Artificial light at night shapes marine early hard-bottom communities in Madeira Island, NE Atlantic

Artificial light at night (ALAN) is increasingly recognised as a pollutant that modifies natural light conditions and negatively impacts terrestrial and aquatic ecosystems. Marine hard-bottom communities are important components of coastal ecosystems, supporting diverse sessile organisms and early successional processes that can reveal community-level responses to environmental changes. This study investigated the influence of ALAN on early successional marine hard-bottom communities in a field experiment conducted in Madeira, Portugal. Simulated outdoor LED lighting systems were used to expose submerged PVC settlement panels to white and yellow light in order to assess their effects on the establishing communities over a three-month period. Light of both colours significantly altered community composition and the abundances of colonisers such as the hydrozoan Pennaria disticha and the red alga Polysiphonia sertularioides in comparison to assemblages that simultaneously established on non-illuminated surfaces. In general, ALAN-exposed communities exhibited a higher species richness but a lower evenness compared to non-exposed assemblages. Our results reveal the potential of ALAN to alter the biodiversity and structure of shallow-water, benthic communities and underscore the need for further research to assess the consequences of anthropogenic light emissions for coastal marine ecosystems.