Investigating the Chemical Composition and Bioavailability of Arctic River Dissolved Organic Matter (DOM) Using Biomarkers

Arctic rivers are the dominant pathways for the transport of terrestrial dissolved organic matter to the Arctic Ocean, but knowledge of sources, transformations and transfer of organic carbon and nitrogen in Arctic river watersheds is extremely limited. This study uses chemical analyses of carbohydrates and enantiomeric amino acids to investigate the bioavailability of dissolved organic matter in five major Arctic river watersheds. In addition, existing measurements of lignin, mono/di-hydroxy benzenes, and radiocarbon contents were integrated to track the decomposition of the major organic constituents of riverine DOM and identify DOM sources. Carbohydrate-based indicators are sensitive to polysaccharide components derived from all plant sources; hydroxyproline and D-amino acids indicate plant and bacterial nitrogen, respectively; and lignin and mono/di-hydroxy benzenes track the phenolic constituents of vascular plants, mosses and lichens. The results show the bioavailability of DOM in Arctic rivers is strongly correlated with seasons, vegetation topography and water residence time in the watersheds. Pulses of bioavailable DOM were observed in the Siberian Rivers during the spring flood, whereas the Mackenzie River showed extensively degraded DOM throughout all stages of the hydrograph. Δ¹⁴C-DOC was not correlated to higher carbohydrate yields possibly indicating export of recently mobilized ancient permafrost organic carbon. Freshet samples showed significant input of plant-derived N, whereas bacterially-derived N dominated the dissolved organic nitrogen (DON) pool during low water discharge.