Abstract. Thermokarst features are widespread in ice-rich regions of the circumpolar Arctic. The rate of thermokarst lake formation and drainage is anticipated to accelerate as the climate warms. However, it is uncertain how these dynamic features impact the terrestrial Arctic carbon cycle. Methane (CH4) and carbon dioxide (CO2) fluxes were measured during peak growing season using eddy covariance and chambers at Illisarvik, a 0.16 km2 thermokarst lake basin that was experimentally drained in 1978 on Richards Island, Northwest Territories, Canada. Vegetation in the basin differs markedly from the surrounding dwarf-shrub tundra and included patches of tall shrubs, grasses, and sedges with some bare ground and a small pond in the centre. During the peak growing season, temperature and wind conditions were highly variable, and soil water content decreased steadily. Basin-scaled net ecosystem CO2 exchange (NEE) measured by eddy covariance was −1.5 [CI95 %±0.2] g C−CO2 m-2d-1; NEE followed a marked diurnal pattern with no day-to-day trend during the study period. Variations in half-hourly NEE were primarily controlled by photosynthetic photon flux density and influenced by vapour pressure deficit, volumetric water content, and the presence of shrubs within the flux tower footprint, which varied with wind direction. Net methane exchange (NME) was low (8.7 [CI95 %±0.4] mgCH4m-2d-1) and had little impact on the growing season carbon balance of the basin. NME displayed high spatial variability, and sedge areas in the basin were the strongest source of CH4 while upland areas outside the basin were a net sink. Soil moisture and temperature were the main environmental factors influencing NME. Presently, Illisarvik is a carbon sink during the peak growing season. However, these results suggest that rates of growing season CO2 and CH4 exchange rates may change as the basin's vegetation community continues to evolve.
Abstract. Thermokarst features are widespread in ice-rich regions of the circumpolar Arctic. The rate of thermokarst lake formation and drainage is anticipated to accelerate as the climate warms. However, it is uncertain how these dynamic features impact the terrestrial Arctic carbon cycle. Methane (CH4) and carbon dioxide (CO2) fluxes were measured during peak growing season using eddy covariance and chambers at Illisarvik, a 0.16 km2 thermokarst lake basin that was experimentally drained in 1978 on Richards Island, Northwest Territories, Canada. Vegetation in the basin differs markedly from the surrounding dwarf-shrub tundra and included patches of tall shrubs, grasses and sedges with some bare ground and a small pond in the centre. During the study period, temperature and wind conditions were highly variable and soil water content decreased steadily. Basin scaled net ecosystem exchange (NEE) measured by eddy covariance was −1.5 [CI95 % ± 0.2] g C-CO2 m−2 d−1; NEE followed a marked diurnal pattern with no trend during the study period. NEE was primary controlled by photosynthetic photon flux density and influenced by vapor pressure deficit, volumetric water content and the presence of shrubs. By contrast, net methane exchange (NME) was low (8.7 [CI95 % ± 0.4] mg CH4 m−2 d−1 and had little impact on the carbon balance of the basin during the study period. NME displayed high spatial variability, sedge areas in the basin were the strongest source of CH4 while upland areas outside the basin were a net sink. Soil moisture and temperature were the main environmental factors influencing NME, having a positive and negative effect respectively.
Growing season surface-atmosphere exchange of carbon dioxide and methane were quantified at Fish Island, a wetland site in the lower Northeast Mackenzie River Delta, NWT, Canada. The terrain consists of low-center polygonal tundra and is subject to infrequent flooding in high water years. Carbon dioxide and methane fluxes were continuously measured using eddy covariance and the relevance of different environmental controls were identified using Neural Networks. Net daily carbon dioxide uptake peaked in mid-July before gradually decreasing and transitioning to net daily emissions by September. Variations in light level and temperature were the main controls over diurnal net carbon dioxide uptake, while thaw depth and phenology were the main seasonal controls. Methane emissions measured at Fish Island were higher than comparable studies on river delta sites in the Arctic and were influenced by the interaction of a large number of factors including thaw and water table depth, soil temperatures and net radiation. Spikes in methane emissions were associated with strong winds and turbulence. The Fish Island tundra was a net sink for carbon during the growing season and methane emissions only slightly reduced the overall sink strength.
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