Marine sessile communities in coastal areas provide a wide range of ecosystem services, including protection of biodiversity, nutrient cycling, water purification and sustainable fisheries. On the other hand, they can be considered as the undesirable accumulation damaging ships and moorings. Artificial light at night (ALAN) is expected to increase along with population growth and urbanisation. Studies investigating the effects of ALAN have mostly targeted single or a few species, focusing on its effects on physiology and behaviour. However, research of the effects of ALAN on marine organisms at a community level is still lacking.
In this study, we investigated the influence of ALAN on marine sessile communities in the early succession phase (< 6 months). Settlement panels made of PVC were deployed from June to September 2023 and May to November 2024 in Akkeshi Bay, Hokkaido. We assessed composition, coverage, biomass and species diversity of colonised organisms on illuminated and unilluminated panels at 12 weeks in 2023 and at every 4 weeks up to 24 weeks in 2024.
Community structures, abundances and species diversity were different between ALAN and control treatments, but the direction of variation and effect size changed seasonally and interannually. In 2024, biomass under ALAN treatment remained significantly lower than that under control from 16 weeks to 24 weeks. Total cover showed a significant difference between treatments at 16 weeks, but no significant difference was observed by 24 weeks. Species richness showed no difference between treatments over 24 weeks. The largest difference in community composition, biomass and coverage between treatments was observed in September for both years where abundance of dominant species (a serpulid worm in 2023 and a hydrozoa in 2024) was lower in ALAN treatment than in the control.
Impacts of ALAN may be specific to certain species or life stage. The underlying causes can be ascribed to the variation in predation intensity or deterrence of larval settlement that can vary with/without ALAN, which should be examined in future studies.