Comprehensive profiling of quinolone antibiotics in the Bohai Sea: Occurrence, source apportionment, and environmental risks.

Quinolone antibiotics (QNs), widely used in human and veterinary medicine, have emerged as contaminants of growing concern in marine environments. This study investigated the occurrence, sources, and risks of 19 QNs in the Bohai Sea, with 15 compounds detected at high frequencies. Total QN concentrations ranged from 71.6 to 336 ng/L, with ofloxacin and marbofloxacin being the most abundant. QN levels were higher in coastal waters, particularly in Laizhou Bay, indicating dominant land-based sources. Vertically, higher concentrations were observed in bottom waters, likely due to sediment resuspension and limited photodegradation. QN distribution showed strong correlations with salinity, conductivity, water depth, and nutrient levels. Source apportionment using positive matrix factorization identified aquaculture as the main contributor (∼29.2 %), followed by pharmaceutical, domestic, and livestock sources. Ecological risk assessment showed that ofloxacin and norfloxacin pose high risks to algae (total risk quotient, RQT >1), while enrofloxacin and ciprofloxacin may promote antibiotic resistance in hotspots such as Laizhou Bay. Although QN levels have declined compared to those in previous studies, ecological and resistance risks remain, highlighting the need for continuous monitoring and improved source control in coastal environments.