Molecular signatures of dissolved organic matter in the surface waters of a glacially influenced Arctic fjord (Young Sound, NE Greenland)

The rapidly melting Arctic glaciers deliver substantial amounts of allochthonous material to the coastal ocean, altering the environment in which biogeochemical processes are taking place. Perturbed aquatic carbon cycling is a particularly troubling outcome of the intensified glacial runoff, with increasing heterotrophy and outgassing of CO₂ among the chief concerns. As the key factors influencing the activity of heterotrophic microbes, the quantity and quality of Arctic coastal organic carbon warrant closer examination. To address the relevant aspects, we investigated the molecular composition of dissolved organic matter (DOM) in the glacial rivers and surface coastal waters of the high Arctic fjord (Young Sound, NE Greenland), where glacial runoff contributes to low primary productivity and increasing heterotrophy. Using ultra-high performance liquid chromatography quadrupole time-of-flight mass spectrometry with electrospray ionization (UHPLC-ESI-qTOF-MS), we conducted a non-targeted analysis of solid-phase extracted DOM. We expected molecular signatures of DOM to differ substantially between the two investigated rivers (Tyroler and Zackenberg River), which contrast in length, glacial water source, and drainage basin characteristics. While unique compounds were indeed observed in each river, considerable overlap in molecular DOM signatures was also detected, and a large proportion of aliphatic formulas was found at each site. Comparisons between riverine, river plume, and the outer fjord samples indicated a rapid loss of the riverine signature, likely due to the combined effects of dilution and swift biological consumption of the glacially derived DOM in the inner fjord. Although further research is needed to better understand the mechanisms and ecosystem implications of carbon utilization in high Arctic fjords, our study offers useful insights into the molecular signatures and fates of glacially derived carbon in these systems.