关键词: 溶解有机质, 吸收光谱, 荧光光谱, 东海, 穿刺锋面

Abstract:

Dissolved organic matter (DOM) plays an essential role in marine carbon cycling by modulating carbon sequestration and ecosystem dynamics through its degradation and transformation. As the largest marginal sea in China, the East China Sea (ECS) is a critical pathway for export of terrestrially derived DOM to the Pacific Ocean. This dynamic system is influenced by multiple environmental factors, including Yangtze River inputs, Kuroshio intrusion, and anthropogenic activities, which collectively contribute to the complex sources and transformations of DOM. However, the distribution of DOM and its underlying driving factors remain understudied in the ECS. Here, measurements of dissolved organic carbon (DOC) and its optical properties (absorbance of chromophoric DOM, CDOM; excitation-emission matrices on the fluorescent DOM, EEMs on the FDOM) were made in ECS water during a cruise in spring 2023 to provide insights into the mechanisms modulating DOM variability. The results showed strong variations in both quantity and quality of DOM, with the highest DOC concentration and absorption coefficient of CDOM (a_CDOM(355)) close to the coast and decreasing offshore. DOC decreased from the surface to the bottom layer, whereas a_CDOM(355) values showed an increasing trend. Four fluorescent DOM components were resolved by parallel factor analysis: two autochthonous protein-like components, C1 and C2; one terrestrial humic-like component, C3, and one marine humic-like component, C4. The fluorescence intensity of each component decreased as distance offshore increased. Elevated fluorescence intensities of C1 and C2 were observed in the surface and bottom layers, whereas minimum values were observed in the middle layer. The fluorescence intensities of C3 and C4 decreased as water depth increased. Principal component analysis (PCA) enabled differentiation of water samples following hydrodynamic gradient. The penetrating front in the ECS resulted in enhanced cross-shelf transport of terrestrially derived DOM, while phytoplankton blooms significantly altered the amount and compositions of DOM. Overall, the DOM in the ECS in spring 2023 was primarily of terrestrial origin, while autochthonous production, microbial transformation, and bottom resuspension were collectively responsible for its variability. This study provides a fundamental framework for characterizing DOM distribution patterns in the ECS during spring.

Key words: dissolved organic matter, absorbance, fluorescence, East China Sea, penetrating front