Optical characterization of CDOM in a marsh-influenced environment in the Changjiang (Yangtze River) Estuary

To elucidate the influence of tidal marshes on the amount and chemical structure of chromophoric dissolved organic matter (CDOM) in adjacent coastal waters, surface water samples were collected along three cross-marsh transects in tidal marshes within the Changjiang Estuary. In addition, three sediment cores were collected from the marshes and incubated for 10 h in the laboratory after adding overlying tidal waters. Nutrients, DOC, absorption, fluorescence excitation–emission matrix (EEM) spectroscopy, and other parameters were measured in these samples, which clearly illustrated releases by the tidal marshes of DOC, and CDOM absorbent and fluorescent molecules. To characterize the chemical nature of the marsh-derived CDOM, variations in a suite of optical indices (including molar absorptivity, absorption spectral slope, fluorescence quantum yield, and fluorescence ratios between certain excitation/emission locations) in these samples were plotted against the offshore distance of the stations as well as the incubation time. From the literature, it has been suggested that these variations are closely associated with molecular weight, aromaticity, and humic/fulvic or allochthonous/autochthonous ratios in the CDOM mixtures. This study showed that the CDOM fluorophores changed to more humic-like and were less protein-like from sea to land across the marsh gradients. Furthermore, subtractions between EEM spectra before and after laboratory incubations were applied to illustrate in greater detail the properties of the pure fluorescent compounds exchanged at the sediment–overlying water interface.