Norfloxacin pollution alters species composition and stability of plankton communities.

Despite recent advances in assessing lethal effects of antibiotics on freshwater organisms, little is known about their potential consequences on community composition and function, which are essential for assessing the ecological risk of these pollutants. Here, we investigated the impact of norfloxacin (NOR) on the short-term (≤ 6 days) dynamics of co-cultured Scenedesmusquadricauda-Chlorella vulgaris and Scenedesmusobliquus-C. vulgaris, and the long-term (≤ 70 days) dynamics of co-cultured S.obliquus-C. vulgaris in experiments with or without grazer Daphnia magna at sublethal antibiotic concentrations (0, 0.5, 2 and 8 mg L). NOR increased the relative abundance of Scenedesmus species in the absence of grazers but exerted opposite effects when Daphnia was present in both short- and long-term experiments due to reduced colony size. Meanwhile, increasing NOR concentrations led to quickly increased total algal density in the initial stage, followed by a sharp decline in the long-term experiment in the absence of grazers; when Daphnia was present, population fluctuations were even larger for both prey and predator species (e.g., grazer extinction at the highest concentration). Thus, NOR affected the outcome of species interactions and decreased temporal stability of plankton ecosystems, suggesting that antibiotics have more extensive impacts than presently recognized.