Climate‐Sensitive Controls on Large Spring Emissions of CH₄ and CO₂ From Northern Lakes

Abstract

Northern lakes are important sources of the climate forcing trace gases methane (CH4) and carbon dioxide (CO2). A substantial portion of lakes' annual emissions can take place immediately after ice melt in spring. The drivers of these fluxes are neither well constrained nor fully understood. We present a detailed carbon gas budget for three subarctic lakes, using 6 years of eddy covariance and 9 years of manual flux measurements. We combine measurements of temperature, dissolved oxygen, and CH4 stable isotopologues to quantify functional relationships between carbon gas production and conversion, energy inputs, and the redox regime. Spring emissions were regulated by the availability of oxygen in winter, rather than temperature as during ice‐free conditions. Under‐ice storage increased predictably with ice‐cover duration, and CH4 accumulation rates (25 ± 2 mg CH4‐C·m−2·day−1) exceeded summer emissions (19 ± 1 mg CH4‐C·m−2·day−1). The seasonally ice‐covered lakes emitted 26–59% of the annual CH4 flux and 15–30% of the annual CO2 flux at ice‐off. Reduced spring emissions were associated with winter snowmelt events, which can transport water downstream and oxygenate the water column. Stable isotopes indicate that 64–96% of accumulated CH4 escaped oxidation, implying that a considerable portion of the dissolved gases produced over winter may evade to the atmosphere.

Clumped Isotopes Link Older Carbon Substrates With Slower Rates of Methanogenesis in Northern Lakes

“…This study only considered ebullition fluxes of CH 4 , and there are no studies comparing Δ 18 with 14 C or flux for dissolved CH 4 . Where comparison is possible, dissolved CH 4 tends to contain younger carbon than ebullition CH 4 (Elder et al, 2019) and is more likely to have its stable isotope composition influenced by methane oxidation (Elder et al, 2019;Jansen et al, 2019). Further study will be needed to assess whether the patterns observed here for ebullition CH 4 also apply to diffusive CH 4 fluxes.…”

“…Further study will be needed to assess whether the patterns observed here for ebullition CH 4 also apply to diffusive CH 4 fluxes. However, for the studied lakes, ebullition fluxes are estimated to be the dominant source of CH 4 emissions in the ice-free season (Jansen et al, 2019;Sepulveda-Jauregui et al, 2015).…”

Drivers of diffusive CH<sub>4</sub> emissions from shallow subarctic lakes on daily to multi-year timescales

“…Q eff (W m −2 ) represents the net heat flux into the mixing layer and is the sum of net shortwave and long-wave radiation and sensible and latent heat fluxes. Penetration of radiation into the water column was evaluated across seven wavelength bands via Beer's law (Jellison and Melack, 1993). An attenuation coefficient of 0.74 was computed for the visible portion of the spectrum from Secchi depth (2.3 m; Karlsson et al, 2010) following Idso and Gilbert (1974).…”

Diverse Arctic lake sediment microbiota shape methane emission temperature sensitivity

“…Ebullition moves CH 4 rapidly from sediments directly to the atmosphere, typically bypassing microbial CH 4 oxidation in the water column 11 . Incoming short-wave radiation and sediment temperature have been identified as strong predictors of ebullitive CH 4 emission from sub-arctic post-glacial lakes on an annual basis, with higher temperature increasing emissions during the ice-free season 2,12 . However, the extent and drivers of spatial variability in this temperature response, particularly within lakes, are poorly understood.…”

“…These lakes are part of the Stordalen Mire complex, a hydrologically interconnected, discontinuous permafrost ecosystem encompassing post-glacial lakes and a mosaic palsa/wetland in approximately equal portions 13 . The lakes contribute ∼55% of the total ecosystem CH 4 loss 2 and are model sites for studying ebullitive emissions, which were collected at lake surfaces for the six summers from 2009-2014 12,14 every 1-3 days 9 . Here, we linked site-specific (lake edge vs. middle) CH 4 emissions to analyses of the microbiota and biogeochemistry in the underlying sediments.…”