Linking Antibiotic Residues and Antibiotic Resistance Genes to Water Quality Parameters in Urban Reservoirs: A Seasonal Perspective

The interaction between antibiotics and antibiotic resistance genes (ARGs) in freshwater ecosystems has become a critical environmental concern. This study investigates seasonal variations of sulfonamide and tetracycline antibiotics and their relationship with ARGs in three urban reservoirs in Nanjing, China: Pingshan Forest Park, Shanhu Lake Wetland Park, and Zhaoqiao Reservoir. Sampling was conducted in May and September 2023 to assess water quality, antibiotic concentrations, and ARG abundance. A total of 30 water samples were analyzed in regard to their physicochemical parameters, heavy metals, and antibiotics. A quantitative PCR assay was used to measure the ARG abundance relative to the 16S rRNA gene. Sulfonamide concentrations ranged from 30 to 120 ng/L, while the concentrations of tetracyclines were 50–160 ng/L. Notably, sulfamethazine decreased significantly in two reservoirs (Shanhu and Zhaoqiao, p < 0.05), while other antibiotics showed minimal variation, indicating persistent contamination from agricultural runoff and wastewater discharge. ARG abundance was lower in May than in September, with sulfonamide resistance genes being lower cumulatively than tetracycline resistance genes. Strong correlations (r > 0.7) were observed between ARGs and parameters like dissolved oxygen and pH. High antibiotic levels were observed in areas without nearby hospitals or pharmaceutical companies, implicating agriculture as a major pollution source. By analyzing sulfonamide and tetracycline antibiotics and their resistance genes across three eutrophic reservoirs in Nanjing, China, we highlight critical environmental drivers of ARG proliferation and propose targeted mitigation strategies.