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Extreme methane emissions from a temperate reservoir: the importance of ebullition

Univ. of Iceland, School of Engineering and Natural Sciences
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  • Ecology
  • Mathematics
  • Musicology
  • Physics


Methane (CH4) is one of the most important greenhouse gases (IPCC, 2005). Lakes and reservoirs have been identified as important, but overlooked, sources to the global CH4 budget. CH4 emission pathways include dissolved gas exchange at the water surface, bubble transport (ebullition), and degassing at the turbines of a hydropower dam or further downstream (Soumis, et al., 2005). Ebullition is an extremely effective pathway as bubbles mostly bypass oxidation at the sediment surface or in the water column and directly emit CH4. The stochastic nature of ebullition, however, makes it incredibly difficult to estimate; thus the aim of this study was to compare the traditional funnel method for measuring ebullition with a mass balance system analysis, atmospheric CH4 measurements, and hydroacoustic surveying. A yearlong CH4 survey was conducted at 2.5 km2 Lake Wohlen, a 90-yr-old run-of-river hydropower reservoir along the Aare River downstream of Bern, Switzerland. Dissolved CH4 ([CH4]d) profiles were measured monthly at the river inflow and at the dam. Sediment surface and water surface CH4 diffusion and CH4 oxidation in the water column were measured and/or calculated. Gas trap funnels measured ebullition near the seabed; drifting chambers captured total surface CH4 emissions. A bubble dissolution model was used to assess fractions of CH4 dissolving into the water and emitted to the atmosphere from bubbles. Complete method details in DelSontro, et al. (2010). Drifting chamber campaigns were accompanied by hydroacoustic surveys using an echosounder (Simrad EK60, 120 kHz). Eddy covariance measurements of atmospheric CH4 fluxes (EC/CH4) over the lake were made in conjunction with a cavity ringdown laser spectrometer (Los Gatos Research DLT-100). For details, see Eugster and Plüss (in press). It was discovered that [CH4]d increased by an order of magnitude along the reservoir and the [CH4]d accumulation was exponentially correlated with water temperature (T) (Figure 1a). The bubble dissolution model predicted that 70% of bubble-conveyed CH4 would reach the atmosphere, resulting in ~470 mg CH4 m-2 d-1 emitted to the atmosphere at T=17°C. Sediment and surface diffusions did not vary much with season and played a much lesser role in CH4 emissions than ebullition. Methane oxidation was negligible in this oxic reservoir with an average 2-day residence time. A system analysis was developed to better constrain the stochastic pattern of ebullition. Assuming no ebullition in winter (T<10°C), sediment diffusion was estimated based on [CH4]d accumulation in water at a given flow rate. The [CH4]d accumulation and T regression was used to estimate [CH4]d from dissolving bubbles at various T regimes which, at T=17°C, agreed well with funnel measurements (140 and 220 mg CH4 m-2 d-1, respectively). Using the bubble dissolution model results, sediment ebullition and atmospheric emissions were calculated and agreed well with empirical results. Considering all CH4 dissolved into the water from rising bubbles will either degas at the turbines or further downstream, Lake Wohlen thus emits ~156 mg CH4 m-2 d-1 on average throughout the year (140 tons/yr; Figure 1b), the highest recorded for a temperate reservoir to date (Soumis, et al., 2005) and of which ~80% is from ebullition. Drifting chambers captured emissions (mean, 855 mg CH4 m-2 d-1) much higher than those estimated with the system analysis at 17°C, but chambers were deployed in a highly active ebullition area. The chamber emissions agreed, however, with the peak CH4 emissions measured by EC/CH4 in the same region and are comparable to emissions estimated via hydroacoustics. These findings further highlight the importance in a potentially warming climate of (1) temperature-correlated CH4 ebullition emissions from temperate water bodies, and (2) these promising techniques for quantifying them.

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