Abstract: Dissolved organic matter (DOM) is a key driver of biogeochemical reactions. The characteristics and sources of DOM components in estuarine and coastal waters significantly influence the marine ecological environment. Qinzhou Bay is a typical semi-enclosed subtropical estuarine bay, comprising an inner bay (Maowei Sea) and an outer bay. The Pinglu Canal discharges into the Maowei Sea via the Qinjiang River. This study focused on the estuary of the Pinglu Canal-Qinzhou Bay system. By integrating ultraviolet-visible (UV-Vis) absorption spectroscopy and excitation-emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC), we systematically investigated the compositional characteristics and sources of DOM in the water body. The results indicated that the dissolved organic carbon (DOC) concentration in the surface water of Qinzhou Bay ranged annually from 1.8 to 9.1mg·L-1. The lowest concentration was observed in February 2025 (average 2.11±0.38mg·L-1), while the highest occurred in October 2024 (average 7.53±1.43mg·L-1). Spatially, DOC exhibited a decreasing trend from the Pinglu Canal estuary towards the inner bay mouth, followed by a further decrease towards the outer bay. Three fluorescent components were identified in the DOM of Qinzhou Bay: humic-like C1 (Ex/Em = 240/430nm), protein-like C2 (Ex/Em = 245/300nm), and protein-like C3 (Ex/Em = 250, 275/350nm). The maximum fluorescence intensity (Fmax) of C2 showed the greatest fluctuation (58.03~37423.30). The Fmax reached its peak value of 72279.81 in September 2024 and its lowest value of 8264.35 in April 2025. From March to June 2025, the relative abundance of the humic-like component (C1) was high (60%~81%), accompanied by significantly elevated SUVA254 (>3.5) and humification index (HIX). This indicates that the DOM during this period was predominantly composed of terrestrial humic substances with high aromaticity and degree of humification, transported by riverine input. From August 2024 to January 2025, the combined abundance of the protein-like components (C2 and C3) exceeded 70%, SUVA254 decreased to very low levels (<1.5), HIX dropped significantly, while the biological index (BIX) increased. This stage was dominated by autochthonous and anthropogenic sources. Correlation analysis revealed significant negative correlations between salinity and DOC concentration (r=-0.715, p<0.01), the humic-like component C1 (r=-0.777, p<0.01), and the tyrosine-like component C2 (r=-0.541, p<0.05), indicating that terrestrial DOM is primarily introduced through riverine input. These findings provide support for understanding the evolution of estuarine carbon cycling under the influence of human activities and offer a scientific basis for the construction of the Pinglu Canal and regional water environment management and pollution control.

Key words: dissolved organic matter, component characteristics, three dimensional fluorescence spectroscopy, Qinzhou Bay, Pinglu Canal