Importance of colored dissolved organic matter (CDOM) inputs from the deep sea to the euphotic zone: Results from the East (Japan) Sea

• Three CDOM components were determined in the East (Japan) Sea.
• The deep sea is an important source of humic-like CDOM in the euphotic zone.
• Changjiang River is an important CDOM source in the surface East Sea.
• Microbial production is an important source of CDOM in the deep layer.

The vertical and horizontal distributions of colored dissolved organic matter (CDOM) in the East (Japan) Sea of the Northwestern Pacific Ocean were determined by excitation–emission matrix spectroscopy (EEMs) with parallel factor analysis (PARAFAC). The PARAFAC model identified two humic-like components and one protein-like component in the study region. The concentration of the protein-like component decreased, whereas that of the humic-like components increased with depth. Significant correlations between the fluorescence intensity of the humic-like CDOM and the apparent oxygen utilization below the thermocline indicate that the humic-like CDOM originates primarily from the in situ biological activity. In addition, the negative relationship observed between the salinity and fluorescence intensity of CDOM in the surface water of the study region, including the East China Sea, Yellow Sea, southern sea of Korea, and East Sea, suggests that CDOM in the “freshwater” fraction of the study region originated predominantly from the Changjiang River. The box model results show that the proportions of the humic-like CDOM transported from the subsurface layer were approximately 33% for C1 and 22% for C3 of the riverine sources. The results of this study highlight the significance of CDOM inputs from the subsurface layer as an unrecognized source of CDOM in the euphotic zone, which may have a significant influence on the biology and biogeochemistry of the ocean.