Seasonally decoupled dynamics of dissolved organic carbon and fluorescent dissolved organic matter in continental shelf waters

Abstract. We investigated seasonal variations in dissolved organic carbon (DOC) and humic-like fluorescent dissolved organic matter (FDOM_H) in the Yellow Sea and East China Sea to better understand the factors shaping dissolved organic matter (DOM) dynamics in continental shelf waters. Multi-year observations revealed that DOC concentrations increased with Radium based water residence time, whereas FDOM_H showed no corresponding accumulation across seasons. This contrasting behavior was particularly evident during summer, when strong solar radiation may promote preferential degradation of photo-labile FDOM_H, while a relatively more photo-resistant DOC remains in shelf waters, indicating a seasonal decoupling between DOC and FDOM_H. The persistent increase in DOC with water residence time suggests that a fraction of DOC can accumulate in shelf waters despite ongoing degradation processes. Based on the observed relationship between DOC and water residence time, we estimated a net DOC accumulation flux of 1.1±0.2 Tg C yr^-1, comparable in magnitude to annual riverine DOC inputs in this region. Our findings highlight the contrasting dynamics of DOC and FDOM_H and suggest that prolonged water residence and selective DOM transformation may contribute to DOC persistence in continental shelf waters, potentially supporting the export of persistent DOC to the open ocean.