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Based on observations in the Bohai Sea (BS) and North Yellow Sea (NYS) obtained during the late summer and early autumn of 2021, the hypoxic characteristics and influencing factors were explored by analyzing the spatial patterns of temperature, salinity, density, dissolved oxygen (DO) and nutrients, revealing the regulation mechanisms of hydro-biogeochemical processes on the distributions of DO and hypoxia. A narrow hypoxic zone (with the minimum DO mass concentration of 2.18 mg/L) in a southwest-northeast orientation, which was characterized by a high apparent oxygen consumption (AOU) (＞4 mg/L), was observed within the bottom cold water-dominated region in the western BS; a low-DO area with a small scope existed in the northeastern region off the Bohai Bay mouth at 10-m layer. DO concentrations in the NYS were overall higher than that in the BS, and a relatively high DO level was maintained within the Bottom Cold Water Mass (BCWM) in the central NYS, although there was a high AOU (＞2.5 mg/L). The low-lying topography and higher stratification intensity within the bottom cold water-dominated region in the western BS provided the basis for the formation and maintenance of bottom hypoxia, and the fronts around this cold water significantly controlled the boundary and extension scope of the hypoxic zone. The low-DO area at 10-m layer in the northeastern region off the Bohai Bay mouth was caused by the uplift of the hypoxic cold water from the bottom. The presence of an anticyclonic eddy in the shallow bank of the central BS led to the formation of a DO-rich water (＞6 mg/L) at bottom. The decomposition of organic matter and related oxygen consumption in the context of stratification was an important material basis for the formation of hypoxia in the western BS, exactly corresponding to a high-nutrient area at bottom layer. In contrast, due to the higher background levels of DO within the BCWM and the lower AOU than that in the bottom layer of the western BS, it was difficult to form hypoxia in the central NYS even under the relatively high stratification strength; moreover, the deeper water depth was another factor responsible for this situation. This study could provide a scientific basis for understanding the multi-scale variations and regulation of DO in the BS and the NYS, laying a foundation for the subsequent refined simulation and prediction of hypoxia in this sea area.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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