Abstract
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.
Highlights
The northern permafrost region stores approximately 50 % of global organic soil carbon in 16 % of the terrestrial land area (Tarnocai et al, 2009)
This study investigated net ecosystem CO2 exchange (NEE), gross primary productivity (GPP), ecosystem respiration (ER), and Net methane exchange (NME) in the Illisarvik experimental drained thermokarst lake basins (DTLBs) using eddy covariance (EC) and chamber data
To our knowledge this is the first such study conducted in a DTLB outside of the Barrow Peninsula or Siberia
Summary
The northern permafrost region stores approximately 50 % of global organic soil carbon in 16 % of the terrestrial land area (Tarnocai et al, 2009). Thermokarst landscapes account for approximately 20 % of the land area in this region and hold about half of its organic soil carbon (Olefeldt et al, 2013). Thermokarst lakes are a prominent landscape feature of the western Canadian Arctic (Mackay, 1999; Marsh et al 2009; Lantz and Turner, 2015). These lakes drain, sometimes catastrophically, forming drained thermokarst lake basins (DTLBs) via bank overflow, ice wedge erosion, coastal erosion, and stream migration (Billings and Peterson, 1980; Mackay, 1999). Lake formation and drainage are a natural part of the thaw lake cycle, but it is anticipated that climate change will accelerate or disturb this cycle, potentially altering the regional carbon balance (Jones et al, 2018)
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