Recently, the team led by Researcher Bai Yan and collaborators have published a research paper entitled Declining particulate organic carbon flux to estuary yet rising oceanic flux over the past 20 years: a case study of the Pearl River Estuary in the top-tier environmental journal Water Research (Impact Factor = 11.5). The first author is Dr. Wang Zhihong, a postdoctoral fellow of our laboratory, and the corresponding author is Researcher Bai Yan. Co-authors include Researcher He Xianqiang, Dr. Bai Ruofeng, Associate Researcher Li Teng and Associate Researcher Jin Xuchen from our institute.
The transport and burial of terrigenous particulate organic carbon (POC) in estuary-shelf areas is a core scientific issue in global carbon cycle research. Nevertheless, accurate estimation and long-term monitoring of riverine POC flux into the sea face great challenges due to the strong hydrodynamic conditions in estuarine zones. To solve this problem, the research team previously established an estimation method for riverine POC flux integrating remote sensing and numerical models, realizing long-term and high-frequency POC flux estimation of the Yangtze River (Wang, Bai* et al., RSE, 2021). On this basis, they further assessed the impacts of extreme gale events on POC flux and transport in the Yangtze River Estuary (Wang, Bai* et al., RSE, 2023).
As China’s second largest river by discharge volume, the Pearl River delivers massive terrigenous materials to the northern South China Sea. Unlike the Yangtze River which features a wide and single estuary, the Pearl River has a dense river network at its estuary with eight narrow outlets showing distinctly different hydrodynamic characteristics, making its seaward flux estimation extremely difficult.
Based on the previous research framework, this study developed an estuarine POC concentration retrieval method using 30-meter high-spatial-resolution Landsat-7 and Landsat-8 satellite data, as well as a POC seaward flux estimation approach combining remote sensing data and the FVCOM numerical model. A long-term monthly mean dataset of Pearl River POC seaward flux from 2001 to 2020 was constructed, and new insights into the variation patterns of river-entering and seaward POC fluxes over the past two decades were summarized.
The main research findings are as follows:
- In the river basin section, the annual average river-entering POC flux of the Pearl River reached 0.13 Tg C/yr in the past 20 years. Affected by the increasing number of reservoirs and dams within the watershed, both the river-entering POC flux and its seasonal variations showed a continuous declining trend.
- In the estuarine section, approximately 25% of the POC flowing into the Pearl River Estuary was deposited locally. Over the recent decade, driven by reduced river-entering POC flux, increased river runoff and higher proportion of northerly winds in the estuary, the local POC sedimentation volume dropped by 47% compared with the prior decade, while the POC flux transporting into the northern South China Sea increased by 8.7%.
It is thus confirmed that the decrease of river-entering POC flux does not necessarily lead to lower seaward POC flux, as a series of hydrodynamic and biogeochemical processes in estuaries can greatly regulate the final POC export flux to the open ocean.
This study reveals the critical regulatory role of the estuarine "filter effect" in terrigenous carbon transport, and proves that traditional carbon flux estimation based on watershed output cannot accurately represent actual seaward carbon flux. The integrated method combining remote sensing and numerical simulation enables more comprehensive and scientific assessment of estuarine POC transport and accurate calculation of effective riverine POC export flux to the ocean.
Fig.1 Monthly estimated river-entering POC flux of the Pearl River from January 2001 to December 2020 (red dotted line). Yellow bars represent river sediment flux sourced from China River Sediment Bulletin.
Fig.2 Schematic diagram of climatological annual mean POC transport flux across sections in the Pearl River and its adjacent sea areas between 2001–2010 (grey figures) and 2011–2020 (black figures). Red arrows stand for river-entering POC flux into the Pearl River Estuary; blue arrows denote POC transported inside the estuary and exported to the northern South China Sea; orange arrows indicate local POC sedimentation flux within the estuary.
Fig.3 POC transport and sedimentation variations in the Pearl River Estuary from 2001 to 2020. Blue dotted line: annual river-entering POC flux into the estuary; red dotted line: annual POC flux exported to the northern South China Sea across transects; yellow bars: annual POC sedimentation flux in the estuary (positive values for deposition, negative values for erosion).
Paper Citation
Zhihong Wang, Yan Bai*, Xianqiang He, Ruofeng Bai, Teng Li, Xuchen Jin. (2025). Declining particulate organic carbon flux to estuary yet rising oceanic flux over the past 20 years: a case study of the Pearl River Estuary. Water Research, 277, 123332.
Related Publications
[1] Zhihong Wang, Yan Bai*, Xianqiang He, Hui Wu, Ruofeng Bai, Teng Li, Bozhong Zhu, Fang Gong. (2023). Assessing the effect of strong wind events on the transport of particulate organic carbon in the Changjiang River estuary over the last 40 years. Remote Sensing of Environment, 288: 113477.
[2] Zhihong Wang, Yan Bai*, Xianqiang He, Bangyi Tao, Teng Li, Xiaoyan Chen, Tianyu Wang, Fang Gong. (2021). Estimating particulate organic carbon flux in a highly dynamic estuary using satellite data and numerical modeling. Remote Sensing of Environment, 252, 112116.
