Environ. Sci. Technol. 2010, 44, 2419–2425
Extreme Methane Emissions from a Swiss Hydropower Reservoir: Contribution from Bubbling Sediments T O N Y A D E L S O N T R O , * ,†,⊥ DANIEL F. MCGINNIS,‡ S E B A S T I A N S O B E K , †,⊥,§ ILIA OSTROVSKY,| AND B E R N H A R D W E H R L I †,⊥ Eawag, Swiss Federal Institute of Aquatic Science and Technology, Seestrasse 79, 6047 Kastanienbaum, Switzerland, Institute for Biogeochemistry and Pollutant Dynamics, ETH, Zurich, Switzerland, IFM-GEOMAR, Leibniz Institute of Marine Sciences at the University of Kiel, Wischhofstrasse 1-3, D-24148 Kiel, Germany, and Israel Oceanographic & Limnological Research, Yigal Allon Kinneret Limnological Laboratory, Midgal, Israel
Received October 14, 2009. Revised manuscript received February 3, 2010. Accepted February 15, 2010.
Methane emission pathways and their importance were quantified during a yearlong survey of a temperate hydropower reservoir. Measurements using gas traps indicated very high ebullition rates, but due to the stochastic nature of ebullition a mass balance approach was crucial to deduce system-wide methane sources and losses. Methane diffusion from the sediment was generally low and seasonally stable and did not account for the high concentration of dissolved methane measured in the reservoir discharge. A strong positive correlation between water temperature and the observed dissolved methane concentration enabled us to quantify the dissolved methane addition from bubble dissolution using a system-wide mass balance. Finally, knowing the contribution due to bubble dissolution, we used a bubble model to estimate bubble emission directly to the atmosphere. Our results indicated that the total methane emission from Lake Wohlen was on average >150 mg CH4 m-2 d-1, which is the highest ever documented for a midlatitude reservoir. The substantial temperature-dependent methaneemissionsdiscoveredinthis90-year-oldreservoirindicate that temperate water bodies can be an important but overlooked methane source.
Introduction There is growing interest and concern regarding greenhouse gas emissions, mainly carbon dioxide (CO2) and methane (CH4), from lakes and reservoirs (1-4). Organic-rich sediments in lakes and reservoirs are thought to be ‘hot spots’ * Corresponding author phone: +41 41 349 2151; fax: +41 41 349 2168; e-mail:
[email protected]. † Eawag. ‡ IFM-GEOMAR. § Present address: Department of Ecology and Evolution, Limnology, Uppsala University, Sweden. ⊥ ETH, Zurich. | Israel Oceanographic & Limnological Research, Yigal Allon Kinneret Limnological Laboratory. 10.1021/es9031369
2010 American Chemical Society
Published on Web 03/10/2010
of methanogenesis in particular, from which CH4 can escape to the atmosphere via four major pathways - ebullition (bubbling), surface diffusion, advection through plants, and exposure of anoxic CH4-rich deep waters to the atmosphere during convective mixing events (turnover) (1). Additionally, gas is emitted from CH4-saturated dam releases (5). Typically, surface diffusion is the dominant atmospheric emission pathway for CO2 and for CH4 from deep water bodies (>50 m) (6, 7). The low solubility of CH4 relative to CO2 means that CH4 bubbles form more readily with typical diameters ranging from 2 to 8 mm (7-9). While these bubbles typically dissolve in deep systems, in shallow waters (40 g C m-2 y-1 (MATM, Figure 4). While this annual carbon emission is several orders of magnitude lower than that of the large tropical reservoirs (6, 27), it is the highest recorded for a temperate reservoir to date, and, in contrast to the tropical reservoirs, the majority of the emission is due to ebullition (∼80%, MBF; Figure 4). In addition, Saarnio et al. (34) estimated CH4 from European water bodies, and the CH4 emission rate of Lake Wohlen is well above that of the average small lake. Finally, at the current yearly average water temperature of 11 °C the reservoir emits ∼0.2 tons of CH4 per day via surface emissions and discharge. The projected temperature increase of 3 °C in the coming century (13) could result in a doubling of CH4 emission from Lake Wohlen, given the exponential dependence of CH4 emission on temperature (Figure 2). Even if this statement is highly speculative, it clearly illustrates the need for further investigations on the temperature dependence of CH4 emissions from lakes and reservoirs. Implications. Our findings indicate that this 90-year-old, temperate reservoir is a very significant CH4 source, almost entirely due to bubbling sediments. In addition, we found that [CH4]d accumulation was highly temperature-dependent in Lake Wohlen and allowed us to estimate methane emissions using the [CH4]d accumulation and flow rate of the reservoir. Even though the results of this study clearly point to a relationship between temperature and methane emission, many questions remain open regarding what is causing such intense methane production and subsequent ebullition in this small oxic reservoir, especially since the 10-year high emission period following damming that has been observed in newly dammed reservoirs is long over (6). A potential explanation is the combination of reservoir characteristics that made Lake Wohlen an easy system to analyze - a flow-through reservoir with a short residence time and minimal oxidation - as well as its high organic carbon load (∼25 tons per day) and fast sedimentation rate (18) all leading to an exceptional rate of methanogenesis, super2424
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saturation, and subsequent ebullition. The high organic carbon load comes from several sources - (1) passage of the Aare River through the capital city, Bern (population >100,000), including input from several wastewater treatment plants; (2) drainage of the large Alpine and agricultural river basin (35) yielding terrigenous carbon; and (3) the input from two large Prealpine lakes, Brienz and Thun. Consequently, similar run-of-river reservoirs lying downstream of such organic carbon sources may also be large CH4 producers. Therefore, the sampling and system analysis methods outlined in this paper are quite appropriate for studying other similar systems and could even be adapted for use in more complex ones. Depending on access to the reservoir, monthly inflow/outflow samplings are not too time-consuming and essential to this method. More detailed campaigns could include coring, funnel, and chamber surveys conducted seasonally when water temperature differences are obvious. On a global scale of methane emissions from reservoirs, temperate ones are often overlooked because the largest reservoirs reside in tropical regions where warm temperatures support methanogenesis year round (2, 3, 36). Our findings indicate that temperate reservoirs with high carbon inputs and burial rates (37) as well as the sources of these carbon inputs should be given some consideration in current greenhouse gas budgets, especially in a changing climate. A more detailed analysis of the biogeochemical and geographic setting of reservoirs in temperate areas is needed when assessing global methane emissions from water bodies.
Acknowledgments A thank you goes to D. Bastviken, M. Schmid, K. Wallmann, and A. Wuest for helpful suggestions and special thanks go to K. Ashe, T. Diem, C. Dinkel, S. Flury, A. Jullian, L. Rovelli, M. Schurter, N. Wongfun, J. Yuen, and A. Zwyssig for help in the lab and field. Also thanks go to T. Schneiter and BKW for allowing dam and data access and three reviewers who helped improve this manuscript. This study was supported by the Swiss National Science Foundation (Nr. 200020-112274 and 200021-120112).
Supporting Information Available An extended methods section, three tables containing measurements from various surveys, and three figures showing gas trap and floating chamber details and temperature relationship with CH4 saturation concentrations and production. This material is available free of charge via the Internet at http://pubs.acs.org.
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