Identifying key environmental factors for enhancing the pollutant removal potential at a river confluence

The confluence area of river networks is a hot spot for pollutant removal. As an essential part of the river ecosystem, sediment bacterial communities played a crucial role in the removal of pollutants. However, how the potential of sediment bacterial communities can be enhanced toward the removal of pollutants remains unclear. Therefore, this study provides a new approach for the identification of key environmental factors that enhance the pollutant removal potential at a river confluence, integrating the bacteria-based index of biotic integrity (Ba-IBI), path model, support vector regression (SVR) model, and sensitivity analysis. The developed Ba-IBI could quantitatively evaluate the differences of both structure and function of bacterial communities before and after the confluence, with a range from 1.52 to 2.78. The flow regime, which was represented by the Froude number, exerted an indirect effect on Ba-IBI mediated through water nutrients and sediment nutrients according to path model results. Sediment nutrients and water nutrients were considered as the main environmental factors that directly affected sediment bacterial communities. A function that could predict the response of sediment bacterial communities to environmental factors in the best possible way was found through SVR modeling, with R² = 0.8357. The results of the sensitivity analysis indicated that the total phosphorus in water and ammonia nitrogen in sediments were key environmental factors for enhancing the pollutant removal potential at the river confluence. The established approach aids the improvement of the bioremediation potential of river confluence area, and might provide a theoretical basis for watershed restoration.