CHRACTERIZATION OF CHROMOPHORIC DISSOLVED ORGANIC MATTER IN THE YANGTZE ESTUARY BY ABSORPTION AND FLUORESCENCE SPECTROSCOPY

Dissolved organic matter (DOM) plays an important role in natural aquatic ecosystems. Fluorescence and UV absorption of chromophoric dissolved organic matter (CDOM) in the Yangtze (Changjiang River) estuary are reported in this study. Water samples were collected mainly from three sections (North Channel, the head of South Channel and the end of South Channel) of the Yangtze Estuary in March, 2006. Three-dimensional excitation emission matrix (3-DEEM) fluorescence spectrums and UV absorption at 355 nm are analyzed for those filtrates through Whatman GF/F filters. The concentrations of dissolved organic carbon (DOC) are also measured. The results showed that absorption coefficients of CDOM at 355 nm, which ranged from 16.4 m-1 to 18.3 m-1, had significant correlations with the concentrations of DOC (2.0∼3.4 mg/L) at high tide and only in bottom water at low tide. The 3DEEM fluorescence scans suggested the terrestrial fluorophores of aromatic protein (peak A: Ex=236±1 nm, Em=364±3 nm), humic acid-like (peak B: Ex=251±3 nm, Em=473±5 nm) are obvious. Another peak of humic acid-like (peak D: Ex=337±6 nm, Em=447±9 nm) is also found. Tryptophan and protein-like (peak C: Ex=282±2 nm, Em=351±5 nm) with strong peaks, which could be derived from both terrestrial and microbial origins, were detected only in South Channel. In addition, the intensities of peak C are stronger at river end member than at marine end member (here is a low salinity area). The higher fluorescence index (FI) values at river end member could be resulted by microbially derived CDOM in the Huangpu River and wastewater discharges from Shanghai. The fluorescence index suggested that the terrestrial signal became stronger at marine end member but microbial signal became weaker, which could be attributed to the mobilization of particulate organic matter (POM) in the riverine suspended sediments with the intrusion of the seawater and the suspension of the sediments. The saltwater intrusion could also increase the contents of microbial CDOM during flooding tide. Therefore, there is autochthonous production of CDOM occurring within the estuary, where chromophoric dissolved organic matter is dominated by mainly terrestrial and microbial sources.