The role of methane transport from the active layer in sustaining methane emissions and food chains in subarctic ponds

Abstract

<p>Shallow groundwater flow from the seasonally thawed active layer is increasingly recognized as an important pathway for delivering methane (CH<sub>4</sub>) into Arctic lakes and streams, but its contribution to CH<sub>4</sub> emissions from thaw ponds has not been evaluated. Furthermore, the potential influence of the shallow groundwater-derived CH<sub>4</sub> on the trophic support and nutritional quality of thaw pond food chains remains unexplored. In this study, we used a radon-mass balance approach to quantify the CH<sub>4 </sub>transport from the active layer into thaw ponds in a sub-Arctic catchment. We analysed stable isotopes and fatty acids of pond macroinvertebrates to evaluate the potential effects of CH<sub>4</sub> inputs through active layer groundwater flows on the aquatic food chains. Our results indicate that CH<sub>4</sub> fluxes from the active layer can sustain CH<sub>4</sub> emissions from the ponds. Consumers in ponds receiving greater CH<sub>4</sub> inputs from the active layer had lower stable carbon isotope signatures that indicates a greater trophic reliance on methane oxidizing bacteria (MOB), and they had lower nutritional quality as indicated by their lower tissue concentrations of polyunsaturated fatty acids. Accurate predictions of CH<sub>4</sub> release from small thaw ponds will thus require improved knowledge of the contributions from various processes including internal production, flow paths of active layer groundwater, and MOB-consumer interactions.</p>

Aged soils contribute little to contemporary carbon cycling downstream of thawing permafrost peatlands

“…We also refitted this model using solid peat at the bottom of the active layer as a terrestrial DIC source instead of shallow groundwater DOC. Peatland soils can also export large quantities of CH 4 to fluvial networks, especially from anaerobic conditions in the active layer (Campeau et al, 2014; Dinsmore et al, 2010; Olid et al, 2021 but see Street et al, 2016). This CH 4 can then be made available for autotrophic growth by methane‐oxidizing bacteria (Grey, 2016; Kohzu et al, 2004; Raghoebarsing et al, 2005).…”

Permafrost and groundwater interaction: current state and future perspective

“…These differences were due to an almost 18-fold higher concentration of CH 4 in groundwater discharged into Landing Lake than into Toolik Lake, which in turn was explained by the shallow active layer and the small amount of OM in the active layer in Toolik Lake (Dimova et al, 2015). The estimated contribution of groundwater to subarctic ponds located in the sporadic permafrost zone of the Stordalen catchment (subarctic region of northern Sweden) ranged from 6% to 46% of the volume of the pond per day (Olid et al, 2021). The thawed subarctic ponds were supersaturated with CH 4 .…”

The highest methane concentrations in an Arctic river are linked to local terrestrial inputs

“…This is a sensical finding, as the dynamic river flow enables the diffusive methane transport and emissions to the atmosphere compared to the emissions across smaller surface areas in highly-stratified, less dynamic and largely anoxic pond environments. The majority of the CH4 produced in thawing permafrost is first locally oxidized before it can be released to the atmosphere (Olid et al, 2021). Thus, 615 the higher relative abundance of CH4-consuming bacteria compared to CH4-producing archaea in the Kolyma River suggests that a considerable fraction of CH4 is already oxidized within the recently thawed active layer.…”

“…As the seasonal progression takes place, deeper water-saturated soil layers are thawed, and substances, microorganisms, and gases, like CH4, are mobilized through the lateral transfer from groundwater discharge into Arctic inland waters, particularly to the 635 fluvial network (Connolly et al, 2020;Harms et al, 2020;Saunois et al, 2020). It has been demonstrated that the majority of the CH4 emitted to the atmosphere from subarctic ponds is sustained by the discharge of CH4 from groundwaters upon the active layer thaw (Olid et al, 2021).…”

“…Trapped or newly formed CH4 can be emitted directly to the atmosphere after the abrupt or gradual permafrost thaw (Olefeldt et al, 2013;Saunois et al, 2020;Turetsky et al, 2020), or be laterally transported into neighboring inland waters via surface hydraulic connectivity or underground drainage (e.g., Dabrowski et al, 2020). Current and projected changes in the Arctic land surface hydrology, vegetation, landscape, and temperature due to permafrost thaw, will modulate CH4 concentrations in Arctic fluvial ecosystems (Harms et al, 2020;Olid et al, 2021).…”