9:30 AM - 9:45 AM
[AOS13-03] Formation and Dynamics of Low-oxygen Waters at the Time Series Station in East China Sea in 2023: A Chemical Approach
Keywords:Low oxygen, middle-bottom waters, temporal variation, East China Sea
The East China Sea (ECS) is vital in supplying biological resources, not only for its local fishery and biodiversity but also for seas around Japan via the Kuroshio Current. Low-oxygen waters during summer months have been widely identified in the outer shelf of the ECS from 2018-2020 and currently in 2023. Low-oxygenated waters are dangerous for marine life and can result from multiple factors such as formation, dynamics, and biological processes. It is thus urgent to clarify the formation and dynamic of low-oxygen waters in a temporal observation. Interestingly, in the 2023 observation, strong positive correlations between low-oxygen bottom (and subsurface) waters and turbidity are much more prominent than in 2018, 2019, and 2020 observations, indicating that biological processes and water dynamics might be the dominant controlling factors of this water. Seawater, sediment, and porewater samples were collected to be analyzed chemically and biologically. Some preliminary results are as follows: (1) based on the simple water mixing ratio calculated by combining temperature, salinity, and Heavy Rare Earth Elements (HREEs), it is found that Kuroshio Subsurface Water (KSSW) and Kuroshio Intermediate Water (KIW) contribute significantly to the middle and bottom water, accounting for 73.5 and 73.2% respectively. Moreover, a small yet significant porewater contribution, accounting for a few percent, was also detected due to its high nutrient concentration. (2) Across the 24-hour time series observation, the dynamics of KSSW and KIW varied slightly. Furthermore, nutrient (DIN, DIP) contributions in these water layers are further evaluated to quantify the material budget in the area.