Abstract
The ecotoxicological effects of Ciprofloxacin hydrochloride (CIP) were tested on population densities of plankton assemblages consisting of two algae (Isochrysis galbana and Platymonas subcordiformis) and a rotifer (Brachionus plicatilis). The I. galbana showed a significant decrease in densities when concentrations of CIP were above 2.0 mg L(-1) in single-species tests, while P. subcordiformis and B. plicatilis were stable in densities when CIP were less than10.0 mg L(-1). The equilibrium densities of I. galbana in community test increased with CIP concentrations after falling to a trough at 5.0 mg L(-1), showed a completely different pattern of P. subcordiformis which decreased with CIP concentrations after reaching a peak at 30.0 mg L(-1). The observed beneficial effect was a result of interspecies interactions of trophic cascade that buffered for more severe direct effects of toxicants. The community test-based NOEC of CIP (2.0 mg L(-1)), embodying the indirect effects, was different from the extrapolated one derived by single-species tests (0.5 mg L(-1)), but all lacked confidence interval. A CIP threshold concentration of obvious relevance to ecological interaction was calculated with a simplified plankton ecological model, achieving a value of 1.26 mg L(-1) with a 95% bootstrapping confidence interval from 1.18 to 1.31 mg L(-1).