Anthropogenic activities drive the distribution and ecological risk of antibiotics in a highly urbanized river basin.

Although antibiotics are widely detected in river water, their quantitative relationships with influencing factors in rivers remain largely unexplored. Here, 15 widely used antibiotics were comprehensively analyzed in the Dongjiang River of the Pearl River system. The total antibiotic concentration in river water ranged from 13.84 to 475.04 ng/L, with fluoroquinolones increasing from 11 % in the upstream to 38 % in the downstream. The total antibiotic concentration was high downstream and significantly correlated with the spatial distribution of population density, animal production, and land-use type. The total risk quotient of antibiotics for algae was higher than that for crustaceans and fish. Based on the optimized risk quotient method, amoxicillin, ofloxacin, and norfloxacin were identified as priority antibiotics. The key predictors of antibiotic levels were screened through Mantel test, correlation analysis, and multiple regression models. Water physicochemical parameters significantly impacted antibiotics and could be used as easy-to-measure surrogates associated with elevated antibiotics. Cropland negatively affected fluoroquinolones and sulfonamides, whereas urban land exerted positive impacts on fluoroquinolones, β-lactam, and sulfonamides. In the main stream, population, animal production, urbanization status, and economic development had key effects on the distribution of florfenicol, norfloxacin, ofloxacin, and sulfadiazine.