Artificial light at night reshapes vertical distributions of lake zooplankton across population and community scales

Artificial light at night (ALAN) can strengthen visually mediated interactions in the pelagial, yet open-water field evidence remains scarce. We tested whether low-intensity ALAN aggregates planktivorous fish and reshapes zooplankton vertical distributions and population structure in a eutrophic, stratified lake. During two new-moon campaigns (May and June 2017), we assessed fish aggregation using hydroacoustics and quantified zooplankton across 0–6 m by day, in natural darkness, and under high-pressure sodium (HPS) illumination. Fish formed dense aggregations in the illuminated epilimnion. Depth-integrated densities tended to be lower under ALAN for large-bodied taxa and gravid females, clearly in May, whereas small cladocerans and copepods showed weak or season-limited differences among diel states. Responses were strongest in Chaoborus flavicans , which shifted deeper under ALAN, followed by large cladocerans (especially Daphnia longispina and gravid females) that showed reduced surface-layer prevalence and deeper nocturnal distributions. Leptodora kindtii occupied intermediate depths. Under ALAN, size-dependent vertical stratification re-emerged at night, consistent with size-structured segregation in surface layers. Seasonal changes in depth-integrated densities matched PEG-type succession, but ALAN altered spatial expression by disproportionately affecting large and reproductive individuals. Together, these results show that moonlight-level ALAN can aggregate planktivores and reorganise pelagic predator–prey landscapes in lakes.