Variability of Particulate Organic Carbon Stock in the South China Sea With Reference to Profile Structures

Abstract Accurate characterization of particulate organic carbon (POC) stock is essential for understanding regional biological pump processes, yet the roles of vertical structure and integration depth in regulating POC stock variability remain unclear in marginal seas. Using a performance‐evaluated 3D POC data set, we assessed POC stock changes in the South China Sea. Column‐mean POC covaries tightly with surface POC ( R > 0.9) in both the euphotic zone and the upper 100 m. From 2003 to 2023, POC in the upper 100 m showed a basin‐wide decline (−0.1 mg·m −3 ·y −1 ). For euphotic‐zone POC stock, vertical structure accounts for only ∼30% of its variability, whereas euphotic depth exerts dominant control, driving a summertime maximum and a positive changing rate (3.68 mg·m −2 ·y −1 ) in 2003–2023. These non‐steady‐state stock changes highlight the need to jointly evaluate POC production, storage, and export when assessing local biological carbon pump dynamics.