Artificial light at night reshapes nocturnal activity patterns in a coastal amphipod at sky-glow relevant irradiances

Coastal habitats are increasingly exposed to artificial light at night, which has been reported to disrupt biological rhythms, trophic interactions, and community structure. However, responses to low, geographically widespread irradiance levels remain poorly understood, particularly within dose-response frameworks that determine the relationship between increasing artificial light intensity and impacts on biological responses. Diel vertical swimming is a widespread behaviour that enriches the water surface by increasing abundance of coastal species, but the effect of artificial light on this behaviour remains largely unexplored. We exposed the intertidal amphipod Marinogammarus obtusatus to six increasing levels of nighttime irradiance (0.0; 0.1; 0.5; 1.0; 1.5 and 2.0 mW m^-2) over a 24-h cycle using a dose-response experimental framework. Amphipod activity was quantified from time-lapse images taken every 5 min, measuring the number of active individuals, the number of swimmers in the water column, and maximum swimming height. Both the number of swimmers and maximum height decreased at night with increasing irradiance, while the number of active individuals remained unaffected. Under control conditions, M. obtusatus exhibited a strong peak of swimming activity shortly after nightfall, followed by smaller peaks throughout the night. At 0.1 mW m^-2, this peak was reduced, and increasing irradiance further suppressed nocturnal swimming. At 2.0 mW m^-2, activity extended across the 24-h cycle. Nocturnal swimming in coastal amphipods is associated with mating and foraging, during which they are vulnerable to predation. This indicates that even sky-glow irradiance levels may disrupt population dynamics and trophic interactions.