Enhancing Surface Methane Fluxes from an Oligotrophic Lake: Exploring the Microbubble Hypothesis

Abstract

Exchange of the greenhouse gases carbon dioxide (CO2) and methane (CH4)across inland water surfaces is an important component of the terrestrial carbon (C) balance. We investigated the fluxes of these two gases across the surface of oligotrophic Lake Stechlin using a floating chamber approach. The normalized gas transfer rate for CH4 (k600,CH4) was on average 2.5 times higher than that for CO2 (k600,CO2) and consequently higher than Fickian transport. Because of its low solubility relative to CO2, the enhanced CH4 flux is possibly explained by the presence of microbubbles in the lake’s surface layer. These microbubbles may originate from atmospheric bubble entrainment or gas supersaturation (i.e., O2) or both. Irrespective of the source, we determined that an average of 145 L m−2 d−1 of gas is required to exit the surface layer via microbubbles to produce the observed elevated k600,CH4. As k600 values are used to estimate CH4 pathways in aquatic systems, the presence of microbubbles could alter the resulting CH4 and perhaps C balances. These microbubbles will also affect the surface fluxes of other sparingly soluble gases in inland waters, including O2 and N2

Biogeochemical diversity and hot moments of GHG emissions from shallow alkaline lakes in the Pantanal of Nhecolândia, Brazil

“…On the other hand, compared to a simple diffusion process, it also increases the transfer rates of methane to the lake surface, as the ascension of the bubbles is faster than the diffusion towards the top of the water column. The microbubble phenomenon has recently been proposed as an important flux pathway for CH 4 in inland aquatic systems (Beaulieu et al, 20 2012;McGinnis et al, 2015;Prairie and del Giorgio, 2013). Here, this process results in a CH 4 emission that is two orders of magnitude greater than previously reported for these environments.…”

The global methane budget 2000–2012

“…Third, another important mechanism stopping methane from reaching the ocean surface is the dissolution of bubbles into the ocean water. Although bubbling is the most efficient way to transfer methane from the seabed to the atmosphere, the fraction of bubbles actually reaching the atmosphere is very uncertain and critically depends on emission depths (< 100-200 m, McGinnis et al, 2015) and on the size of the bubbles (> 5-8 mm; James et al, 2016). Finally, surface oceans are aerobic and contribute to the oxidation of dissolved methane (USEPA, 2010a).…”

Methane Production in Oxic Lake Waters Potentially Increases Aquatic Methane Flux to Air

“…However, even in the latter case, there is still uncertainty about how much methane will reach the atmosphere and how much is oxidized. 54,55 With the methane source located in the upper oxic layer instead of the bottom (Figure 1), methane needs to be transported over only a much shorter distance to reach the water−air interface. Additionally, shallow water mixing (convection), which often occurs diurnally, both exposes higher methane concentrations to the air−water interface and enhances k. 53 These fluxes would be particularly important during periods of colder weather and higher winds during the stratified season and would be further elevated by microbubbles.…”