Seasonal-Interannual Variations in Surface Dissolved Oxygen Concentrations Along Chinese Coastal Areas and Their Influencing Factors

Based on multi-source in-situ observations and reanalysis datasets spanning from 2017 to 2024, this study systematically investigates the seasonal-interannual variations and underlying drivers of surface dissolved oxygen (DO) concentration in Chinese coastal waters using multiple linear regression and the Shapley value decomposition methods. The results reveal that there is a pronounced latitudinal gradient on the surface of costal, with DO concentrations decreasing progressively from north to south. And DO concentration exhibits significant seasonal-interannual variation characteristics across all regions. Seasonally, the variation amplitude of DO concentration is the strongest in the Bohai Sea and the Yellow Sea, moderate in the East China Sea, and weakest in the South China Sea. Regionally, DO concentration reaches its maximum in spring, minimum in summer, and maintains intermediate level in autumn. In addition, the DO concentration exhibits substantial spatial differences in interannual trends. In spring, the increase of DO concentration is observed in the Bohai Bay, the Liaodong Bay, most of the Yellow Sea, and southern areas of the Yangtze River Estuary, while the decrease of DO concentration occurs in the Laizhou Bay, the northern areas of the Yellow Sea, and the southwestern areas of the East China Sea. In summer, DO concentration increases in southern areas of the Bohai Bay, east areas of the Yangtze River Estuary, and the northern areas of the South China Sea, but decreases in northern areas of the Bohai Bay, coastal areas of the Shandong Peninsulas, the northern areas of the Yellow Sea, and northern areas of the Yangtze River Estuary. In autumn, DO concentration rises in the Bohai Bay, the Yangtze River Estuary, and most of the South China Sea, whereas it declines in most areas of the Yellow Sea and the East China Sea. Annual average data indicates that the upward trend of DO concentration is observed in southern areas of the Bohai Bay, the Liaodong Bay, areas near the Yangtze River Estuary, and most of the South China Sea, and the downtrend is identified in the Laizhou Bay, northern areas of the Bohai Bay and the Yellow Sea, as well as the northern and southern regions of the Yangtze River Estuary. Regarding influencing factors, sea surface temperature (SST) is the dominant control factor on the spatial distribution of DO concentration in spring and summer, while the DO concentration distribution is jointly influenced by SST and photosynthetically active radiation in autumn. The regions with substantial interannual DO concentration variation are as follows: the Laizhou Bay and the southwestern areas of the East China Sea in spring; most of the Bohai Sea, the northern areas of the Yellow Sea, and areas adjacent to the Shandong Peninsulas in summer; northern areas of the Bohai Bay, most of the Yellow Sea and the East China Sea, together with the northern areas of the South China Sea in autumn. For annual average data, the El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) modes show significant relevance with surface DO concentration in the Laizhou Bay, northern areas of the Bohai Bay and the Yellow Sea. Notably, when the Niño3.4 index and the IOD index are both in strong positive phases, SST is also at a relatively high level, coinciding with the lower DO concentration. This comprehensive analysis elucidates the seasonal-interannual variation characteristics and regulatory mechanisms of surface DO concentration in Chinese coastal waters, providing a scientific basis for region-specific and category-based water quality management and ecological conservation strategies.