Seasonal and spatial variations of dissolved organic matter in the East China Sea: Insights from optical spectroscopy and ultrahigh-resolution mass spectrometry
Zekai Ji, Hong-Sheng Cao, Fang Cao
ABSTRACT
Marginal continental shelves store abundant dissolved organic matter (DOM), whose cycling and transformation strongly modulate global carbon budgets. We sampled surface, mid-layer and bottom waters in the East China Sea during spring (May) and autumn (October) cruises in 2023. Combined bulk dissolved organic carbon (DOC) measurements, optical spectroscopy and ultrahigh-resolution mass spectrometry were used to resolve seasonal and spatial DOM variations. Surface waters held the highest DOC concentrations, with steeper cross-shelf gradients in spring. Chromophoric DOM (CDOM) accumulated in nearshore areas: CDOM rose with depth in spring but decreased downward in autumn, and bottom CDOM maxima overlapped with the offshore Zhejiang-Fujian mud belt. Autumn offshore CDOM featured low molecular weight and aromaticity, indicating autochthonous marine origins. Spring protein-like fluorophores peaked in nearshore and bottom waters via sediment resuspension, while autumn counterparts distributed evenly across surface and mid layers. Humic-like signals were nearshore-enriched and decreased with depth year-round, primarily of terrestrial origin. Spring DOM heterogeneity was controlled by terrestrial inputs, frontal upwelling and sediment resuspension, generating richer, more diverse DOM molecules. Lower terrestrial loading and enhanced in-situ production in autumn homogenized DOM chemistry and improved its stability. Salinity mixing governed horizontal DOM patterns in spring, while vertical environmental gradients structured autumn DOM vertical profiles. This work improves our knowledge of shelf DOM biogeochemistry and provides observational data to assess the East China Sea’s carbon sink capacity.