Norfloxacin disrupts Daphnia magna-induced colony formation in Scenedesmus quadricauda and facilitates grazing

•The effect of norfloxacin on Scenedesmus quadricauda-Daphnia magna interaction has been investigated.•Norfloxacin could inhibit grazer-induced colony formation in S. quadricauda..•Norfloxacin could inhibit feeding behavior of D. magna..•More S. quadricauda cells are consumed by D. magna with increasing norfloxacin concentrations.

Environmental pollutants can negatively affect grazer-induced defenses in phytoplankton species and lead to the disruption of phytoplankton-zooplankton relationships. However, it is still not known whether norfloxacin (NOR), a widely used antibiotic, has such effects on plankton systems when released into the environment. Here, we investigated the colony-forming ability of the green alga Scenedesmus quadricauda as well as the behavior (hopping frequency and duration of quiescence) and grazing rate of the cladoceran Daphnia magna, when exposed to different NOR concentrations (0, 50, 500, 5000 and 50,00 ug L−1), to determine the effects of NOR on the S. quadricauda-D. magna relationship. Colony size was generally unaffected by NOR but significantly increased when subjected to the stress of adding grazers; however, the combined effects of adding grazers and increasing NOR concentrations led to consistently decreased colony size compared to only adding grazers. The hopping frequency and the time spent on swimming of D. magna individuals decreased while their grazing rate increased with increasing NOR concentrations. Thus, NOR seemed to inhibit grazer-induced colony formation in S. quadricauda, resulting in more algal cells being consumed by grazers, despite inhibiting grazers' feeding behavior. Considering that phytoplankton-zooplankton interaction plays an important role in determining ecological process of aquatic ecosystems, ecosystem function might be more affected by NOR contamination than previously recognized.