Molecular Signatures of Dissolved Organic Matter Across the Glacial, Proglacial, and Fjord Continuum in NE Greenland

Abstract Rapidly melting Arctic glaciers deliver increasing amounts of allochthonous material to the coastal ocean, altering carbon cycling and promoting heterotrophy. As key factors influencing the activity of heterotrophic microbes, the quantity and quality of Arctic coastal organic carbon warrant closer examination. We investigated the molecular composition of dissolved organic matter (DOM) in two rivers and surface waters of Young Sound, NE Greenland—a high Arctic fjord where glacial runoff contributes to low primary productivity and increasing heterotrophy. Using ultra‐high‐performance liquid chromatography–tandem mass spectrometry (UHPLC‐qTOF‐MS), we conducted a non‐targeted analysis of solid‐phase extracted DOM (SPE‐DOM). We expected DOM composition to differ between the two studied rivers (Tyroler and Zackenberg), which contrast in length, glacial water source, and catchment characteristics, and to reflect the salinity gradient in fjord waters. Both rivers carried a strong glacial imprint, with DOM enriched in aliphatic constituents typically associated with higher bioavailability, yet the proglacial Zackenberg River also exhibited unique compositional features that were more unsaturated and aromatic in character. Comparisons along a salinity gradient, from river plumes to outer fjord and open sea, revealed limited contrasts beyond the most glacially influenced section, with SPE‐DOM composition showing high similarity across sites. Although multiple factors may contribute to this similarity, dilution and rapid processing of glacially derived DOM are likely to play a role. While further research is needed to understand carbon cycling in high Arctic fjords, our findings offer relevant insight into the molecular characteristics and potential ecological roles of DOM in this environment.