Study On The Photodegradation Of Dissolved Organic Matter In The Yangtze Estuary And Its Adjacent Sea

Dissolved organic matter (DOM) plays a key role in the global carbon cycling. However, less attention has been paid to the effect of salinity on DOM photobleaching. Data was employed to examine the sources and estuarine mixing behavior of DOM in the Yangtze estuary in July 2014. And also, the photochemical process and the influence factor were explored to provide the basic data for the model of carbon cycling in estuaries and marginal seas.The result indicated that the horizontal distribution of DOM in the studied area are mainly controlled by fluvial transportation, and then diffused following the direction of the Yangtze Diluted Water. CDOM abundance (as quantified by the absorption coefficient at 355 nm,a(355)), the absorption spectral slope over 275-295 nm (S275-295), and the specific UV absorbance at 254 nm (SUVA254) were all generally conservative across the freshwater-saltwater transitional zone, A localized elevation of CDOM occurred downstream of the mouth of the Huang Pu River, revealing a subtle difference in the mixing pattern of CDOM between the North Port and the Sourth Port, whereas the constitutive property between the two Ports was similar. Significant relationships were found in this study among a(355), Fs(355) and DOC concentration ([DOC]). Furthermore, [DOC] can be predicted from the CDOM absorption coefficients at 275 nm (a(275)) and 295 nm (a(295)). The regression results demonstrated CDOM absorbance could be used as the DOC tracer along the Yangtze estuary. Moreover, the Fs(355) seemed to be a good tracer, too.The PARAFAC analysis technology was employed for the fluorescence excitation-emission matrix, according to which three humic-like components (C2, C4 and C5) and three protein-like components (C1, C3 and C6) were identified from the bulk samples. The humic-like components possessed similar origins and correlated with a(355) and salinity. The protein-like components C1 and C6 were not significant correlated to salinity and a(355), suggesting the protein-like components were closely related to the in situ microbial activities. Fluorescence intensity showed a difference between summer and winter, which suggestes the influence caused by Huang Pu River was more significant in summer than in winter, especially on the protein-like components in the water column. The organic matter carried by the Huang Pu River might provide sufficient carbon source and energy for microbial activity.The result of photobleaching experiments of DOM illustrated that all the selected samples with different salinities could be degradated completely under the solar simulator in about one week, with the e-folding time<1 day. By contrast, the degradation rate of DOC was lower than CDOM, which indicated that TDOM was resistant to UV and visible irradiance to some extent relative to CDOM. Another possible pathway is CDOM was partly transformed into TDOM during bleaching.In this study, the salinity impact on the CDOM photodegradation was also investigated for humic acid, fulvic acid and natural seawater samples. The results demonstrated CDOM photodegradation rate was declined with increasing salinity, with a sharp peak at salinity-15. The ionic strength (≈0.31) might be responsible for this distinct phenomenon, i.e. at salinity～15, the hydrous status of DOM was unique and hence the photoreactivity of DOM was enhanced.