Colored Dissolved Organic Matter (CDOM) is a crucial constituent of estuarine and coastal Dissolved Organic Matter (DOM), which can be utilized to track the transport and transformation of DOM during various water masses mixing and transportation. In this study, we investigated the sources, composition and distribution of CDOM in the Changjiang Estuary and adjacent waters in August 2018. We measured the UV-visible absorption spectra and three-dimensional fluorescence characteristics of CDOM in the different layers of water column, combined with physical and chemical parameters such as temperature, salinity and dissolved oxygen, etc. In particular, we discussed the influence of allochthonous inputs and biological activities. Our results showed that the dissolved organic carbon (DOC) content and CDOM characteristic parameters (such as the aromaticity index SUVA254
, absorption coefficient a350
, and humification index HIX) in the Changjiang Estuary were significantly higher than those in the adjacent sea areas. These parameters decreased gradually from the surface layer to the middle layer and then to the bottom layer, indicating that the primary source of CDOM in the Changjiang diluted water was from land-based inputs. The values of these parameters in the outer sea area of the Changjiang Estuary and the coastal area near Zhejiang-Fujian were relatively low, and the spectral slope S275-295
of DOM molecular weight and the biological index BIX were relatively high, indicating that there were more biological activities in these areas. This may be related to the abundant nutrients brought by the Changjiang and Taiwan warm currents, and the subsurface water of the Kuroshio. The low HIX values and high BIX values in the study area indicated that CDOM was mainly contributed by autochthonous sources. The results of principal component analysis showed that pH, dissolved oxygen, temperature and chlorophyll a
were the main factors affecting the concentration and composition of DOC and CDOM. However, this influence was weaker in the surface layer compared to the other layers. Based on the parallel factor analysis (PARAFAC), we identified three fluorescent components in CDOM, namely humic-like components C1 Ex/Em, 240(285)/352 nm and C3 Ex/Em, 265(335)/428 nm, and protein-like components C2 (Ex/Em, 270/306 nm). C1 was jointly controlled by marine autochthonous and terrestrial sources, while C3 was mainly from terrestrial sources. C1 and C3 were mainly distributed in the Changjiang Estuary, Hangzhou Bay and the coastal area near Zhejiang-Fujian, while C2 was mainly distributed at the entrance and outside Hangzhou Bay. In surface water, the components of C2 had higher values at some middle-high salinity stations, corresponding to the higher concentrations of chlorophyll a
and lower apparent oxygen utilization (AOU) in the study area, indicating that C2 mainly originated from phytoplankton production in situ. At certain stations with lower salinity levels, the C2 components exhibited lower values, whereas the C1 components displayed increased values. Additionally, some stations exhibited elevated concentrations of chlorophyll a
, which suggested that the increased primary production promoted microbial activities, resulting in the decomposition and consumption of certain protein components and the production of C1 humic-like components. This study has enhanced our understanding of the composition, distribution, and influencing factors of CDOM in the Changjiang Estuary and adjacent waters, which is beneficial for gaining further insight in the transport and transformation of DOM in large river estuaries and adjacent sea areas.