Spatiotemporal variations and controlling mechanism of low dissolved oxygen in a highly urbanized complex river system

Study region: Dongjiang River Network (DJRN), a complex urbanized river network in the Pearl River Basin, China. Study focus: Low-oxygen conditions have been expanding in urbanized river systems, whereas a clear and quantitative understanding on the deoxygenation processes is still lacking. This study utilized a well-validated physical-biogeochemical model to investigate the oxygen dynamics combined with river ecosystem metabolisms over an annual cycle and explicitly quantify the contribution of major oxygen-depleting substances from different sources to low-oxygen conditions. New hydrological insight for the region: Our results showed significant spatiotemporal variations in low-oxygen extents and oxygen source-sink patterns in the DJRN, where the underlying control mechanisms varied across stream order due to the intricate geographic and hydrological regime shifts in conjunction with diverse pollution stressors. Ascribed to the seasonal variations in anthropogenic pollution and water temperature, the entire DJRN shifted to a completely heterotrophic system with severe oxygen deficits during the late summer and early autumn. Scenario simulations indicated that in line with the substantial wastewater control in the DJRN region, local pollutant loads played a trivial role in the low-oxygen generation, which instead was primarily fueled by organic matter from transboundary delivery and in-situ primary production. Our findings underscored the necessity of co-regional collaborative management on pollutant emissions and the importance of eutrophication mitigation for the sake of oxygen recovery in the urbanized river network.