplored. In order to fill this gap, a one-month experimental approach was set up under controlled conditions to understand the singular and combined effects of two specific urban stresses (ALAN and chemical contamination by dodecylbenzyldimethylammonium chloride BAC12) on aquatic biofilms. The BAC12 is a biocide used in many virucidal products that can potentially be transferred to urban aquatic ecosystems. The bioaccumulation of BAC12 in biofilms exposed or not to ALAN, and the response of the latter in terms of taxonomic composition, photosynthetic activity, and fatty acid composition (including essential fatty acids) were analyzed. Initial results indicate that ALAN impacts diurnal photosynthetic yield and chlorophyll production, and that exposure to BAC12 at environmental concentrations induces lipid peroxidation. No interaction between the two stresses were found on the parameters measured