Assessing the Dynamics of Dissolved Organic Matter in the Changjiang Estuary with Absorption and Fluorescence Spectroscopy

Changjiang Estuary is the largest estuary in China and is significantly influenced by anthropogenic perturbation and depositional processes. The south branch of this estuary is the dominant pathway of Changjiang river discharge, while the shallower north branch is dominated by salty water, and the water residence time is much longer, especially in the dry season. To examine the dynamics of dissolved organic matter (DOM) in such complex environments, two optical methods (absorption and fluorescence spectroscopy) combined with dissolved organic carbon (DOC) analysis were used to characterize the properties of DOM samples collected in different seasons (April, July, and October, 2011). The DOC concentration, absorption coefficient, and fluorescence intensities were highest in the Huangpu River (the last major tributary of south branch) that receives large amounts of sewage from Shanghai. Such input leads to not only an increase in the DOM level but also a notable change in the chemical composition (and hence reactivity) of DOM in the south branch. In the north branch, significant additions of DOM were observed in the dry season, which probably results from the release of water-soluble organic matter via intense resuspension. These results suggest that anthropogenic inputs from megacities and additions from resuspended sediments should be seriously considered in assessing the flux and chemical composition of DOM being discharged into the ocean.