Spatiotemporal variability, size and photoreactivity of chromophoric dissolved organic matter in the Bohai Sea and the northern Yellow Sea

The distribution, composition, and photoreactivity of chromophoric and fluorescent dissolved organic matter (CDOM and FDOM) in the Bohai Sea (BS) and the northern Yellow Sea (NYS) were investigated during three cruises (summer 2017, winter 2017, and spring 2018) by combining size fractionation through a series of micro- and ultra-filtrations and irradiation experiments. A total of 484 excitation-emission matrix spectra were modeled by parallel factor analysis to yield four humic-like (C1–3 and C5) and three protein-like (C4 and C6–7) fluorescent components. CDOM in the BS and the NYS was primarily affected by terrestrial inputs. Microbial degradation processes and autochthonous production were considered to be important factors driving the dynamics of fluorescence components. A total of 73 triple CDOM absorption and fluorescence spectra (n = 219) from the study regions were measured on 0.2, 0.45, and 0.7 μm filters in winter; water samples were also collected from the Yellow River Estuary (YRE) to the NYS, which was further size fractionated through ultrafiltration membranes (10 kDa and 1 kDa) to investigate variability between spectra of different filter pore sizes. The significant mean difference of absorption coefficient and fluorescence spectra, and no consistent trend between spectral slope (S_{275 – 295}, S_{350 – 400}), slope ratio (S_R) from these filtrates suggested that the choice of different filter pore sizes considerably influenced the CDOM optical assessment. Higher values of CDOM and FDOM in the particulate phases (0.2–0.7 μm) fraction were more distributed in the NYS where terrestrial inputs were less and DOM dynamics were more linked with phytoplankton production. As pore size decreased, the percentage of humic-like fluorescence increased suggesting that humic-like components of CDOM were more dominated by low-molecular-weight (LMW) materials compared with the protein-like components. Consistent with its higher terrestrial CDOM content, photodegradation resulted in losses of CDOM and FDOM and increasing of the spectral slope ratios (S_R). These finding suggested that CDOM molecules were photochemically degraded to LMW substances. Our results indicated that the proportion of the LMW CDOM decreased from estuary to open sea, which likely resulted from high-molecular-weight (HMW) organic material produced during phytoplankton production in off-shore regions, and LMW substances produced during CDOM photochemical processes in coastal waters.