Keywords: 
• norfloxacin /
• Brachionus calyciflorus /
• chronic toxicity /
• antibiotics /
• food density /
• life table

Abstract: Norfloxacin is a widely used antibiotic, but its toxicity to rotifers remains unknown. In order to investigate the chronic toxicity of sublethal concentrations of norfloxacin to rotifers, and its relation with algal food level, and to compare the relative sensitivity of various endpoints to norfloxacin exposure, the effects of different concentrations (5, 20, 35, 50, 65 and 80 mg·L^-1) of norfloxacin on the life-table demographic parameters of Brachionus calyciflorus cultured at three densities (1.0×10⁶, 2.0×10⁶ and 4.0×10⁶ cells·mL^-1) of Scenedesmus obliquus were studied by life-table method. The results showed that at 1.0×10⁶, 2.0×10⁶ and 4.0×10⁶ cells·mL^-1 of S. obliquus, and compared with the controls, the rotifers exposed to norfloxacin at 5-80 mg·L^-1 had significantly longer life expectancy at hatching and generation time, and higher net reproductive rate and intrinsic rate of population increase. With the rise of algal density, the prolonged magnitudes of life expectancy at hatching and generation time of the rotifers exposed to norfloxacin at 5 mg·L^-1 increased successively from 39.4% to 59.6% and 62.2%, and from 36.4% to 59.4% and 62.5%, but the mean values of the six treatments decreased successively from 85.9% to 83.1% and 63%, and from 84.8% to 83.1% and 64.6%, respectively. Accordingly, the increased magnitudes of net reproductive rate and intrinsic rate of population increase of the rotifers exposed to norfloxacin at 5 mg·L^-1 increased successively from 164.6% to 296.3% and 353.2%, and from 59.1% to 69.2% and 94.2%, but the mean values of the six treatments changed from 404.5% to 384.6% and 500.4%, and from 84.7% to 75.7% and 115.2%, respectively. At 1.0×10⁶ cells·mL^-1 of S. obliquus, norfloxacin concentration did not significantly affect the proportion of sexual offspring of the rotifers (P>0.05). However, at 2.0×10⁶ cells·mL^-1 of S. obliquus, the rotifers exposed to norfloxacin at 5-35 and 80 mg·L^-1 had significantly lower proportion of sexual offspring. When Scenedesmus density was 4.0×10⁶ cells·mL^-1, the rotifers exposed to norfloxacin at 5, 35 and 80 mg·L^-1 had significantly lower proportion of sexual offspring. When Scenedesmus density was 1.0×10⁶ and 2.0×10⁶ cells·mL^-1, there were significant dose-effect relationships between norfloxacin concentration and life expectancy at hatching, generation time, net reproductive rate as well as intrinsic rate of population increase. When Scenedesmus density was 4.0×10⁶ cells·mL^-1, there were significant dose-effect relationships between norfloxacin concentration and net reproductive rate as well as intrinsic rate of population increase. The results indicated that sublethal concentrations of norfloxacin are beneficial to survival, development, asexual reproduction and population growth, and the affecting magnitudes are significantly affected by algal density.