ИСТИНА

The Arctic Shelf seas receive large fluxes of dissolved organic matter (DOM) from major Siberian rivers—Ob, Lena, Yenisei, Kolyma and Indigirka—whose discharge is increasingly influenced by permafrost degradation. This study aimed to assess spatial variability in DOM composition and optical properties across three key river–shelf systems: Ob–Kara Sea, Lena–Laptev Sea, and Indigirka–East Siberian Sea. A total of 109 DOM extracts were obtained aboard the R/V Akademik Mstislav Keldysh during the AMK-82 expedition (October 2020) and analyzed using 21 T Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). DOM samples from the Kara Sea exhibited the lowest compositional heterogeneity, with 11,327 ± 2,484 molecular formulas dominated by CHO compounds (74.1 ± 6.2%), higher average molecular weights (Mn = 629 ± 48 Da), and elevated double bond equivalents (DBEₙ = 14.6 ± 1.5), indicating a prevalence of condensed aromatic structures. In contrast, DOM from the Laptev and East Siberian seas showed increased molecular diversity (12,486 ± 1,697 and 12,400 ± 2,110 formulas, respectively), lower CHO contributions (~65%), and greater abundance of nitrogen- and sulfur-containing formulas (CHON: 24–26%; CHONS: 9–10%).Samples from the Lena river discharge zone were particularly enriched in CHON (32.5 ± 5.6%) and CHONS (14.8 ± 5.0%) formulas and showed reduced CHO contribution (54.5 ± 7.7%), lower molecular weights (Mn = 530 ± 40 Da), and DBEₙ (11.6 ± 1.0). These patterns suggest inputs of aliphatic, biolabile compounds—such as peptides, amino acids, and heterocycles—likely derived from thawing permafrost deposits mobilized by riverine flow. Hydrolyzable tannins were the dominant class across all regions (40–60%), followed by lignins (20–45%), terpenoids (2–15%), and carbohydrates (2–10%). Significant differences in chemotype distribution between regions were confirmed by ANOVA and Tukey’s test (p < 0.05). DOM near the Lena River outflow showed lower contributions of condensed (2.6 ± 1.0%) and hydrolyzable tannins (48.2 ± 3.7%) and elevated proportions of lignins (30.2 ± 0.2%), terpenoids (10.2 ± 1.5%), and carbohydrates (7.3 ± 1.0%) compared to other shelf areas, reflecting a shift toward more aliphatic composition. Principal component analysis (PCA) of molecular data further distinguished the Lena river influenced DOM samples along PC1 (53.08% of variance), driven by higher H/C ratios and aliphatic components. PC2 (11.49%) reflected variation in aromaticity and oxidation state. These findings suggest that the molecular signature of DOM in the Lena River discharge area may reflect inputs influenced by thawing permafrost, contributing biogeochemically distinct compounds to the shelf waters.