Artificial light at night intensifies effects of a parasitic flatworm on the water flea Daphnia magna.

Artificial light at night can strongly alter organismal traits, but its role in shaping species interactions remains poorly understood, especially so in aquatic ecosystems. By capitalizing on a recently discovered antagonistic interaction between a brood-parasitic flatworm and Daphnia magna water fleas, we tested whether this interaction depends on exposure to artificial light at night. During a 19 day laboratory population growth experiment, we manipulated flatworm presence and night-time light conditions in a full-factorial design. We confirmed the negative effects of flatworm predation on Daphnia abundance at the population level. Importantly, we showed that the flatworm-caused reduction in the final population size of Daphnia under artificial light at night was twice as strong (81%) compared to under dark-night conditions (39%). Our findings are relevant when it comes to assessing the impact of artificial light at night on the development of Daphnia populations and thus top-down control of phytoplankton. Freshwater ecosystems in urbanized areas, where this parasitic interaction was first encountered, may be especially at risk, as these are typically exposed to high levels of stress factors, including light pollution.