Recently, the team headed by researcher Bai Yan of the SIO and the co-authors published the research paper entitled “Three stages in the variation of the depth of hypoxia in the California Current System 2003-2020 by satellite estimation” in the environmental science and ecology TOP journal Science of The Total Environment (IF=10.75). The first author of the paper is Zhang Yifan, a doctoral student jointly trained by the Laboratory of the SIO and Shanghai Jiao Tong University, the corresponding author is researcher Bai Yan of the SIO, and the co-authors include researcher He Xianqiang, Dr. Li Teng, Dr. Jiang Zhiting, and senior engineer Gong Fang from the SIO.

Oxygen (O2) content in the ocean is the restrictive factor of ecosystem productivity, biodiversity and biogeochemical cycle. Under steady-state conditions, O2 exists as dissolved oxygen (DO) in the seawater environment, and DO is very sensitive to changes in the biological cycle, so it is considered to be an important characteristic signal of changes in the marine miogeochemical cycle. At present, the impact of global climate change is evident, and the O2 concentration in the global oceans is continuously declining; For the middle ocean, the abnormal changes of the marine environment and human activities will produce complex biogeochemical responses. One of its impacts can lead to the shallow formation depth of the oxygen minimum zones (OMZs) in the middle ocean, which will have a fatal impact on the stability of ecosystem and the development of fishery economy, especially the coastal marine ecosystem. The depth of hypoxia (DOH, based on DO=60 μmol kg−1) in the middle ocean can be used to determine the degree of longitudinal expansion of OMZ in the water column, evaluate its impact on the middle marine ecosystem, and help identify and predict changes in favorable habitats for aquatic organisms. However, the amount of in-situ DO measurement data is still insufficient to complete the study on the evolution characteristics of DOH in long time series and high temporal-spatial resolution, especially in complex coastal waters.

Satellite remote sensing can provide long time series and large-area synchronous marine ecological environment change information. In this study, a DOH nonlinear polynomial regression inversion model was developed to estimate the DOH in the California Current System (CCS), based on the dissolved oxygen profile observations detected by the BGC-Argo float (from November 18, 2012 to August 31, 2016) deployed at CCS (Figure 1). In the construction process, the physical vertical mixing process regulating the DO balance, and phytoplankton photosynthesis were considered, and satellite-derived net community production and SST remote sensing data were applied. After verification with measured data, the overall result shows R2=0.82 and RMSE=37.69 m (n=80). Then, it was used to reconstruct the variation in satellite-derived DOH in the CCS from 2003 to 2020. From 2003 to 2013, the DOH showed a significant shallowing trend due to the intense subsurface O2 consumption caused by strong phytoplankton production in the CCS coastal region (p<0.05, n=132); The trend was interrupted by two successive strong climate oscillation events from 2014 to 2016 (2014~2015 East Pacific anomalous warming event and 2015~2016 El Nino event), which led to a significant deepening of the DOH and a slowing, or even reversal, of the variations in other environmental parameters. After 2017, the effects of climate oscillation events gradually disappeared, and the shallowing pattern in the DOH recovered slightly. However, by 2020, the DOH had not returned to the pre-2014 shallowing characteristic (Figure 2). In the absence of further human disturbance, the ecosystem will likely gradually return to the pre-2014 environmental pattern, implying the great possibility of the shallowing of the DOH of CCS in the future. The satellite inversion model of DOH in the CCS developed in this study provides a new perspective for exploring the long time series high-resolution marine ecosystem and the spatio-temporal rules of hypoxia change.

Figure 1 (a) Flow field and major economic fish ranges in the CCS (modified from Checkley and Barth, 2009). (b) The working trajectory of BGC-Argo No. 5904021. The movement period is from November 18, 2012 to August 31, 2016. The color represents the observation time, and the green and red asterisks represent the start and end points of buoy operation, respectively. The Box in the figure is 2°×2°.

Figure 2 The satellite retrieved change distribution map of DOH in the CCS from 2003 to 2020, which can be divided into three stages: (a) normal climate stage (2003 - 2013), (b) climate oscillation stage (2014 - 2016) and (c) climate restoration stage (2017 - 2020). The position of Box in Figure (a) is consistent with Figure 1 and coincides with the running trajectory of BGC-Argo. Figures (d-f) show the long time series changes in mean SST, NCT and DOH for three boxes, which are also divided into three stages. The linear fitting result of the change trend p<0.05 is represented by a solid line, and vice versa, it is represented by a dashed line. The gray shadow in the figure shows the running time of BGC-Argo.

Paper Citation:

Zhang, Y., Bai, Y.*, He, X., Li, T., Jiang, Z., & Gong, F. (2023). Three stages in the variation of the depth of hypoxia in the California Current System 2003–2020 by satellite estimation. Science of The Total Environment, 162398. https://doi.org/10.1016/j.scitotenv.2023.162398