Light pollution suppresses diel vertical migration across rural and urban Daphnia magna populations

Artificial light at night (ALAN) is a stressor of growing ecological concern. It alters the light conditions that organisms have adapted to over evolutionary time and, hence, can influence their physiology and behaviour. Zooplankton use light as a cue for diel vertical migration, moving to surface layers at night to feed when predation risk is low. We investigated if ALAN alters diel migration of a common freshwater water flea, Daphnia magna, and whether urban populations have adapted to the light. We cultivated clonal lineages from three urban and three rural ponds that have historically differed in light exposure during the last century and assessed their vertical migration when exposed to ALAN, white LEDs at 2.2 lx. We found no difference between urban and rural populations in migration behaviour; all reduced migration towards the illuminated surface layer when exposed to LEDs. The results indicate that the reaction norm to nighttime illumination, which most likely evolved in response to variation in moonlight and cloudiness, has been maintained under urban conditions. This may be due to the urban populations not yet having adapted to white LEDs, given the recent transition to LEDs from older light sources, or lack of selection for differentiation because of similar cost-benefit ratio of migration in urban and rural ponds. The recorded suppression of diel vertical migration in response to ALAN in both urban and rural populations could have wider ecological consequences. This is especially likely if different species respond differently to ALAN, potentially disrupting species interactions and ecological functions.