Vertical Distribution and Controlling Factors of Dissolved and Particulate Organic Carbon in the Kuroshio Extension of the Northwestern Pacific

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.