Dissolved organic matter characterization in soils and streams in a small coastal low-arctic catchment

Abstract. Ongoing climate warming in the western Canadian Arctic is leading to thawing of permafrost soils and subsequent mobilization of its organic matter pool. Part of this mobilized terrestrial organic matter enters the aquatic system as dissolved organic matter (DOM) and is laterally transported from land to sea. Mobilized organic matter is an important source of nutrients for ecosystems as it is available for microbial breakdown, and thus a source of greenhouse gases. We are beginning to understand spatial controls on the release of DOM as well as the quantities and fate of this material in large arctic rivers. Yet, these processes remain systematically understudied in small, high-arctic watersheds, despite the fact that these watersheds experience the strongest warming rates in comparison. Here, we sampled soil (active layer and permafrost) and water (porewater and stream water) from a small catchment along the Yukon coast, Canada, during the summer of 2018. We assessed the organic carbon (OC) quantity (using dissolved (DOC) and particulate OC (POC) concentrations and soil OC content), quality (δ13C-DOC, optical properties, source-apportionment), and bioavailability (incubations, optical indices such as slope ratio (Sr) and humification index (HIX)) along with stream water properties (T, pH, EC, water isotopes). We classify and compare different landscape units and their soil horizons that differ in microtopography and hydrological connectivity, giving rise to differences in drainage capacity. Our results show that porewater DOC concentrations and yield reflect drainage patterns and waterlogged conditions in the watershed. DOC yield (in mg DOC g soil OC−1) generally increases with depth but shows a large variability near the transition zone (around the permafrost table). Active layer porewater DOC generally is more labile than permafrost DOC, due to various reasons (heterogeneity, presence of a paleo-active layer, and sampling strategies). Despite these differences, the very long transport times of porewater DOC indicate that substantial processing occurs in soils prior to release into streams. Within the stream, DOC strongly dominates over POC, illustrated by DOC/POC ratios around 50, yet storm events decrease that ratio to around 5. Source-apportionment of stream DOC suggests a contribution of around 50 % from permafrost/deep-active layer OC, which contrasts to patterns observed in large arctic rivers (12 ± 8 % Wild et al., 2019). Our 10-day monitoring period demonstrated temporal DOC patterns on multiple scales (i.e. diurnal patterns, storm-events, and longer-term trend) underlining the need for high-resolution long-term monitoring. First estimates of Black Creek annual DOC (8.2 ± 6.4 t DOC yr−1) and POC (0.21 ± 0.20 t yr−1) export allowed us to make a rough upscaling towards the entire Yukon Coastal Plain (447 ± 313 t DOC yr−1 and 8.95 ± 9.7 t POC yr−1). With raising arctic temperatures, increases in runoff, soil OM leaching, permafrost thawing and primary production are likely to increase the net lateral OC flux. Consequently, altered lateral fluxes may have strong impacts on the arctic aquatic ecosystems and arctic carbon cycling.