Articles | Volume 13, issue 1
Biogeosciences, 13, 27–44, 2016

Special issue: Freshwater ecosystems in changing permafrost landscapes

Biogeosciences, 13, 27–44, 2016

Research article 14 Jan 2016

Research article | 14 Jan 2016

Detection and spatiotemporal analysis of methane ebullition on thermokarst lake ice using high-resolution optical aerial imagery

P. R. Lindgren1, G. Grosse1,2, K. M. Walter Anthony3, and F. J. Meyer1 P. R. Lindgren et al.
  • 1Geophysical Institute, University of Alaska Fairbanks, Fairbanks, USA
  • 2Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Potsdam, Germany
  • 3Water and Environmental Research Center, University of Alaska Fairbanks, Fairbanks, USA

Abstract. Thermokarst lakes are important emitters of methane, a potent greenhouse gas. However, accurate estimation of methane flux from thermokarst lakes is difficult due to their remoteness and observational challenges associated with the heterogeneous nature of ebullition. We used high-resolution (9–11 cm) snow-free aerial images of an interior Alaskan thermokarst lake acquired 2 and 4 days following freeze-up in 2011 and 2012, respectively, to detect and characterize methane ebullition seeps and to estimate whole-lake ebullition. Bubbles impeded by the lake ice sheet form distinct white patches as a function of bubbling when lake ice grows downward and around them, trapping the gas in the ice. Our aerial imagery thus captured a snapshot of bubbles trapped in lake ice during the ebullition events that occurred before the image acquisition. Image analysis showed that low-flux A- and B-type seeps are associated with low brightness patches and are statistically distinct from high-flux C-type and hotspot seeps associated with high brightness patches. Mean whole-lake ebullition based on optical image analysis in combination with bubble-trap flux measurements was estimated to be 174 ± 28 and 216 ± 33 mL gas m−2 d−1 for the years 2011 and 2012, respectively. A large number of seeps demonstrated spatiotemporal stability over our 2-year study period. A strong inverse exponential relationship (R2 >  =  0.79) was found between the percent of the surface area of lake ice covered with bubble patches and distance from the active thermokarst lake margin. Even though the narrow timing of optical image acquisition is a critical factor, with respect to both atmospheric pressure changes and snow/no-snow conditions during early lake freeze-up, our study shows that optical remote sensing is a powerful tool to map ebullition seeps on lake ice, to identify their relative strength of ebullition, and to assess their spatiotemporal variability.

Short summary
We mapped and characterized methane ebullition bubbles trapped in lake ice, and estimated whole-lake methane emission using high-resolution aerial images of a lake acquired following freeze-up. We identified the location and relative sizes of high- and low-flux seepage zones within the lake. A large number of seeps showed spatiotemporal stability over our study period. Our approach is applicable to other regions to improve the estimation of methane emission from lakes at the regional scale.
Final-revised paper