Role of gas ebullition in the methane budget of a deep subtropical lake: What can we learn from process‐based modeling?

Abstract

We analyzed the processes affecting the methane (CH 4 ) budget in Lake Kinneret, a deep subtropical lake, using a suite of three models: (1) a bubble model to determine the fate of CH 4 bubbles released from the sediment; (2) the one-dimensional physical lake model Simstrat to calculate the mixing dynamics; and (3) a biogeochemical model implemented in Aquasim to quantify the CH 4 sources and sinks. The key pathways modeled include diffusive and bubble release of CH 4 from the sediment, aerobic CH 4 oxidation, and atmospheric gas exchange. The temporal and spatial dynamics of dissolved CH 4 concentrations observed in the lake during 3 years could be well represented by the combined models. Remarkably, the relative contributions of ebullition and diffusive transport to the accumulation of CH 4 in the hypolimnion during the stratified period could not be accurately constrained based only on the observed evolution of CH 4 concentrations in the water column. Importantly, however, our analysis showed that most ($99%) of the CH 4 supplied to the water column by bubble dissolution and diffusive transport from the sediment is aerobically oxidized, whereas a substantial fraction ($60%) of the sediment-released bubble CH 4 is directly transported to the atmosphere. Ebullition is thus responsible for the bulk of the emissions from Lake Kinneret to the atmosphere. Therefore, as in all freshwaters, ebullition quantification is crucial for accurately assessing CH 4 emissions to the atmosphere. This task remains challenging due to high spatio-temporal variability, but combining in situ measurements with a process-based modeling can help to better constrain flux estimates.

Influence of water column stratification and mixing patterns on the fate of methane produced in deep sediments of a small eutrophic lake

“…When CH 4 leaves via ebullition (instead of diffusion across the SWI), less CH 4 is stored in the hypolimnion and diffused across the AOI and more can directly reach the atmosphere (Schmid et al ). In Soppensee, the proportion of CH 4 production leaving via ebullition was higher in 2017 than in 2016 (Table ) and is likely partly caused by the CH 4 concentration accumulation in the bottom waters (Boehrer et al ; Horn et al ).…”

“…The daily change in CH 4 storage below 8 m (; mmol d −1 ) is described as: where F ebu,SWI (mmol m −2 d −1 ) is the ebullition of CH 4 leaving the sediments, α is the dissolution factor of CH 4 from bubbles into the hypolimnetic water (0.3; determined from the bubble simulation), F diff,SWI (mmol m −2 d −1 ) is the diffusion of CH 4 across the SWI, F diff,AOI (mmol m −2 d −1 ) is the diffusion from hypolimnion to the epilimnion across the AOI, A sed (m 2 ) is the sediment area, and A AOI (m 2 ) is the area at the AOI (8 m). As the hypolimnion was anoxic during the integration period, we assumed that CH 4 oxidation was insignificant compared to the other fluxes (Schmid et al ). However, we acknowledge that anaerobic CH 4 oxidation (Reed et al ) and microaerobic CH 4 oxidation (Blees et al ) might however have occurred ( see Discussion section for implication on the results).…”

Coupling Water Column and Sediment Biogeochemical Dynamics: Modeling Internal Phosphorus Loading, Climate Change Responses, and Mitigation Measures in Lake Vansjø, Norway

“…Empirical lake models have been developed for internal P loading (e.g., Schauser et al, 2006;Bryhn & Haakanson, 2007), but their generalization is unlikely as their applicability tends to be site-specific. Popular approaches to couple sediment processes to lake water column models have included the incorporation of an empirical bottom flux boundary (Schmid et al, 2017) and vertically integrated submodules (e.g., oxic and anoxic layers; Janssen et al, 2015;Matzinger et al, 2010;Mooij et al, 2011;Schmid et al, 2017). Several well-established lake models, such as FABM-PCLake (Hu et al, 2016), DYRESM-CAEDYM (Trolle et al, 2008), CE-QUAL-W2 (Zhang et al, 2015), GLM (Hipsey et al, 2017), and DELWAQ (Smits & van Beek, 2013), were built on variations of those approaches in order to represent sediment-water interactions.…”

What the bubble knows: Lake methane dynamics revealed by sediment gas bubble composition

“…); however, it does not give any quantitative information on the CH 4 production or fluxes. Combined with a basin‐scale hypolimnetic CH 4 mass balance (e.g., Schmid et al ()), the ebullition and diffusive fluxes can be easily determined. If a second parameter, together with bubble gas content, or any two parameters shown in Table are determined, then the hypolimnetic lake CH 4 mass balance and flux pathways can be estimated with very good approximation (note that the combination of f E and are not independent and need a third parameter).…”

“…Other than gas removal (stripping), we do not consider other physical effects of bubbles in the sediment or their release (i.e., no bubble‐mediated pore‐water advection; see Flury et al ). We assume that bubbles leaving the sediment do not interact notably with the surrounding sediment and pore water while leaving the sediment (i.e., no additional mass transfer once they begin to exit the sediment). Advective transport is not considered (i.e., only cohesive sediments are considered). Bioturbation is not explicitly considered. CH 4 is the only gas produced in the sediments, i.e., no N 2 production (Kipphut and Martens ; Martens et al ; Wilkinson et al ; Schmid et al ). Boundary conditions: At the SWI, the concentrations of dissolved pore‐water gas are equal to the concentrations in the lake and N 2 is in equilibrium with the atmosphere. At the bottom of the sediment, a zero flux condition is applied.…”