Dissolved oxygen variation in the North China Plain river network region over 2011-2020 and the influencing factors

The 2011-2020 data of dissolved oxygen (DO) and nutrients-total phosphorus (TP), solution reactive phosphorus (SRP), total nitrogen (TN), ammoniacal nitrogen (NH₃-N), and chemical oxygen demand (COD_Cr)-in the North China Plain river network region were analyzed. Moreover, the DO variation trend and the influencing factors were investigated. The results showed that between 2011 and 2020, the DO concentration steadily increased from <3 mg L^-1 to >5 mg L^-1. Negative relationships were observed between the DO and COD_Cr (R = -0.34, p < 0.01), TN (R = -0.41, p < 0.01), NH₃-N(R = -0.40, p < 0.01), TP (R = -0.28, p < 0.01), and SRP (R = -0.19, p < 0.01), indicating that a reduction in the nutrient input promoted the increase in the DO concentration in the past decade. The DO concentration in the rainy season was lower than that in the dry season, which suggests that nonpoint-source pollution caused by heavy storm runoff was the main factor affecting the water quality. The average DO concentration in the suburban rivers (4.88 mg L^-1) was higher than that in urban rivers (3.41 mg L^-1). Furthermore, comprehensive analysis indicated that the loss of riparian buffer, river solidification, pollutant input, and sluice dam operations are the main factors affecting DO concentration decrease and water quality deterioration. Finally, measures for water improvement and DO recovery in the river network region, namely pollutant input control, reduction in the impact of water conservancy projects, and river ecological restoration, were examined. Overall, this study shows that water quality has improved over the past decade. The study results provide a reference for the continuous improvement of water quality and the continuous recovery of DO in river network regions.