Artificial light is a widespread human-induced change that disrupts natural light regimes and transforms nighttime environments in urban and suburban areas. Incrising number of studies report ecological effects of light pollution over a broad range of species, trophical levels and ecosystems, raising concerns for conservation of biodiversity and ecosystem services. The importance of light availability for growth and development of autotrophs is well known and the variation in light quality and quantity potentially accounts for much of the variation in the physiology, population growth and community structure of freshwater producers. However, the understanding of ecological implications of low-level artificial light on aquatic primary producers is still scarce. Primary production of many rivers and streams is mainly driven by benthic autotrophs that develop complex periphyton communities closely interacting with bacteria, protozoa, fungi and detritus. They play major ecological roles in food web structuring, nutrient turnover, sediment stabilization and recovery after physical disturbances. We simulated conditions of a light-polluted alpine stream in an outdoor flume mesocosm experiment and investigated how artificial light-naive periphyton respond to alterations of natural light regime by applying low-level LED light at night. We here present the effects of such exposure on community composition of periphytic autotrophs.