Variation in chemical composition of dissolved organic matter during the winter to spring transition in the northern Barents Sea

Molecular characterization of dissolved organic matter (DOM) from the Arctic Ocean is scarce, especially during the winter, which is a crucial period for water mass mixing and carbon cycling. The northern Barents Sea extending into the Arctic Ocean is experiencing global warming at a rate 5–7 times faster than the global average, leading to drastic chemical, physical and ecosystem changes. We sampled a transect along this region during early winter (December), late winter (March) and spring (May) and analyzed seawater samples using high-resolution mass spectrometry. Our results show significant changes in DOM composition driven by biological seasonality and water circulation such as lateral and vertical water transport, whereas water masses did not exhibit significant correlations with DOM composition. Our mass spectrometry-based results indicate that ionizable DOM compounds in early winter contained a greater proportion of unsaturated compounds relative to late winter and spring, as shown by weighted average hydrogen to carbon atomic ratios (H/C_wa) (−0.029, Mann-Whitney U test, p < 0.001). Higher DOM lability in late winter was associated with higher nitrogen containing formulas which could be a result of DOM products from viral processes. Deep waters in the Arctic Basin and on the Barents Sea shelf break show greater lability in spring suggesting an influence of water circulation from the biologically active shelf regions. In early winter, higher weighted average aromaticity index (AI_mod), double-bond equivalents (DBE) and relative intensities of CHO formulas over heteroatom (N, S)-containing formulas were observed, thus supporting the presence of DOM with higher recalcitrance. Early winter also exhibited a significantly higher number of terrigenous peaks (t-Peaks) (p < 0.001), suggesting seasonal removal of these riverine markers. This DOC may be transported to deeper ocean layers during winter water mixing. Our findings bridge the gap in winter DOM molecular characterization, which allows for future assessments of potential changes in the Arctic DOM reactivity.