Pan‐Arctic Riverine Dissolved Organic Matter: Synchronous Molecular Stability, Shifting Sources and Subsidies

Abstract

Climate change is dramatically altering Arctic ecosystems, leading to shifts in the sources, composition, and eventual fate of riverine dissolved organic matter (DOM) in the Arctic Ocean. Here we examine a 6-year DOM compositional record from the six major Arctic rivers using Fourier-transform ion cyclotron resonance mass spectrometry paired with dissolved organic carbon isotope data (Δ 14 C, δ 13 C) to investigate how seasonality and permafrost influence DOM, and how DOM export may change with warming. Across the pan-Arctic, DOM molecular composition demonstrates synchrony and stability. Spring freshet brings recently leached terrestrial DOM with a latent high-energy and potentially bioavailable subsidy, reconciling the historical paradox between freshet DOM's terrestrial bulk signatures and high biolability. Winter features undiluted baseflow DOM sourced from old, microbially degraded groundwater DOM. A stable core Arctic riverine fingerprint (CARF) is present in all samples and may contribute to the potential carbon sink of persistent, aged DOM in the global ocean. Future warming may lead to shifting sources of DOM and export through: (1) flattening Arctic hydrographs and earlier melt modifying the timing and role of the spring high-energy subsidy; (2) increasing groundwater discharge resulting in a greater fraction of DOM export to the ocean occurring as stable and aged molecules; and(3) increasing contribution of nitrogen/sulfur-containing DOM from microbial degradation caused by increased connectivity between groundwater and surface waters due to permafrost thaw. Our findings suggest the ubiquitous CARF (which may contribute to oceanic carbon sequestration) underlies predictable variations in riverine DOM composition caused by seasonality and permafrost extent.

A new conceptual framework for the transformation of groundwater dissolved organic matter

“…This corresponds to a significant decrease in the % RA of IOS formulae in groundwater over time ( p = 3.4 × 10 −4 ). These findings contrast marine waters where decreases in H/C and increases in NOSC have been reported with increasing DOM residence times 37 , 38 , and are associated with an overall increase in IOS formulae over time in non-groundwater environments 36 , 37 , 41 ( p = 5.0 × 10 −7 , Fig. 3c ).…”

“…Subsequently, a larger group of CRAM molecules, termed the Island of Stability (IOS), was shown to increase in relative abundance in aging marine DOM 37 . Increasing relative abundance of molecules lying within the H/C and O/C bounds of the IOS have since been shown to correlate with increasing DOC age in other non-groundwater aquatic environments 36 , 41 , thereby raising the question of whether the degradation trajectory of natural DOM towards intermediate H/C and O/C ratios may be consistent, irrespective of environmental conditions. This was further supported by observations of a cascade of degradation processes driving the accumulation of a stable background DOM of similar molecular formulae in marine and lake samples (i.e., molecular-level convergence) 35 .…”

Controls on Riverine Dissolved Organic Matter Composition Across an Arctic‐Boreal Latitudinal Gradient

“…Molecular formulae that were significantly associated with SUVA 254 were largely aromatic (Figure 6e), further confirming the utility of SUVA 254 as a proxy for aromaticity. Additionally, during spring freshet there was an increase in the relative abundance of aliphatic molecular formulae, this higher relative abundance of aliphatic formulae during freshet may explain high DOC bioavailability previously observed during freshet (Behnke et al., 2021; Holmes et al., 2008; Textor et al., 2019; Wickland et al., 2012).…”

Merging Satellite and in situ Data to Assess the Flux of Terrestrial Dissolved Organic Carbon From the Mackenzie River to the Coastal Beaufort Sea

“…a decrease in May-June and an increase in July-November, may have implications on both the physical and biogeochemical settings of the coastal Beaufort Sea. In the Mackenzie Delta, tDOC has a high potential for biological degradation at the spring freshet (Gareis and Lesack, 2020;Behnke et al, 2021). A decrease of the riverine flux in May-June would imply that a lesser fraction of bioavailable tDOC would be delivered and potentially processed by heterotrophic bacteria within the marine ecosystem (Colatriano et al, 2018;Vaqué et al, 2019).…”

“…Because the interannual variability of this seasonal shift is high, such biogeochemical responses might be exacerbated in the future. This is particularly true in a context of Arctic warming, which is expected to alter both the river discharge seasonality and the quality of the tDOC exported (see Behnke et al, 2021).…”