Integrated Monitoring of Water Quality, Metal Ions, and Antibiotic Residues, with Isolation and Optimization of Enrofloxacin-Degrading Bacteria in American Shad (Alosa sapidissima) Aquaculture Systems

This study investigated water quality, metal ion concentrations, and antibiotic residues specifically enrofloxacin (ENR) and its metabolite ciprofloxacin (CIP), across six American shad (<i>Alosa sapidissima</i>) aquaculture sites over a one-year period. Water and sediment samples were analyzed to determine contamination levels, and ENR-degrading bacteria were isolated from the culture environment to explore their potential use in bioremediation. Findings showed that NH₃-N and total suspended solids (TSS) exceeded recommended standards at all sampling sites. Elevated levels of Li, Na (except S1), Fe, Ni (except S2 and S4), Sr, and Cu were found at site S3. Site S5 recorded the highest concentrations of Al, As, and Pb, while Cd was most abundant at S6. In sediments, S5 showed higher levels of Mg, K (except S3), Ca, Cr, Mn, Fe, Ni, As, Pb, Cu, and Zn (except S3). ENR and CIP were detected in all water and sediment samples, with a 100% detection rate. The highest ENR (16.68-3215.95 mg·kg^-1) and CIP (3.90-459.60 mg·kg^-1) concentrations in water occurred at site S6, following a seasonal pattern of autumn > winter > summer > spring. In sediments, the maximum ENR (41.43-133.67 mg·kg^-1) and CIP (12.36-23.71 mg·kg^-1) levels were observed in spring. Two ENR-degrading bacterial strains were successfully isolated and identified as <i>Enterococcus</i> and <i>Bacillus</i>. Optimal degradation was achieved at 30 °C, pH 8.0, 6% inoculum, and 3000 Lux, resulting in a 64.2% reduction in ENR after 72 h. Under slightly different conditions (25 °C, pH 10), degradation reached 58.5%. This study provides an efficient strain resource for the bioremediation of ENR pollution in the aquaculture water of American shad.