Distinct differences in surface-water dissolved organic matter between the East China Sea and Okinawa Trough: Source and hydrological effects

Abstract

Cross-shelf transport plays an important role in marine carbon cycles. In this study, we examined the concentration and optical composition of dissolved organic matter (DOM) in 33 surface water samples collected along a transect from the East China Sea (ECS) to the Okinawa Trough (OT) in spring 2021. Compared to OT waters, ECS waters are characterized by higher dissolved organic carbon (DOC) concentration (111.55 ± 13.30 vs. 72.52 ± 5.01 μmol L−1), chromophoric DOM concentration (0.44 ± 0.15 vs. 0.12 ± 0.03 m−1 for a350), aromaticity (1.07 ± 0.11 vs. 0.64 ± 0.08 L mg−1C m−1 for SUVA254), and humification index (1.04 ± 0.25 vs. 0.58 ± 0.07). Fluorescence components C1, C3, and C4 displayed similar regional trends with the highest intensity in the inner shelf, followed by the outer shelf and then OT, whereas C2 had the highest intensity in the outer shelf, followed by the inner shelf and then OT. Comparison of the inner and outer ECS shelves revealed similar DOC concentrations, but different optical parameters (i.e., a350, S275–295, HIX, BIX, C1-C4 intensity). Our study suggests that: 1) active carbon cycling alters surface water DOM compositions in the ECS shelf, which is not reflected by total DOC concentrations, likely due to an offset effect between terrestrial DOM removal and marine DOM production; and 2) DOM is well mixed in the ECS shelf, but hardly passes across the OT owing to the strong barrier effect of the Kuroshio Current during the sampling season. Investigation of DOM biogeochemistry in other seasons and different water depth is necessary as the Changjiang Diluted Current and Kuroshio Current are weaker in winter and stronger in summer, respectively, which affects the spatial and temporal distribution of water DOM in the ECS-OT regions.