Ecotoxicity of four prevalent antibiotics to saltwater species and their ecological risks in global estuaries.

Estuaries, where upstream freshwater tributaries converge with marine systems, serve as accumulation hotspots for anthropogenic pollutants. With rising antibiotic usage, concerns over their hazardous impacts in estuaries are growing. Risk reduction and management strategies for pollutants rely on water quality benchmarks. However, saltwater benchmarks for antibiotics are lacking due to insufficient saltwater ecotoxicity data for constructing species sensitivity distribution (SSD) models. This study evaluated the ecotoxicity of four prevalent antibiotics (tetracycline, oxytetracycline, trimethoprim and norfloxacin) on eight saltwater species, generating new data to derive robust saltwater predicted no-effect concentrations (PNECs) using SSDs. Their ecological risks in global estuaries were also assessed. Results revealed that the four algal species were sensitive to tetracycline, oxytetracycline and trimethoprim (EC50: 8.34 - 30.13 mg/L), but showed low sensitivity to norfloxacin. The rotifer and copepod species showed varied sensitivities (LC50: 7.24 - 139.10 mg/L), while the fish species was tolerant to all four antibiotics. Hazard rankings based on derived chronic saltwater PNECs were: oxytetracycline (PNEC: 0.05 mg/L) > trimethoprim (0.07 mg/L) > tetracycline (0.08 mg/L) > norfloxacin (0.09 mg/L). Although ecological risk assessments indicated very low risk from these antibiotics in most global estuaries, a limited number of estuaries (n = 15) primarily in Asia and Africa showed risks ranging from low to medium. This study provides essential saltwater ecotoxicity data for establishing interim saltwater quality criteria for four broadly applied antibiotics and offers a timely overview of their ecological risks in estuaries worldwide, supporting the risk management of antibiotic contamination in global estuaries.