Antibiotic-induced multi-trophic effects and their cascades in a sub-tropical freshwater ecosystem

Antibiotics are commonly detected in aquatic ecosystems worldwide due to their extensive use and excretion by humans and animals, posing potential risks to the health of these ecosystems. This study aimed to assess the ecological effects of the antibiotic ciprofloxacin on both structural (microbes, phytoplankton, zooplankton, and macroinvertebrates) and functional endpoints of a sub-tropical freshwater ecosystem. Ciprofloxacin was applied at concentrations of 0, 0.5, 5, 50, and 500 µg/L for 21 consecutive days in outdoor mesocosms, followed by a five-week recovery period. Ciprofloxacin significantly affected the structure of microbial, phytoplankton, and zooplankton communities, with calculated NOECs of 5, 0.5, and 5 µg/L, respectively. Notably, the microbial community composition, particularly taxa within the phyla Cyanobacteria and Bacteroidetes, exhibited marked shifts. Among phytoplankton, the filamentous cyanobacterium Cylindrospermopsis sp. exhibited the largest negative response to ciprofloxacin, while Microcystis sp. displayed the largest increase in abundance. Ciprofloxacin exposure also indirectly led to significant increases in zooplankton populations belonging to Cladocera, Copepoda, and Rotifera. Significant effects of ciprofloxacin on physicochemical parameters related to carbon and nitrogen cycling were observed. Structural equation models revealed that ciprofloxacin induced both direct and indirect effects across multiple trophic levels through cascading effects, further impacting ecosystem-level endpoints. Overall, this study provides an understanding of the potential ecological risks posed by antibiotic pollution on freshwater ecosystems.