Eddy covariance flux measurements confirm extreme CH<sub>4</sub> emissions from a Swiss hydropower reservoir and resolve their short-term variability

Abstract

Abstract. Greenhouse gas budgets quantified via landsurface eddy covariance (EC) flux sites differ significantly from those obtained via inverse modeling. A possible reason for the discrepancy between methods may be our gap in quantitative knowledge of methane (CH 4 ) fluxes. In this study we carried out EC flux measurements during two intensive campaigns in summer 2008 to quantify methane flux from a hydropower reservoir and link its temporal variability to environmental driving forces: water temperature and pressure changes (atmospheric and due to changes in lake level). Methane fluxes were extremely high and highly variable, but consistently showed gas efflux from the lake when the wind was approaching the EC sensors across the open water, as confirmed by floating chamber flux measurements. The average flux was 3.8 ± 0.4 µg C m −2 s −1 (mean ± SE) with a median of 1.4 µg C m −2 s −1 , which is quite high even compared to tropical reservoirs. Floating chamber fluxes from four selected days confirmed such high fluxes with 7.4 ± 1.3 µg C m −2 s −1 . Fluxes increased exponentially with increasing temperatures, but were decreasing exponentially with increasing atmospheric and/or lake level pressure. A multiple regression using lake surface temperatures (0.1 m depth), temperature at depth (10 m deep in front of the dam), atmospheric pressure, and lake level was able to explain 35.4 % of the overall variance. This best fit included each variable averaged over a 9-h moving window, plus the respective short-term residuals thereof. We estimate that an anCorrespondence to: W. Eugster (eugsterw@ethz.ch) nual average of 3 % of the particulate organic matter (POM) input via the river is sufficient to sustain these large CH 4 fluxes. To compensate the global warming potential associated with the CH 4 effluxes from this hydropower reservoir a 1.3 to 3.7 times larger terrestrial area with net carbon dioxide uptake is needed if a European-scale compilation of grasslands, croplands and forests is taken as reference. This indicates the potential relevance of temperate reservoirs and lakes in local and regional greenhouse gas budgets.

Reviews and syntheses: Effects of permafrost thaw on Arctic aquatic ecosystems

“…While proving the feasibility of EC measurements in freshwater ecosystems, aquatic EC work also highlights challenges related to the application of this terrestrially optimized approach to aquatic systems (Vesala et al, 2006;Eugster et al, 2011). Overall, EC shows great promise with respect to the following: (1) integration of all gas flux pathways from the lake sediments to the atmosphere; (2) continuous flux monitoring over time, enabling the capture of episodic ebullition events of CH 4 in lakes (Eugster et al, 2011); and (3) the analysis of dynamic responses of lake-atmosphere carbon fluxes to temporal (including diurnal) changes in environmental variables (Eugster, 2003;Vesala et al, 2006;Podgrajsek et al, 2014b).…”

“…Overall, EC shows great promise with respect to the following: (1) integration of all gas flux pathways from the lake sediments to the atmosphere; (2) continuous flux monitoring over time, enabling the capture of episodic ebullition events of CH 4 in lakes (Eugster et al, 2011); and (3) the analysis of dynamic responses of lake-atmosphere carbon fluxes to temporal (including diurnal) changes in environmental variables (Eugster, 2003;Vesala et al, 2006;Podgrajsek et al, 2014b). A significant portion of gaseous carbon emissions from seasonally ice-covered lakes appears to occur during spring ice-thaw (e.g.…”

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

“…The component drivers respond differently to slow and fast changes in meteorological covariates (Baldocchi et al, 2001;Koebsch et al, 2015) such that different mechanisms may explain the diel and seasonal variability of the flux. For example, temperature affects emissions through convective mixing on short timescales and through the rate of sediment methanogenesis on longer timescales; the diurnal cycle of insolation may have a limited effect on production because the heat capacity of the water buffers the temperature signal (Fang and Stefan, 1996). Similar phase lags and amplifications may lead to hysteretic flux patterns, such as cold season emission peaks due to release of gases from the hypolimnion in dimictic lakes (Encinas Fernández et al, 2014;López Bellido et al, 2009) or thermal inertia of lake sediments (Zimov et al, 1997).…”

Extreme organic carbon burial fuels intense methane bubbling in a temperate reservoir

“…The water column is generally well-mixed and permanently oxygenated [DelSontro et al, 2010]. Studies based on gas traps, mass balance calculations and eddy covariance showed that CH 4 emission from Lake Wohlen to the atmosphere was the highest ever documented for a temperate reservoir, and mainly attributable to ebullition [DelSontro et al, 2010;Eugster et al, 2011].…”