Distribution characteristics and source analysis of dissolved organic matter with different molecular weights in the coastal waters of Zhejiang Province.

Dissolved organic matter (DOM) is a crucial component of the carbon cycle in ecosystems, with its composition, functional groups, and chemical structures varying significantly depending on source and molecular weight (MW). In this study, the content, composition characteristics, and sources of DOM in the coastal waters of Zhejiang Province were investigated using total organic carbon analysis, ultraviolet-visible (UV-Vis) absorption spectroscopy, and excitation-emission matrix (EEM) spectroscopy combined with parallel factor analysis (PARAFAC). The results indicated regional differences in the content and composition of DOM. Hangzhou Bay exhibited the highest DOM content (1.81 ± 0.06) mg/L, fluorescence intensity (F), and proportion of protein-like components (83.30-92.68%). Three PARAFAC fluorescence components (C1, C2, C3) were identified in each region, classified into protein-like and humic-like substances. UV and fluorescence characteristic parameters showed that the primary sources of DOM differed among the regions: Hangzhou Bay was significantly influenced by anthropogenic sources, while Yueqing Bay was more affected by terrestrial inputs. However, allochthonous inputs were important in all regions, primarily consisting of terrestrial materials transported by rivers, urban wastewater, and agricultural runoff. Ultrafiltration divided the DOM into four MW fractions: 30 kDa-0.45 μm, 10-30 kDa, 1-10 kDa, and <1 kDa. It was found that the DOM in each region predominantly consisted of low MW fractions (<1 kDa), accounting for 43.35-45.02%, with the proportion decreasing as MW increased. In the same investigated sea area, <1 kDa and <0.45 μm fractions exhibited similar fluorescence characteristics; the DOM in the 1-10 kDa range showed relatively higher F values, indicating the highest fluorescent DOM (FDOM) content in this fraction. This study highlighted the importance of anthropogenic and terrestrial inputs on DOM in coastal waters.