Metabolism overrides photo‐oxidation in CO₂ dynamics of Arctic permafrost streams

Abstract

Global warming is enhancing the mobilization of organic carbon (C) from Arctic soils into streams, where it can be mineralized to CO2 and released to the atmosphere. Abiotic photo‐oxidation might drive C mineralization, but this process has not been quantitatively integrated with biological processes that also influence CO2 dynamics in aquatic ecosystems. We measured CO2 concentrations and the isotopic composition of dissolved inorganic C (δ13CDIC) at diel resolution in two Arctic streams, and coupled this with whole‐system metabolism estimates to assess the effect of biotic and abiotic processes on stream C dynamics. CO2 concentrations consistently decreased from night to day, a pattern counter to the hypothesis that photo‐oxidation is the dominant source of CO2. Instead, the observed decrease in CO2 during daytime was explained by photosynthetic rates, which were strongly correlated with diurnal changes in δ13CDIC values. However, on days when modeled photosynthetic rates were near zero, there was still a significant diel change in δ13CDIC values, suggesting that metabolic estimates are partly masked by O2 consumption from photo‐oxidation. Our results suggest that 6–12 mmol CO2‐C m−2 d−1 may be generated from photo‐oxidation, a range that corresponds well to previous laboratory measurements. Moreover, ecosystem respiration rates were 10 times greater than published photo‐oxidation rates for these Arctic streams, and accounted for 33–80% of total CO2 evasion. Our results suggest that metabolic activity is the dominant process for CO2 production in Arctic streams. Thus, future aquatic CO2 emissions may depend on how biotic processes respond to the ongoing environmental change.

Limited Presence of Permafrost Dissolved Organic Matter in the Kolyma River, Siberia Revealed by Ramped Oxidation

“…This could help explain the lack of convergence in biodegraded permafrost (t28) E to that of the Kolyma River (t0), if photochemical reactions were to significantly influence permafrost DOM. The controlling processes in Arctic streams remains an area of ongoing debate (Cory & Kling, 2018;Cory et al, 2013;Rocher-Ros et al, 2020); however, no FT-ICR MS compositional parameter increased or decreased for degraded permafrost beyond that of the Kolyma River to account for the overall shift to higher E post-degradation, obscuring insight into the possible effects of photo-modification in our data set (Table 1). Future studies will be necessary to determine the effects of photochemistry on thermostability as no literature currently documents these effects.…”

“…Assessing how permafrost thaw DOC is contributing to, and changing with respect to Arctic riverine export, has been the focus of numerous recent studies (Bowen et al., 2020; Drake et al., 2018; Ewing et al., 2015; Mann et al., 2015; Rocher‐Ros et al., 2020; Spencer et al., 2015; Wickland et al., 2018). Abrupt thaw through thermokarst formation including thaw slumps has shown to be a major contributor of permafrost carbon release with a lasting impact on aquatic carbon concentrations (Abbott et al., 2014, 2015; Turetsky et al., 2020).…”

Environmental effects of stratospheric ozone depletion, UV radiation, and interactions with climate change: UNEP Environmental Effects Assessment Panel, Update 2021

“…release of CO 2 ) compared to biological processes [ 135 – 137 ]. It is possible that previous research has overestimated the extent of photomineralisation in Arctic rivers because data have been modelled with a gas transfer velocity typical of lakes, an assumption that recent studies proved inaccurate [ 136 ]. The possible role of photofacilitation was not investigated in these studies.…”

Does Photomineralization of Dissolved Organics Matter in Temperate Rivers?

“…Previous studies have additionally shown for both rivers and estuaries that photobleaching or photo‐bleachable DOC represents a major portion (>50%) of the DOC pool, despite possibly minor DOC photomineralization (Clark et al, 2020; Fichot & Benner, 2014; Yoon et al., 2021). We further note that our study does not directly compare photomineralization rates with bio‐mineralization (respiration) rates, which has been a metric used in past studies to identify photo‐alteration as an important (Cory et al., 2014) or unimportant process (Rocher‐Ros et al, 2021). We can make some broad conclusions by comparing our very low uptake velocities for photomineralization (mean = 0.0054 m day −1 ) with those for bio‐mineralization or bulk DOC processing in other New England rivers (e.g., Wollheim et al.…”