Dissolved organic matter sources in glacierized watersheds delineated through compositional and carbon isotopic modeling

Abstract

Climate change is decreasing watershed glacial coverage throughout Alaska, impacting the biogeochemistry of downstream ecosystems. We collected streamwater fortnightly over the glacial runoff period from three streams of varying watershed glacier coverage (0–49%) and a subglacial outflow to assess how glacier recession impacts the relative contributions of glacier and terrestrial plant derived dissolved organic matter (DOM) inputs to streams. We show an increase in the fraction of old dissolved organic carbon (up to ∼ 3200 yr old radiocarbon age) with increasing glacial meltwater contribution to streamflow. We use a dual isotopic mixing model (δ13C and Δ14C) to quantify the relative contribution of terrestrial and glacial sources to streamwater DOM. The endmember contributions were further compared to DOM molecular compositional data from Fourier‐transform ion cyclotron resonance mass spectrometry to assess whether DOM composition can be linked to streamwater DOM source in watersheds with varying contributions of glacial runoff. This approach revealed the glacial fraction was positively correlated with percent relative abundance of heteroatom‐containing DOM molecular formulae, aliphatics, and peptide‐like formulae, while the terrestrial fraction was positively correlated with condensed aromatics and polyphenolics. These results provide information about how the retreat of mountain glaciers will impact the composition and thus biogeochemical role of DOM delivered to downstream ecosystems. Our findings highlight that combining an isotopic mixing model and ultrahigh resolution mass spectrometry data can provide novel insights into how changes in watershed landcover impact the source and chemical properties of streamwater DOM.

The evolution of stream dissolved organic matter composition following glacier retreat in coastal watersheds of southeast Alaska

“…Concomitantly, the aliphatic and peptide-like contributions decreased (Table 1, Figure 4). This was in broad agreement with previously reported values in southeast Alaska, where glacier outflows have a far greater relative abundance of aliphatics and fewer condensed aromatic and polyphenolic compounds than old growth forest watersheds (Behnke et al 2020;Stubbins et al 2012). Sugar-like compounds contributed very little DOM (0.02 -0.31 % RA) to all streams and thus are not discussed further (Table 1).…”

“…For example,  18 O values in southeast Alaskan glacier outflows and old growth forest streams ranged from -15.6 to -17.0 ‰ (n = 8) and -12.6 to -13.8 ‰ (n = 7), respectively (Fellman et al 2014). In past studies, DOC concentrations ranged from 0.13 -0.37 (n =14) and 1.10 -14.01 (mean 7.3 ± 4.6, n = 15) mg C L -1 for glacier outflow and old growth forest watersheds, respectively (Behnke et al 2020;Fellman et al 2014;Nagorski et al 2014;Spencer et al 2014b). The DOC concentration for OGF1393 was at the lower end of previously reported values for old growth forest watersheds; this could be a function of soil properties, soil organic carbon stocks and/or the low streamflow during the sampling period, since transport of DOC to the stream would have been limited (Canadell et al 2019;Hood et al 2020).…”

“…In past studies, DOC concentrations ranged from 0.13 -0.37 (n =14) and 1.10 -14.01 (mean 7.3 ± 4.6, n = 15) mg C L -1 for glacier outflow and old growth forest watersheds, respectively (Behnke et al 2020;Fellman et al 2014;Nagorski et al 2014;Spencer et al 2014b). The DOC concentration for OGF1393 was at the lower end of previously reported values for old growth forest watersheds; this could be a function of soil properties, soil organic carbon stocks and/or the low streamflow during the sampling period, since transport of DOC to the stream would have been limited (Canadell et al 2019;Hood et al 2020). The marked similarity in  18 O and DOC values of our endmember samples (GO0 and OGF1393, where n = 1) with other similar watersheds in southeast Alaska, highlights that our watersheds and sampling period were representative.…”

“…GO0 had the most depleted  14 C-DOC value (-246.1 ‰ ≈ 2,220 ybp; Figure 3b). Globally, DOC in bulk glacier runoff has been shown to be ancient, with  14 C-DOC ranging from -639 to -108 ‰, n = 36 (Andrews et al 2018;Bhatia et al 2013;Csank et al 2019;Spencer et al 2014a) and in southeast Alaska glacier outflows  14 C-DOC ranges from -355 to -182 ‰, n = 10 ( Arimitsu et al 2018;Behnke et al 2020;Hood et al 2009;Spencer et al 2014b). Furthermore, glacier DOC is usually more  13 C enriched than DOC derived from forested ecosystems, including those in southeast Alaska (-26.35 to -23.7 ‰, n = 14 compared to -28.7 to -28.0 ‰, n = 7; Fellman et al 2014;Hood et al 2009;Raymond and Bauer 2001).…”

“…Late succession (LS100-210) and OGF1393  14 C-DOC approached modern values (mean -22.2 ± 48.2 and 13.7 in LS100-210 streams and OGF1393, respectively; Table 1, Figure 3b). Recently leached, modern DOC is characteristic of non-glacial streams in southeast Alaska (Δ 14 C-DOC of 33 to 69 ‰, n = 4, respectively; Behnke et al 2020;Hood et al 2009) and undisturbed, forested watersheds without permafrost influence (Butman et al 2015;Raymond and Bauer 2001).…”