Dissolved organic carbon (DOC) and Particulate organic carbon (POC) in the ocean are key components of the global carbon cycle, and their distribution and transformation profoundly influence the ocean carbon sequestration. The Kuroshio Extension region of the northwestern Pacific harbors one of the world's strongest western boundary current systems, where organic carbon cycling processes are particularly complex. In May, 2023, this study conducted simultaneous observations of vertical variations in DOC and POC concentrations at two typical stations in this region and combined analysis of key environmental parameters (including temperature, salinity, and nutrient concentrations) to clarify the distribution characteristics and dominant controlling factors of DOC and POC. The results show that both DOC and POC concentrations exhibited a consistent trend of significant decrease with increasing depth. DOC concentrations stabilized below approximately 1 000 m, while POC concentrations stabilized below about 300 m. This distribution pattern is jointly driven by phytoplankton primary production and microbial degradation.The vertical distributions of DOC and POC showed distinct north-south differences: in the waters above 1 000 m, DOC concentrations at the Kuroshio-dominated station in the south were generally higher than those at the Kuroshio-Oyashio mixed-water station in the north; the high-value areas of POC were observed in the 75~300 m water layer at the southern station, but confined to the waters above 75 m at the northern station. Moreover, there was a significant positive correlation between DOC and POC concentrations. This correlation at the southern station was mainly regulated by physical processes such as water column stratification and microbial-dominated biogeochemical processes, whereas at the northern station it was further modulated by the phytoplankton-driven classical food web on the basis of the aforementioned processes. Overall, the distribution and interaction of organic carbon in the Kuroshio Extension region are shaped by regional differences in hydrological and biological processes. This study identifies the key controlling factors of variations in DOC and POC concentrations across different hydrographic settings, providing an important basis for further advancing the understanding of carbon cycling processes and mechanisms in the open ocean.

Coastal bays are important carbon sink areas. The productivity and community structure of the main primary producer, phytoplankton, are significantly affected by human activities. Understanding the long-term change patterns of phytoplankton productivity and community structure in typical bays is an important basis for clarifying the influence of phytoplankton on the carbon sink function of bays. Sanggou Bay is one of the most representative farming areas in China. However, the long-term change patterns and controlling factors of phytoplankton productivity and community structure under different aquaculture conditions in Sanggou Bay are still unclear. In this study, we analyzed sediment cores at four stations in Sanggou Bay (i.e., stations B2 and C2 in the shellfish culture area, station B4 in the shellfish-kelp integrated culture area, and station A6 in the area without aquaculture) to investigate the long-term change patterns and controlling factors of phytoplankton productivity and community structure. Our results showed that phytoplankton productivity in the shellfish culture area showed an increasing trend, which might be related to the terrestrial input and the increased activities of shellfish culture (accelerated dissolved inorganic nitrogen (DIN) cycling). Phytoplankton productivity in the shellfish-kelp integrated culture area showed a hump-shaped change, which was significantly influenced by the change in DIN (p < 0.05) and the integrated culture of shellfish and kelp. Phytoplankton productivity in the area without aquaculture was relatively low, while it showed an upward trend from the 5 cm sediment layer to the surface, which might be attributed to the increased DIN and changes in hydrodynamics. Overall, there were significant regional differences in phytoplankton productivity, that is, phytoplankton productivity was significantly higher in the shellfish-kelp integrated culture area than the area without aquaculture during the last 40 years (p < 0.05), and it was significantly higher in the shellfish-kelp integrated culture area than the shellfish culture area during the last 10 years (p < 0.05). Our results indicate that the shellfish-kelp integrated culture model has significant potential for oceanic organic carbon sink. In addition, the productivity ratio of diatoms to dinoflagellates in Sanggou Bay showed an overall upward trend, which may be influenced by the decreased ratio of nitrogen to phosphorus (N/P), the increased ratio of silicon to nitrogen (Si/N), the increased dissolved organic phosphorus (DOP), the decreased temperature, aquaculture activities, and hydrodynamic changes. The above findings provide a scientific basis for further elucidating the carbon sink potential of aquaculture bays.

The interaction of riverine inputs and ocean current systems causes complex spatiotemporal variations in phytoplankton dynamics in marginal seas of the northwest Pacific Ocean, yet quantitative assessments of these variations and their causes remain limited. Here we evaluate phytoplankton biomass and community structure changes using lipid biomarkers, accompanying ocean circulation and nutrient variations in surface waters collected in spring and summer of 2017–2018 at 118 sites in the East China Sea off the Zhejiang coast. High biomass of diatoms, inferred from brassicasterol concentrations, shifted from the south in spring to the north in summer, while high dinoflagellate biomass, inferred from dinosterol concentrations, occurred mainly in the Changjiang (Yangtze) River plume and adjacent areas in both seasons. Seasonal variation in phytoplankton distribution was linked to the spatial extents of water masses such as the Changjiang Diluted Water (CDW) and the intrusion of the Kuroshio Subsurface Water (KSSW). A three end-member mixing model was applied to quantify water mass contributions. The results showed that an increase in the KSSW (from 0 to 40%) and a decrease in the CDW (from 100 to 20%) resulted in a significant (20%) increase in diatom proportions and a significant (20%) decrease in dinoflagellate proportions. Dinoflagellate proportions were highest in the CDW-dominated region, while diatoms and total phytoplankton biomass were higher in the CDW–KSSW mixing region and the KSSW-dominated region. This study highlights the dynamic response of the phytoplankton community to water mass changes in marginal seas that can aid coastal ecosystem management.