<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>
Climate warming and shifts in precipitation regimes are particularly 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 (CO 2) and methane (CH 4) emissions (Holgerson & Raymond, 2016;
<p>Boreal water courses are large emitters of carbon dioxide (CO<sub>2</sub>) to the atmosphere. In Sweden, a high share of these water courses are man-made ditches, created to improve drainage and increase forest productivity. Previous studies from boreal regions have mainly suggested that terrestrial sources sustain the CO<sub>2</sub> in these ditches and with variability in hydrology as the main temporal control. However, few studies have quantified ditch CO<sub>2</sub> dynamics in harvested catchments. An altered hydrology, increased nutrient export and light availability upon forest harvest are all factors that potentially can change the main source control. Thus, there is a strong need to better understand how clear-cut forestry affects the ditch CO<sub>2</sub> dynamics in boreal regions.</p><p>Here, high-frequency (30 min) CO<sub>2</sub> concentration dynamics together with other hydro-chemical variables were studied in a forest ditch draining a fully harvested catchment in the Trollberget Experimental Area, northern Sweden. Data were collected during the snow-free season from May to October. Ditch CO<sub>2</sub> concentrations displayed a clear seasonal pattern with higher CO<sub>2</sub> during summer than in spring and autumn. Concentrations were ranging from 0.41 to 3.99 mg C L<sup>-1</sup> (median: 1.69 mg C L<sup>-1</sup>, corresponding to partial pressures (pCO<sub>2</sub>) of 2553 &#956;atm, IQR = 1.08 mg C L<sup>-1</sup>). Strong diel cycles in CO<sub>2</sub> were developed during early summer, with daily amplitudes in the CO<sub>2</sub> reaching up to 2.1 mg C L<sup>-1</sup>. These daily cycles in CO<sub>2</sub> were likely driven by aquatic primary production consuming CO<sub>2</sub> during daytime. In addition, individual high-flow events in response to rainfall had a major influence on the ditch CO<sub>2</sub> dynamics with generally a diluting effect, but the strength in the CO<sub>2</sub>-discharge relationship varied among seasons and between events. It was evident from the study that growing season CO<sub>2</sub> dynamics in forest ditches affected by clear-cut forestry are high and controlled by a combination of hydrological and biological factors. These high dynamics and the associated controls need to be considered when scaling ditch CO<sub>2</sub> emissions across boreal landscapes affected by clear-cut forestry.</p>
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