This study assessed the effects of typhoons on sea surface pCO₂ and CO₂ flux in the northern South China Sea (SCS). During the passage of three major typhoons from May to August 2013, sea surface pCO₂, surface seawater temperature (SST), and other meteorological parameters were continuously measured on a moored buoy. Surface water in the region was a source of CO₂ to the atmosphere with large variations ranging from hours to months. SST was the primary factor controlling the variation of surface pCO₂ through most of the time period. Typhoons are seen to impact surface pCO₂ in three steps: first by cooling, thus decreasing surface pCO₂, and then by causing vertical mixing that brings up deep, high‐CO₂ water, and lastly triggering net uptake of CO₂ due to the nutrients brought up in this deep water. The typhoons of this study primarily impacted air‐sea CO₂ flux via increasing wind speeds. The mean CO₂ flux during a typhoon ranged from 3.6 to 5.4 times the pretyphoon mean flux. The magnitude of the CO₂ flux during typhoons was strongly inversely correlated with the typhoon center distance. The effect of typhoons accounted for 22% of the total CO₂ flux in the study period, during which typhoons occurred only 9% of the time. It was estimated that typhoons enhanced annual CO₂ efflux by 23–56% in the northern SCS during the last decade. As such, tropical cyclones may play a large and increasingly important role in controlling CO₂ fluxes in a warmer and stormier ocean of the future.

Sediment oxygen consumption (SOC) is important in modulating the oxygen budget in the East China Sea where seasonal hypoxia occurs. Porewater advection, molecular diffusion and bioturbation supply oxygen for sedimentary organic matter degradation. A pelagic‐benthic coupled model was applied to quantify the SOC. A comparison with observations showed good model performance in reproducing the hydrographic and ecological environments, particularly for the interannual variation in the hypoxic zone. Simulation results show that porewater‐advection‐induced flux is the predominant component of the SOC in sandy areas on the Changjiang bank and outer shelves, while the bioturbation‐induced flux is predominant at mud depocenters. By comparing SOC to the water oxygen consumption (WOC) below the pycnocline, the contribution of SOC is generally below ∼40% in the hypoxic zone. The spatial distribution of SOC in summer is relatively steady from year to year, while the high WOC patches explain more about the interannual variation in the hypoxic zone. WOC rather than SOC drives the variation of hypoxia. Particularly on the Changjiang bank, milder hydrodynamics are favorable for both the higher WOC and bioturbation‐induced benthic oxygen flux but substantially suppress the porewater advective flux, which results in the net lower contribution of SOC to hypoxia. This finding may shed light on other pelagic‐benthic coupling processes in coastal shelf seas where hypoxia occurs on permeable sediments.

The sea surface partial pressure of CO₂ (pCO₂) and air-sea carbon flux in estuarine and bay areas, influenced by both natural and anthropogenic factors, remain poorly understood and inadequately assessed. This study, based on seasonal underway observations conducted in 2024, analyzed the seasonal variations in surface seawater pCO₂ and air-sea CO₂ flux in the high-turbidity coastal waters of Zhejiang, including Hangzhou Bay (HZB), Xiangshan Bay (XSB), Sanmen Bay (SMB), and the nearshore waters (NSW). The results indicate that the pCO₂ in the study area ranged from 194 to 739 μatm throughout the year, exhibiting significant spatiotemporal heterogeneity. In HZB, the lowest pCO₂ was observed in winter, averaging 453 μatm, whereas the values in spring and summer were around 600 μatm, with a subsequent decline to 481 μatm in autumn. In XSB, pCO₂ reached its minimum in winter (194 μatm), attributed to vigorous biological activity, and peaked in spring, averaging 639 μatm. In SMB, pCO₂ was relatively lower in autumn and winter (~470 μatm), and higher in spring and summer (~640 μatm). In the NSW, pCO₂ was lower in winter and spring (~445 μatm), and increased to ~510 μatm in summer and autumn. The pCO₂ was predominantly regulated by sea surface temperature and horizontal mixing, while other factors like biological activity also had significant impacts. The annual average CO₂ flux was 6.0±3.7 mmol m^-2 d^-1 in HZB, 1.2±2.3 mmol m^-2 d^-1 in XSB, 7.0±3.2 mmol m^-2 d^-1 in SMB and 5.2±5.9 mmol m^-2 d^-1 in the NSW. Higher wind speeds in autumn and winter, coupled with elevated the pCO₂ difference between the surface water and the atmosphere (ΔpCO₂) in spring and summer, collectively drove the seasonal variations in CO₂ flux. On an annual scale, both the estuarine and bay areas and the nearshore regions functioned as carbon sources.

Elucidating the compositions, sources and mixing processes of dissolved organic matter (DOM) is crucial for a gaining deeper understanding of the coastal carbon cycle and global carbon budget. Hangzhou Bay (HZB), a vital estuary in China, receives freshwater inputs in the upper bay, borders the Changjiang River Estuary (CRE) to the north and is adjacent to Zhoushan Islands Region (ZIR) to the east. In HZB, the DOM sources and their compositions in estuaries remain unclear due to the complexity of this dynamic environment. In this study, we aimed to explore the chemical composition and sources of the DOM in the HZB and its adjacent coastal waters based on chromophoric DOM, fluorescent DOM indices and other hydrochemical parameters in the winter. The results showed that the DOM compositions in HZB have significant differences in the upper bay, middle bay and lower bay. The highest concentration of DOC was found in the CRE, close to the northern lower HZB, with high humification index (HIX), low biological index (BIX) and high proportion of humic-like fluorescent component (C1), indicating terrestrial inputs. In contrast, the DOM in the upper bay had high BIX and low HIX, being dominated by protein-like fluorescent components (C2 and C3), indicating an autochthonous source. The DOM in the middle bay showed mixed composition characteristics indicated by the chromophoric DOM (CDOM) and fluorescent DOM (FDOM) indices. Moreover, the terrestrial DOM transported via CDW intrusion accounted for a large proportion of the DOM in Northern HZB. Our study shows that, even in coastal estuaries with very strong hydrodynamics, the DOM composition can still retain its unique source signal, which, in turn, affects its migration and transformation processes. The results of this study provide supplement insights into the global carbon cycle and carbon budget estimation.