Abstract
AbstractGroundwater discharge from the seasonally thawed active layer is increasingly recognized as an important pathway for delivering methane (CH4) into Arctic lakes and streams, but its contribution to CH4 emissions from thaw ponds and its influence on the trophic support and nutritional quality of pond food chains remains unexplored. We quantified the transport of CH4 from the active layer through groundwater discharge into thaw ponds in a subarctic catchment in northern Sweden, using radon (222Rn) as groundwater tracer. We analyzed stable isotopes and fatty acids of pond macroinvertebrates to evaluate the potential effects of groundwater‐mediated CH4 inputs on the aquatic food chains. Our results indicate that active layer groundwater discharge flows are nontrivial (range 6%–46% of pond volume per day) and the associated CH4 fluxes (median 339 mg C m−2day−1, interquartile range [IQR]: 179–419 mg C m−2 day−1) can sustain the diffusive CH4 emissions from most of the ponds (155 mg C m−2 day−1, IQR: 55–234 mg C m−2 day−1). Consumers in ponds receiving greater CH4 inputs from the active layer had lower stable carbon (C) 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. Overall, this work links physical (CH4 transport from the active layer), biogeochemical (CH4 emission), and ecological (MOB‐consumer interaction) processes to provide direct evidence for the role of active layer groundwater discharge in CH4 cycling of subarctic thaw ponds.
Highlights
Climate warming and shifts in precipitation regimes are strong in arctic and subarctic regions (IPCC, 2013), causing thawing of permafrost and the formation of small water basins (Bouchard et al, 2014; O'Donnell et al, 2012)
Our results indicate that active layer groundwater discharge flows are nontrivial and the associated CH4 fluxes can sustain the diffusive CH4 emissions from most of the ponds (155 mg C m−2 day−1, IQR: 55–234 mg C m−2 day−1)
Ponds in Storflaket and Stordalen were separated along PC2 that mainly correlated with dissolved oxygen (DO), conductivity, CH4, CO2, and Dissolved inorganic carbon (DIC)
Summary
Climate warming and shifts in precipitation regimes are strong in arctic and subarctic regions (IPCC, 2013), causing thawing of permafrost and the formation of small water basins (Bouchard et al, 2014; O'Donnell et al, 2012). These thaw (thermokarst) lakes and ponds are ubiquitous in the permafrost landscape and hotspots for carbon dioxide (CO2) and methane (CH4) emissions (Holgerson & Raymond, 2016; Kuhn et al, 2018; Laurion et al, 2010; Wik et al, 2016). Little is known regarding CH4 emissions from small thaw ponds and, especially, their environmental drivers
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