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General Session 1: Physical oceanic processes: Dynamics and physical-biological-biogeochemical interactions |
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Mechanisms leading to the frequent occurrences of hypoxia off the Changjiang estuary in summer
GS1-36
Qinsheng Wei* , First Institute of Oceanography, State Oceanic Administration
Baodong Wang, First Institute of Oceanography, State Oceanic Administration
Zhigang Yu, Ocean University of China
Jianfang Chen, Second Institute of Oceanography, State Oceanic Administration
Presenter Email: weiqinsheng@fio.org.cn
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| Based on literature data and shipboard observations, this study investigated the main environmental characteristics of the seafloor topography, current field, front, and upwelling that are closely related to hypoxia occurrence off the Changjiang estuary. The physical processes of the plume front and upwelling off the Changjiang estuary in summer were coupled. The vertical distribution pattern of the plume front was closely related to the upwelling. By reviewing and analyzing the historical summer hypoxia events off the Changjiang estuary, we statistically demonstrated the spatial structure of the horizontal distribution of the hypoxic zone and investigated the location details of the occurrence zone of the hypoxia. We found that the dissolved oxygen (DO) concentration on the inner continental shelf off the estuary showed a ¡°V¡± shape in relation to station depth. Therefore, we noted that the hypoxic water on the inner continental shelf mostly occurred on the slopes with steep seafloor topography. Based on the current understanding of the hypoxic mechanisms off the Changjiang estuary, we analyzed the biogeochemical mechanisms that could cause the steep terrain off the Changjiang estuary to become the main areas prone to summer hypoxia and explained the internal relations between the location of the hypoxic zone on the slopes and the plume front and upwelling. The plume front and upwelling off the Changjiang estuary and their coupling were important driving forces of summer hypoxia. The continuous supply of nutrients affected by the interaction of the plume front extension of the Changjiang Diluted Water (CDW) and upwelling and the favorable light conditions were important mechanisms causing the phytoplankton blooms and benthic hypoxia off the Changjiang estuary in summer. By analyzing oxygen utilization, organic carbon mineralization, and nutrient regeneration in the hypoxic zone, we observed that the significant oxygen utilization process off the Changjiang estuary in summer also mainly occurred near the steep slopes with front and upwelling features and confirmed the apparent nutrient loss in the benthic hypoxic zone. Meanwhile, our analysis revealed that the sediment resuspension in the benthic boundary layer in the mud areas off the Changjiang estuary could also affect the oxygen utilization and mineralization of organic carbon and nutrient recycling and regeneration. |
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