Does light pollution affect benthic primary producers in shallow streams?

The spread of the electrical artificial lighting worldwide continues to introduce artificial light at night into the environments that have not experienced nighttime illumination before, such as freshwater ecosystems adjacent to urban settlements. Light properties greatly influence photosynthesis and community structure and phenology of primary producers in aquatic systems, therefore changes in light quality, duration, intensity and natural light/dark patterns can potentially have numerous effects on these communities. We simulated night-time irradiance levels comparable with near shore street lighting in outdoor flume simulations to explore how artificial light at night may affect benthic primary producers. We measured biomass and community composition of autotrophs in biofilms at two different colonization stages in two seasons (spring and fall) and evaluated their susceptibility to altered light conditions. Biofilms at early colonization stages responded to nighttime illumination treatment with reduced biomass and increase in diatom content, compared to the biofilms grown under natural dark-light cycles. The responses were highly seasondependent. Contrastingly, pre-established communities were found to be resilient to the illumination treatment. Having in mind the important role of autotrophs in primary production and food webs, we conclude that aquatic systems dominated by biofilms in early colonization stages, such as shallow streams recovering after physical disturbances, may be particularly sensitive to light pollution.