Table_2_Hypoxia in the Bohai Bay, China: distributions, causes and mechanisms.xlsx

The Haihe River system is a major conduit of land-based pollution into the Bohai Sea in North China, This study presented the first-ever observation of hypoxia in the Haihe River estuary, providing valuable insights into the complex interplay between physical and biochemical factors influencing hypoxia in estuarine systems. The research found seasonal variations in DO distribution in the estuaries of the Haihe and Yongdingxin Rivers. Hypoxia emerged in summer at the bottom of the Haihe River estuary and dissipated in autumn, with the minimum DO concentration of 1.85 mg/L recorded at the bottom of the HH-2 station. Within the hypoxic zone, the average DO concentration was 4.02 mg/L, and the average of DO saturation was less than 0.4. This study also identified the primary drivers of summer hypoxia as a combination of physical and biochemical factors, with water stratification due to hydrodynamic factors limiting exchange and reoxygenation between different water layers. In addition, the decomposition of organic matter in the bottom consumed a significant amount of dissolved oxygen.The significantly higher DO concentration and saturation in autumn compared to summer suggested the possible existence of seasonal hypoxia in the Haihe estuary. This study of hypoxia in the Haihe River estuary contribute to a better understanding of the factors that influence seasonal hypoxia. The findings have important implications for the management and remediation of hypoxia in the Haihe estuary and other estuarine systems.

海河流域是中国北方陆源污染输入渤海的主要通道。本研究首次观测到海河口的缺氧现象（hypoxia），为解析河口系统中影响缺氧的物理与生物化学因子间的复杂相互作用提供了重要见解。研究发现海河与永定新河口的溶解氧（Dissolved Oxygen, DO）分布存在季节变化特征：夏季海河口底层出现缺氧现象，秋季则消散；HH-2站位底层记录到的最低溶解氧浓度为1.85 mg/L。在缺氧区域内，溶解氧平均浓度为4.02 mg/L，平均溶解氧饱和度低于0.4。本研究同时明确了夏季缺氧现象的主要驱动因子为物理与生物化学因子的共同作用，其中水动力因素引发的水体层化限制了不同水层间的物质交换与复氧过程；此外，底层有机质的分解会消耗大量溶解氧。秋季溶解氧浓度与饱和度显著高于夏季，表明海河口可能存在季节性缺氧现象。本项针对海河口缺氧现象的研究，有助于更深入地解析影响季节性缺氧的各类因子，其研究结果对于海河口及其他河口系统的缺氧治理与修复工作具有重要的指导意义。