BOOK OF ABSTRACTS 31 IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 Monday, 24 August 2026 / Hall B+C Analysis and Characterization size-exclusion chromatography (SEC), UV–Vis absorbance, and fluorescence spectroscopy. Biodegradation experiments were also performed under dark conditions at 20°C following phosphate amendment, with time-course sampling for fractionation and chemical analyses. Results and Discussion SEC-TOC analysis revealed the presence of HMW components (>100 kDa) in Lake Biwa DOM. Membranes with molecular weight cut-offs of ≥10 kDa were used, and these HMWDOM could be effectively isolated by the ultrafiltration. In contrast, LMW components exhibited continuous distributions in SEC and were not completely removed even after repeated diafiltration, indicating the involvement of partitioning, diffusion, and adsorption– desorption processes. To account for this behavior, the permeation model was applied to correct for permeating solutes retained in apparent fractions, resulting in a more accurate quantification of DOM size distribution. Using this approach, the DOM in Lake Biwa was classified into at least three functionally distinct fractions: HMW, retained LMW, and permeating LMW. These fractions exhibited clear differences in chemical properties (e.g., SUVA, fluorescence characteristics, and C/N ratios) and biodegradability. The HMW fraction showed biopolymerlike characteristics with relatively high biodegradability, and the two LMW fractions showed clear differences in chemical composition and reactivity. Notably, part of the LMW-DOM included nitrogen-containing compounds likely associated with microbial production and accumulation. These results demonstrate that the integrated UF-NFDF and permeation modeling approach provides a robust framework for resolving DOM size structure and linking it to biogeochemical properties in aquatic systems. References 1. AW Decho, T Gutierrez, Frontiers in Microbiology 8 (2017) 922. 2. A Nebbioso, A Piccolo, Z. Author, Analytical and Bioanalytical Chemistry 4085 (2013) 109. Acknowledgement This work was supported by JSPS KAKENHI Grant Numbers JP22H00382, JP24K03078, JP24K15283 and JP25K03248.
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