Norfloxacin Pollution Reshapes Interspecific Relationships in Zooplankton: Impacts of Reduced Phytoplankton Defence and Altered Zooplankton Motility Across Individual, Population and Community Scales

ABSTRACT Recent studies indicate that antibiotics can inhibit the formation of phytoplankton colonies. This may affect zooplankton of varying sizes differently, potentially altering interspecific competition. To investigate these effects, we established resource (predator‐free) and apparent competition (with a larval damselfly as predator) systems by co‐culturing Daphnia magna and Moina affinis with the algae Scenedesmus quadricauda as food. We examined the effects of norfloxacin (NOR, 0, 31.25, 500 and 8000 μg L −1 ) on the colony formation of S. quadricauda and on both individual‐ and population‐level traits of zooplankton, along with interspecific competitive dynamics. NOR impaired the ability of S. quadricauda to form large colonies, increasing grazing rates and boosting population expansion of both grazer species, particularly enhancing the competitive advantage of the smaller species, M. affinis , in the resource competition system. However, NOR caused oxidative damage and reduced mobility, which impacted M. affinis more severely, increasing its predation risk by damselfly larvae and reducing its dominance in the apparent competition system. Our findings indicate that zooplankton interspecies competition is highly sensitive to antibiotic contamination. Moreover, the combined effects of differences in individual body size and sensitivity determine the potential hazards antibiotics pose to zooplankton communities, highlighting the need for concern.