Acidification state and interannual variability in marginal sea: A case study of the Bohai and the Yellow Seas surface waters in April 2023.

The acidification of the marginal seawater was a more intricate process than the ocean. Although some studies have been done on seasonal acidification in the bottom water of Chinese marginal seas, research on surface water acidification has still been insufficient. We analyzed the acidification properties and controlling factors in the Bohai Sea (BS) and Yellow Sea (YS) surface water during April 2023. The observation showed that the average surface water pH of the BS, North Yellow Sea (NYS), and South Yellow Sea (SYS) were 8.09 ± 0.06, 8.13 ± 0.05, and 8.15 ± 0.05. Phytoplankton significantly impacted pH and Ωarag, while riverine inputs and biological activity played a vital role in controlling DIC and TA. The Yellow River significantly impacted the BS. The North Yellow Sea Cold Water Mass had a limited impact on acidification, while the South Yellow Sea Cold Water Mass significantly affected the SYS. Regarding seasonal fluctuations, Ωarag was significantly higher in summer than in other seasons. DIC and TA showed different patterns in both the BS and YS, with a minimal fluctuation in pH. Over the last two decades, the pH in the BS showed a slight annual decline, and the rate of change was (-1.45 ± 2.19) × 10-5 yr-1. In contrast, the NYS and SYS have slightly risen, with rates of change of (2.39 ± 1.24) × 10-5 and (1.23 ± 0.76) × 10-5 yr-1. We believed that surface water acidification in the BS and YS did not follow the expected trend of significant acidification observed in open oceanic regions. Instead, the acidification process in these marginal seas was dominated by local factors such as riverine inputs, biological activity, and cold water masses, resulting in minimal pH changes over the last two decades.

Nutrient characteristics driven by multiple factors in large estuaries during summer: A case study of the Yangtze River Estuary.

Nutrients directly control the level of primary productivity and are crucial for the stability of marine ecosystems. Focusing on the survey results in August 2020 of the Yangtze River Estuary, this study elucidated the distribution characteristics and controlling factors of three nutrients: NO3-N, PO4-P, SiO3-Si. The results showed that the concentrations of NO3-N, PO4-P, SiO3-Si in the study area were generally higher near the shore than far shore, with average concentrations of 11.40, 0.70, and 23.73 µmol/L, respectively. The ocean currents drove the distribution of nutrients, and the transport of CDW and YSCC increased the nutrient levels. The resuspension of sediment caused by factors such as terrain and weather may lead to an abnormal increase in nutrients in the bottom waters. The main controlling factors of the three nutrients were different. NO3-N was significantly affected by human activities, PO4-P and SiO3-Si were mainly affected by natural factors.

Different fates of particulate matters driven by marine hypoxia: A case study of oxygen minimum zone in the Western Pacific.

The oxygen minimum zone (OMZ) is an important representative of marine hypoxia in the open ocean, and it is developing rapidly under the context of global warming. However, the research on OMZ in the Western Pacific is still deficient. This study focused on its basic characteristics and impact on the degradation of particulate matters in the M4 seamount of Western Pacific. The results showed that the OMZ is located at 290-1100 m, just below the high-salinity area and thermocline. The M4 seamount has a weak impact on the OMZ, and only the bottom waters contacting with the seamount have a weak decrease in dissolved oxygen (DO). With the increase of water depth, particulate nitrogen and phosphorus decrease first above and in the OMZ and then increase below the OMZ, while particulate organic carbon (POC) gradually decreases. The low-DO environment in the OMZ is not conducive to the degradation of particulate matters, which promotes the transport of particulate matters to the deep sea, and most particulate matters have the lowest degradation rate here. The waters above the OMZ have the fastest change rate of particulate matters, in which particulate organic phosphorus (POP) and particulate inorganic phosphorus (PIP) are preferentially degraded, and the degradation rate of them is significantly higher than particulate organic nitrogen (PON) and particulate inorganic nitrogen (PIN). The particulate nitrogen and phosphorus in the waters below the OMZ continue to increase, while PON/total particulate nitrogen (TPN) and POP/total particulate phosphorus (TPP) increase significantly, and the increase rate of PIN and PIP is far lower than PON and POP, indicating that the increase of organic matters in particulate matters is more significant. It is speculated that this phenomenon might be related to the input of Antarctic Bottom Water or the in-situ production by microorganisms. This study revealed the relationship between OMZ and different particulate matters, which may provide a valuable pathway for the biogeochemical effects of OMZ in the Western Pacific.