Vertical hydrological processes act as the primary driver of organic matter dynamics in frontal zones

Abstract Previous studies of organic matter (OM) dynamics in frontal zones have emphasized biological and chemical controls, whereas the role of vertical physical processes remains insufficiently understood. Here, we integrated an isopycnic mixing model and principal component analysis with biogeochemical measurements to investigate OM cycling in the Luzon Strait frontal system. Four subregions were identified: cyclonic eddy core (CEC), cyclonic bend (CB), anticyclonic eddy core (AEC), and anticyclonic bend (AB), each characterized by distinct vertical regimes. Each subregion exhibited distinct OM transformation pathways governed by different mechanisms. (1) Upwelling vs. Downwelling : Upwelling (strong in CEC, weak in CB) enhanced photosynthesis by supplying nutrient‐rich waters to the euphotic zone, whereas downwelling (strong in AEC, weak in AB) suppressed it; AB showed concurrent strong upwelling and downwelling due to submesoscale fronts. (2) Stratification vs. Mixing : Stratification promoted photosynthesis and semi‐labile dissolved OM (DOM) accumulation by trapping chlorophyll‐ a and balancing light and nutrients, while mixing accelerated fresh particulate OM (POM) and semi‐labile DOM degradation by prolonging oxygen exposure. Stratification and mixing coexisted in CEC, CB, and AEC, but AB was dominated by intense mixing from submesoscale dynamics. (3) Total suspended matter ( TSM ) modulation : Elevated TSM facilitated DOM‐to‐POM conversion via mineral adsorption, with mesoscale eddies enhancing TSM in CEC and AEC by trapping terrigenous inputs. Our findings demonstrate that vertical hydrological processes are fundamental in controlling the transformation, degradation, and ultimate sequestration of organic carbon in frontal systems, providing a mechanistic basis for refining carbon cycle models and assessing ocean‐based carbon management strategies.