Mixing behavior, biological and photolytic degradation of dissolved organic matter in the East China Sea and the Yellow Sea

Highlights: • Significant seasonal variations of DOM occurred in the YS and the ECS. • Mixing and degradation processes co-regulated DOM parameters dynamics. • Terrestrial input was the dominant source of DOM in the study region. • Autochthonous DOM was more prone to biodegradation than terrestrial DOM. • Photodegradation processes act preferentially on CDOM rather than colorless DOM. Optical properties of dissolved organic matter (DOM) were used as an indicator of the quantitative and qualitative changes occurring in marine DOM. The spatiotemporal distribution, bioavailability, and photoreactivity of chromophoric DOM (CDOM) and dissolved organic carbon (DOC) were investigated in the East China Sea (ECS) and the Yellow Sea (YS) during spring and summer using absorption spectroscopy and fluorescence excitation-emission matrix–parallel factor analysis. Over a 4-month laboratory study, we measured changes in six commonly used optical indices, including spectral slope (S_275–295), slope ratio (S_R: S_275–295/S_350–400), specific ultraviolet absorbance (SUVA₂₅₄), ratio of the sum of protein-like components to the sum of humic-like components (C_protein/C_humic), biological index (BIX), and humification index (HIX) to determine their changes following biological and photochemical degradation processes. Significant seasonal variations were observed in the spectral characteristics of CDOM in the ECS and the YS, indicating a stronger influence of the terrestrial origin and highly aromatic content of DOM in summer than in spring; this result was likely the consequence of an increase in the Changjiang River discharge, phytoplankton production, and biological activity, resulting in an increase in DOM production. Significant correlation between salinity and optical parameters (SUVA₂₅₄, S_275–295, S_350–400, C_protein/C_humic) indicated that water mixing strongly influenced the distributions of these optical parameters. The bioreactivity and photoreactivity of DOM varied depending on the source material, and the autochthonous protein-like DOM was more prone to biodegradation than the terrestrial DOM. The photodegradation processes acted preferentially on the CDOM than the colorless DOM. These results demonstrated that the optical parameters exhibited distinct changes during the mixing and the biodegradation and photodegradation processes and explained the seasonal distribution of DOM in the ECS and the YS.