Artificial light at night (ALAN) may modify visually mediated interactions in the pelagial, yet field evidence remains scarce. We tested whether moderate-intensity ALAN aggregates planktivorous fish and reshapes zooplankton vertical distributions in a eutrophic 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 illumination. Fish formed dense aggregations in the illuminated epilimnion. Depth-integrated densities showed limited and taxon-specific responses to ALAN, whereas vertical distribution shifted consistently across multiple taxa. Responses were most pronounced in Chaoborus flavicans, which shifted deeper under ALAN, followed by large cladocerans, especially Daphnia longispina, including their gravid females, which showed reduced surface-layer prevalence and deeper nocturnal distributions. Leptodora kindtii occupied intermediate depths. Under ALAN, size-dependent vertical stratification re-emerged at night, partially counteracting nocturnal homogenisation of the upper water column. Differences between campaigns were interpreted as context-dependent variation rather than seasonal effects, but ALAN altered their spatial expression by disproportionately affecting large and reproductive individuals. Together, these results show that ALAN is primarily associated with changes in vertical distribution and predator-prey overlap rather than consistently reducing total zooplankton abundance within the sampled water column.
