Spatial changes in molecular composition of dissolved organic matter in the Yangtze River Estuary: Implications for the seaward transport of estuarine DOM.

The complexity of dissolved organic matter (DOM) limits our understanding of the estuarine carbon cycle. This study adopted a combination of bulk carbon isotope, optical techniques and ultra-high resolution mass spectrometry to study the spatial heterogeneity and compositional variations of DOM across a latitudinal transect of the Yangtze River Estuary (YRE). Results show that the whole section of YRE received high abundance of protein-like C4 fluorescent component (0.66 ± 0.08 R.U.) and high relative abundance of aliphatic compounds and peptides (8.28 ± 1.46%) from phytoplankton, which would contribute to the bioavailable DOM pool of the Eastern China Sea (ECS). However, multivariate analysis indicated that polycyclic aromatics and polyphenols from the Yangtze River experienced a significant decrease of 5% within the turbidity zone, creating a significant decrease of 0.08 in aromaticity index and modulating DOM content and compositions within the YRE. 1837 molecular formulae were identified to track dynamic behaviors of terrestrial DOM in the YRE. Molecular imprints showed the removal of terrestrial molecules in the turbidity zone indicated by the decrease of 753 in molecular quantity, when water masses mixing diluted the abundance of aromatic compounds. Adsorption and flocculation could serve important mechanisms to remove terrestrial DOM, promoting the burial of terrestrial DOM within estuarine sediments. Besides, some terrestrial molecular formulae were also detected in the ECS, suggesting the potential contribution of terrestrial DOM to the carbon stock of open seas after experiencing physical and photochemical transformations. This research provides a comprehensive insight into spatial variations of estuarine DOM composition, underlining the important role of estuaries in sorting and transporting DOM.