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

Abstract

Climatic changes are transforming northern high-latitude watersheds as permafrost thaws and vegetation and hydrology shift. These changes have implications for the source and reactivity of riverine dissolved organic matter (DOM), and thus biogeochemical cycling, across northern high-latitude systems. In this study, we use a latitudinal gradient from the interior to the North Slope of Alaska to evaluate seasonal and landscape drivers of DOM composition in this changing Arctic environment. To assess DOM source and composition, we used absorbance and fluorescence spectroscopy to measure DOM optical properties, lignin biomarker analyses to evaluate vascular plant contribution to the DOM pool, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to assess DOM compositional changes. We found that seasonal inputs of DOM at elevated discharge during the freshet were typically more aromatic in nature with higher lignin concentrations and carbon-normalized yields. Landscape characteristics were a major control on dissolved organic carbon (DOC) yields and DOM composition. More northern watersheds, which were steeper, underlain by continuous permafrost, and exhibited a mix of barren and lichen/moss vegetation cover, exported less DOC with relatively more aliphatic DOM compared to more southern basins. Watersheds with deeper active layers exported DOM that was more aromatic with higher polyphenolic and condensed aromatic relative abundances and lignin yields, likely sourced from shallow subsurface flow during high discharge periods. However, contributions from deeper groundwater to streamflow is expected to increase, which would increase interactions of groundwater with mineral soils and decrease aromatic DOM contributions during periods of low discharge.Plain Language Summary Northern high-latitude rivers are undergoing changes linked to thawing permafrost, warming temperatures, and altered hydrology due to climate change. These changes will impact the source and cycling of carbon in these rivers. Here, we analyzed dissolved organic matter (DOM) composition from watersheds across a gradient of permafrost and vegetation coverage to assess how northern high-latitude rivers will respond to change. This gradient of time and space enabled us to project how watersheds might respond to climatic changes. We hypothesized that sites with continuous permafrost north of the tree line export less dissolved organic carbon and DOM that is less terrestrial, due to longer subsurface residence that enhances processing of terrestrial DOM. Consistent with this hypothesis, the sparsely vegetated, steep northern watersheds underlain by continuous permafrost contained DOM characteristic of snowmelt or groundwater. Conversely, more forested, southern basins contained more terrestrial DOM. As these watersheds change the contribution of shallow subsurface flow might increase DOC exports and the contribution of terrestrial DOM, with the caveat that deeper groundwater contributions with altered hydrology can decrease DOC and terr...

Hydrologic and Landscape Controls on Dissolved Organic Matter Composition Across Western North American Arctic Lakes

“…SUVA 254 was also comparable to other lakes from the Yukon Flats (0.9–4.4 L mg C −1 m −1 ; Johnston et al., 2020), Mackenzie Delta (2–4 L mg C −1 m −1 ; Tank et al., 2011), and Alaskan rivers (2.0–4.5 L mg C −1 m −1 ; Spencer et al., 2008; Johnston et al., 2021); however, CDOM absorbance was greater than in Alaskan rivers (mean = 16.3 m −1 ; Johnston et al., 2021). Finally, spectral slope properties ( S 275‐ 295 ) were in ranges similar to lakes from the Yukon Flats (0.015–0.035 nm −1 ; Johnston et al., 2020), but spectral slopes were generally steeper than other northern high‐latitude streams (0.012–0.018 nm −1 ; Frey et al., 2016; Johnston et al., 2021) and major Arctic rivers (0.012–0.021 nm −1 ; Mann et al., 2016).…”

“…Most lakes regions, excluding Daring and the North Slope, had DOC concentrations comparable to those measured from the Yukon Flats (10–40 mg C L −1 ; Johnston et al., 2020), the Mackenzie Delta (5–16 mg C L −1 ; Tank et al., 2011), and across boreal forests (median = 15.3 mg C L −1 ; Stolpmann et al., 2021), but concentrations were typically higher than many northern high‐latitude and thawing permafrost‐fed fluvial sites (0.7–27 mg C L −1 ; Wologo et al., 2021; Johnston et al., 2021). SUVA 254 was also comparable to other lakes from the Yukon Flats (0.9–4.4 L mg C −1 m −1 ; Johnston et al., 2020), Mackenzie Delta (2–4 L mg C −1 m −1 ; Tank et al., 2011), and Alaskan rivers (2.0–4.5 L mg C −1 m −1 ; Spencer et al., 2008; Johnston et al., 2021); however, CDOM absorbance was greater than in Alaskan rivers (mean = 16.3 m −1 ; Johnston et al., 2021). Finally, spectral slope properties ( S 275‐ 295 ) were in ranges similar to lakes from the Yukon Flats (0.015–0.035 nm −1 ; Johnston et al., 2020), but spectral slopes were generally steeper than other northern high‐latitude streams (0.012–0.018 nm −1 ; Frey et al., 2016; Johnston et al., 2021) and major Arctic rivers (0.012–0.021 nm −1 ; Mann et al., 2016).…”

Global Riverine Export of Dissolved Lignin Constrained by Hydrology, Geomorphology, and Land‐Cover

“… Locations of data. Red dots denote data from this study, and blue dots represent data from the literature (Aufdenkampe et al., 2007; H. Y. Bao et al., 2015; H. Bao et al., 2019; Benner & Kaiser, 2011; Besemer et al., 2009; Bouillon et al., 2012; Duan et al., 2007; Eckard et al., 2007, 2017; Fellman et al., 2014; Godin et al., 2017; Gonsior et al., 2011; Hedges et al., 2000; Hernes et al., 2008, 2017; Johnston et al., 2018, 2021; Liu et al., 2021; Mann et al., 2016; S. P. Opsahl & Zepp, 2001; Shen et al., 2012; Spencer, Hernes, et al., 2010; Spencer et al., 2008, 2012, 2016; Wang et al., 2021; Ward et al., 2012), see Section 2.4 for the data source of land‐cover. …”

“…is compiled. Existing data are mainly from the Arctic (Johnston et al, 2018(Johnston et al, , 2021Mann et al, 2016;Spencer et al, 2008), North America (Bianchi et al, 2013;Eckard et al, 2007;Ward et al, 2012), South America and Africa (Hedges et al, 2000;Hernes et al, 2017;Spencer et al, 2016). Only limited data are available from Asia (H. Y.…”

The Transformation and Export of Organic Carbon Across an Arctic River‐Delta‐Ocean Continuum

“…The organic carbon that is exported from rivers to the ocean is variable in composition and source and depends largely on the hydrology and geomorphology of the rivers (Lynch et al., 2019 ). Terrigenous DOC is generally enriched in chromophoric dissolved organic matter (CDOM), which is a reliable tracer of soil and vascular plant derived DOC such as lignin phenols (e.g., Johnston et al., 2021 ; Mann et al., 2016 ; Spencer et al., 2008 ). The POC pool varies substantially across river systems with estimated mean radiocarbon ages ranging from 2,000 to 5,500 years while DOC is generally young (Barnes et al., 2018 ; Campeau et al., 2020 ; Karlsson et al., 2016 ) although recent radiocarbon evidence shows permafrost DOC mobilization in the Mackenzie River (Schwab et al., 2020 ).…”