Seasonal variability of distribution and mixing behavior of chromophoric dissolved organic matter in the Changjiang River Estuary and adjacent areas

The distribution and mixing behavior of chromophoric dissolved organic matter (CDOM) in the Changjiang River Estuary and adjacent areas were analyzed in March, July and October of 2019. Through discussing the salinity, absorption spectral slope S_275-295, absorption coefficient a_CDOM(355) and Chl a, it was shown that the terrestrial input and a_CDOM(355) had the highest value in July; a_CDOM(355) was affected by terrestrial input and phytoplankton production activities in March, and the value in March was higher than that in October in the Changjiang River Estuary low salinity area. In addition, in high-salt offshore regions, a_CDOM(355) was similar in March and July, and both were lower than that in October, these distribution of CDOM was mainly influenced by phytoplankton production activities. Four fluorescent components of CDOM were identified by EEMs-PARAFAC technique: one proteinlike component C1 (280/330 nm) and three humic-like components C2 (300/350 nm), C3 (260/465 nm) and C4 (320/410 nm) respectively. Four fluorescent components declined from the inside to the outside of the Changjiang River Estuary in March, July and October. Affected by terrestrial input and phytoplankton production activities, the seasonal variation of the average fluorescence intensity from large to small was July, October and March. The fluorescence components of CDOM deviated from the theoretical dilution line in these three seasons. It showed that CDOM source (terrestrial input, sediment resuspension and on-site biological activities) and sink (adsorption of particulate matter, photodegradation and bacterial degradation) were complex and variable, revealing the non-conservative mixed behavior of CDOM in the Changjiang River Estuary.