Evening methane emission pulses from a boreal wetland correspond to convective mixing in hollows

Abstract

[1] Spatial and temporal heterogeneity of methane flux from boreal wetlands makes prediction and up-scaling challenging, both within and among wetland systems. Drivers of methane production and emissions are also highly variable, making empirical model development difficult and leading to uncertainty in methane emissions estimates from wetlands. Previous studies have examined this problem using point-scale (static chamber method) and ecosystem-scale (flux tower methods) measurements, but few studies have investigated whether different processes are observed at these scales. We analyzed methane emissions from a boreal fen, measured by both techniques, using data from the Boreal Ecosystem-Atmosphere Study. We sought to identify driving processes associated with methane emissions at two scales and explain diurnal patterns in emissions measured by the tower. The mean methane emission rates from flux chambers were greater than the daytime, daily mean rates measured by the tower, but the nighttime, daily mean emissions from the tower were often an order of magnitude greater than emissions recorded during the daytime. Thus, daytime measurements from either the tower or chambers would lead to a biased estimate of total methane emissions from the wetland. We found that the timing of nighttime emission events was coincident with the cooling and convective mixing within hollows, which occurred regularly during the growing season. We propose that diurnal thermal stratification in shallow pools traps methane by limiting turbulent transport. This methane stored during daytime heating is later released during evening cooling due to convective turbulent mixing.Citation: Godwin, C. M., P. J. McNamara, and C. D. Markfort (2013), Evening methane emission pulses from a boreal wetland correspond to convective mixing in hollows,

An assessment of natural methane fluxes simulated by the CLASS-CTEM model

“…Wetland methane emissions are known to be spatially highly heterogeneous and temporally intermittent (e.g. Godwin et al, 2013) and CLASS-CTEM does not represent physical processes that govern methane emissions at small spatial and temporal scales. Instead the model is designed for operation at large spatial scales (> 100 km) and implementation within an Earth system model and as such only temperature, soil moisture, and substrate availability (through heterotrophic respiration) are taken into account.…”

High net CO<sub>2</sub> and CH<sub>4</sub> release at a eutrophic shallow lake on a formerly drained fen

“…No diurnal pattern (e.g. Rinne et al, 2007;Forbrich et al, 2011;Herbst et al, 2011) occurred especially at sites without large open water areas (Godwin et al, 2013). We assume the process of convective mixing of the water column (e.g.…”

“…We assume the process of convective mixing of the water column (e.g. Godwin et al, 2013;Poindexter and Variano, 2013;Podgrajsek et al, 2014;Sahlée et al, 2014;Koebsch et al, 2015) to be crucial for the diurnal pattern of CH 4 emissions at our study site. This is indicated by the concurrent timing of convective mixing and daily peak CH 4 emissions and a generally high fractional source area coverage of the open water, which shows higher rates of CH 4 release than emergent vegetation.…”

“…Due to the heat release of the surface water to the atmosphere in the night the surface water cools down, initiating convective mixing of the water column down to the bottom. Diffusion is enhanced due to the buoyancy-induced turbulence, the associated increased gas transfer velocity at the air-water interface (Eugster et al, 2003;MacIntyre et al, 2010;Podgrajsek et al, 2014) as well as the transport of CH 4 enriched bottom water to the surface (Godwin et al, 2013;Podgrajsek et al, 2014). In addition, ebullition can be triggered by turbulence due to convective mixing Read et al, 2012).…”

Comparison of floating chamber and eddy covariance measurements of lake greenhouse gas fluxes

“…Atmospheric concentrations of greenhouse gases, such as methane (CH 4 ) and carbon dioxide (CO 2 ), have increased significantly since pre-industrial times (Forster et al, 2007). Knowledge of both natural and anthropogenic sources and sinks of these greenhouse gases is needed for a better understanding of the global carbon cycle.…”