Evolution of oxygen-consuming pollution and dissolved organic carbon export in Dongjiang Lake Basin

Oxygen-consuming pollution in waters threatens the balance of aquatic ecosystems and affects aquatic carbon transport and regional carbon cycling. Based on continuous monitoring data from 2015 to 2024, this study employed the Pearson correlation coefficient method, Spearman rank correlation coefficient method, and Mann-Kendall trend test to systematically analyze the spatiotemporal evolution patterns and influencing factors of oxygen-consuming pollution in the inflow rivers, lake area, and outlet sections of the Dongjiang Lake Basin, a critical strategic drinking-water source, over the past decade. Oxygen-consuming indicators and the respiratory quotient were used to estimate dissolved organic carbon (DOC) concentration to further quantitatively assess its export flux and relative role in regional carbon budgets. The results showed that the ranges of dissolved oxygen (DO), permanganate index (COD_Mn), five-day biochemical oxygen demand (BOD₅), and ammonia nitrogen (NH₃-N) at the five river sections were 4.2-11.7, 0.2-4.3, 0.1-3.9, and 0.01-0.92 mg/L, respectively; while at the two lake sections, they were 6.2-11.5, 0.3-2.0, 0.1-2.4, and 0.02-0.42 mg/L, respectively. Although oxygen-consuming pollution remained generally stable, it occasionally exceeded Class Ⅰ (for lakes) or Class Ⅱ (for rivers) water quality targets of Environmental Quality Standards for Surface Water (GB 3838-2002) in some months. Oxygen-consuming substance content was higher in the rivers than in the lake and showed a decreasing trend; however, the COD_Mn content in the lake did not decrease synchronously and even increased, suggesting potential contributions from endogenous sources. Temperature was significantly and negatively correlated with DO at most sections, dominating the seasonal variation in DO, whereas the influence of oxygen-consuming substances on DO variations lacked systematic regularity at the specific section scale. DOC concentrations in the waters of the basin were 0.10-0.31 mg/L (calculated as carbon), with a unit DOC export of 0.098-0.292 t/(km²·a), which was relatively low in the Yangtze River basin. DOC export from the basin was estimated to be 453.7-1 353.4 t/a, accounting for 0.1%-0.3% of the net ecosystem productivity (NEP) of the basin, highlighting a characteristic of "strong terrestrial carbon sequestration-weak aquatic carbon export". This study provides a scientific basis for the integrated management of oxygen-consuming pollution and regional carbon budgets.