Optical Characteristics Of CDOM And Tracer To Sources Of DOC In Changjiang Estuary And Adjacent Sea

In this study,52 samples were collected through Changjiang estuary and adjacent sea of the four surveys in August 2008, December 2009, March 2011 and August 2011. The optical characteristics of chromophoric dissolved organic matter (CDOM) are studied to know its spatial and temporal dynamics. The relationship between CDOM and dissolved organic matter (DOC) is also discussed. The major results are as followed:(1) Over a salinity range of 0.18-34.48‰, CDOM absorption coefficients at 355 nm (a(355)) ranged from 0.046 to 3.742 m-1, with a mean value is 0.906 m-1, which were lower than those in several other foreign estuaries and almost the same as other our country’s estuaries. The maximum value appeared in Huangpu River estuary, which indicats the big influence of Huangpu River to the concentration of CDOM. The minimum appeared in the place far away from the estuary. The more close to the Changjiang estuary, the greater the absorption coefficient. There is obvious influence of Changjiang diluted water to the concentration of CDOM in estuary area with the seasonal change. The value of a(355) in summer (0.427-1.440 m-1) is greater than that in winter (0.269-1.154 m-1) from Luchaogang Dock to Shengsi Island. The absorption spectral slop (Sg), ranged between 0.0115 and 0.0232 nm-1, with a mean value of 0.0184 nm-1. The seasonal change also has an effect on Sg value, like the a(355), Sg in summer (0.0192-0.0216 nm-1) is greater than that in winter (0.0115-0.0175 nm-1) from Luchaogang Dock to Shengsi Island, which indicates that CDOM constructional materials changes in different season, maybe the humic acid proportion is lower in summer.(2) Six EEMs peaks are identified in all samples, including two humic acid fluorescence peaks (C and M) in visible range, a humic acid fluorescence peak in UV range (A), two tryptophan-like fluorescence peaks (T1 and T2) and a tyrosine-like fluorescence peak B. From estuary to open sea, the number of fluorescent group is decreased, fluorescence intensity trails off and the main composition of CDOM is changing from humic-like to protein-like. There is a good linear relationship between a(355) and fluorescence (Fs(355)) (R2=0.89), which validates that CDOM fluorescence can be used for absorption monitoring.(3) The good negative linear correlation between a(355) and salinity (R2=0.89, P＜0.001) indicates that Changjiang runoff is the main source of CDOM. On the other hand, there are good positive linear correlations between fluorescence peaks (except peak B) and Chla, which indicates that the phytoplankton contribute to the concentration of CDOM at opean sea. Otherwise, submarine sediments release because of disturbance and microbiological degradation is also a way to the increase of CDOM concentration.(4) The concentration of DOC in research area is 0.764 to 2.644 mg·L-1, with a mean value of 1.308 mg·L-1. There is a good negative linear correlation between salinity and DOC (R2=0.89, P<0.001), which indicates the conservative behaviour of DOC in Changjiang estuary. CDOM absorption coefficient a(355) is significantly correlated with DOC (R2=0.87, P<0.001). The fluorescence intensity of CDOM to the concentration of DOC is higher near the Changjiang estuary, which shows the changing constitutes of CDOM at different areas. We also find a good positive linear correlation between Fs(355) and DOC (R2=0.88, P<0.001), which domenstrates that CDOM fluorescence can be used to DOC monitoring and would play an important part in the research of global carbon cycling.