Spatio-temporal variation of Dissolved Organic Matter (DOM) in the Bohai Sea and the Yellow Sea

The seasonal and spatial variations of dissolved organic carbon (DOC), chromophoric dissolved organic matter (CDOM), and fluorescent dissolved organic matter (FDOM) were evaluated in the Bohai Sea (BS) and the Yellow Sea (YS) during three cruises. Two fluorescence components, such as marine humic-like (C1) and visible humic-like (C2), were identified using excitation-emission matrix fluorescence spectroscopy (EEMs) and parallel factor analysis (PARAFAC). The average values of DOC concentration, CDOM absorption coefficient (a_CDOM(355)) and fluorescence component intensities (C1 and C2) were significantly higher in the BS (2.14 mg C L⁻¹, 0.77 m⁻¹, 0.035 RU, 0.026 RU, respectively) than those in the YS (1.55 mg C L⁻¹, 0.50 m⁻¹, 0.025 RU, 0.019 RU, respectively). The average DOC value was highest in winter (2.16 mg C L⁻¹) and lowest in spring (1.62 mg C L⁻¹), while a_CDOM(355) was the opposite (winter: 0.45 m⁻¹; spring: 0.62 m⁻¹; summer: 0.58 m⁻¹). The average C1 (0.042 RU) and C2 (0.028 RU) values were highest during the winter and summer respectively, and lowest during spring (C1: 0.024 RU; C2: 0.016 RU). DOC and CDOM were mainly derived from terrestrial input, during the winter and spring, while they were primarily derived from phytoplankton production during the summer. C1 and C2 were not correlated with salinity or chlorophyll a during the three seasons. The SUVA₃₅₅ values generally decreased with increasing salinity during the spring and summer, indicating that sources and sinks of CDOM and DOC were uncoupled. DOC, CDOM, and FDOM were dominated by terrestrial input and phytoplankton production activities and their spatial distributions and seasonal variations were also affected by currents, microbial activities, and photobleaching.