Spatial variation in the optical and molecular properties of dissolved organic matter in the Yellow Sea and East China Sea

Marine dissolved organic matter (DOM) plays a crucial role in the global carbon cycle. Carbon cycling in marginal seas is highly complex because it is regulated by numerous source-sink processes. Here, we investigated the spatial variability of the bulk, optical properties, and molecular compositions of DOM in the Yellow Sea (YS) and East China Sea (ECS) and its influencing factors. The DOM in the study area was dominated by protein-like component, highly unsaturated and phenolic compounds, and CHO formulas. The dissolved organic carbon (DOC), chromophoric DOM, and fluorescent DOM decreased with increasing salinity and depth, and within the three regions they were ordered: northern YS > southern YS > ECS. Due to the greater influence of anthropogenic inputs and phytoplankton production in the YS, it contained significantly more heteroatomic compounds and labile compounds (aliphatic, peptide-like, and sugar-like compounds) than the ECS. Moreover, unlike in the open ocean, none of the compound categories exhibited significant variability with depth in these shelf seas. Redundancy analysis suggested that dilution was a main factor determining the spatial distributions of DOM. The polycyclic aromatics and polyphenolics decreased in the seaward direction, but they were still present offshore, indicating they contributed to the open-ocean carbon pool. Anthropogenic inputs obscured potential correlations among the labile compounds and salinity and phytoplankton production. In addition, compounds with high molecular weights and low unsaturated CC functionalities were preserved during the carbon cycle, due to their recalcitrant nature. By providing optical and molecular data, this study helps to better understand the biogeochemical cycling of DOM in marginal seas.