Geophysical Research Letters

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

Authors

  • Sebastian Sobek,

    1. Eawag: Swiss Federal Institute of Aquatic Science and Technology, Kastanienbaum, Switzerland
    2. Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland
    3. Now at Department of Ecology and Genetics, Limnology, Uppsala University, Uppsala, Sweden
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  • Tonya DelSontro,

    1. Eawag: Swiss Federal Institute of Aquatic Science and Technology, Kastanienbaum, Switzerland
    2. Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland
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  • Nuttakan Wongfun,

    1. Eawag: Swiss Federal Institute of Aquatic Science and Technology, Kastanienbaum, Switzerland
    2. Institute for Water Education, UNESCO-IHE, Delft, Netherlands
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  • Bernhard Wehrli

    1. Eawag: Swiss Federal Institute of Aquatic Science and Technology, Kastanienbaum, Switzerland
    2. Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland
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Abstract

[1] Organic carbon (OC) burial and greenhouse gas emission of inland waters plays an increasingly evident role in the carbon balance of the continents, and particularly young reservoirs in the tropics emit methane (CH4) at high rates. Here we show that an old, temperate reservoir acts simultaneously as a strong OC sink and CH4 source, because the high sedimentation rate supplies reactive organic matter to deep, anoxic sediment strata, fuelling methanogenesis and gas bubble emission (ebullition) of CH4from the sediment. Damming of the river has resulted in the build-up of highly methanogenic sediments under a shallow water column, facilitating the transformation of fixed CO2 to atmospheric CH4. Similar high OC burial and CH4 ebullition is expected in other reservoirs and natural river deltas.

1. Introduction

[2] In spite of their relatively small areal extent, inland waters play an important role in the carbon balance of the continents, both as carbon sinks due to the burial of organic carbon (OC) in the sediments, and as carbon sources due to the emission of the greenhouse gases carbon dioxide (CO2) and methane (CH4) to the atmosphere [Aufdenkampe et al., 2011; Battin et al., 2009; Cole et al., 2007; Tranvik et al., 2009]. Even though inland water CH4 emission is small in terms of carbon units compared to CO2 emission, the 25 times higher global warming potential of CH4 as compared to CO2 [Intergovernmental Panel on Climate Change, 2007] implies that CH4 emission from inland waters is relevant for the Earth's climate. Accordingly, it was recently estimated that inland water CH4 emission offsets about 25% of the carbon sink on land [Bastviken et al., 2011]. Particularly recently constructed hydroelectrical reservoirs in the tropics have been shown to emit CH4 at high rates [Abril et al., 2005; Kemenes et al., 2007; Soumis et al., 2005; St. Louis et al., 2000; Tremblay et al., 2005].

[3] In aquatic systems, OC burial and CH4 production are largely confined to the sediments. Of the OC being deposited onto the sediment surface, a portion will be mineralized to either CO2 or CH4 with the remainder buried in the sediment over geologic timescales. Knowledge about factors regulating the OC fate in freshwater sediments is particularly relevant for reservoirs, since they have been estimated to bury more OC in their sediments than the entire ocean [Tranvik et al., 2009] and, at the same time, to be responsible for about 20% of the total inland water CH4 emission [Bastviken et al., 2011]. Moreover, the global area of reservoirs is increasing [Downing et al., 2006], augmenting their role as OC sinks and CH4 sources. Despite recent advances in our understanding of OC fate in freshwater sediments [Gudasz et al., 2010; Sobek et al., 2009], large gaps remain. Importantly, the share of sediment OC that is converted to CH4 and vented to the atmosphere can currently not be predicted. This is at least partially because CH4 is emitted from sediments not only via diffusion, but also via ebullition, which is frequently a major emission pathway to the atmosphere and difficult to assess due to its stochastic nature [Bastviken et al., 2004]. We report here results from studies of Lake Wohlen, a small hydroelectric reservoir in Switzerland, allowing us to relate sediment properties to both OC burial, and, for the first time, to CH4 ebullition.

2. Materials and Methods

2.1. Site Description

[4] We conducted studies at Lake Wohlen, a small (2.5 km2) and shallow (mean depth, 9 m) hydroelectric reservoir in Switzerland, constructed in 1920. Lake Wohlen is a mesotrophic to eutrophic run-of-river reservoir along the Aare river, and has an average water retention time of ∼2 days [Albrecht et al., 1998]. 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 CH4 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].

2.2. Sediment Sampling and Analyses

[5] The sediments of Lake Wohlen were sampled with a gravity corer at 8 sites, from the dam upstream towards the inflow area (Figure S1 in the auxiliary material). At each sampling site, multiple sediment cores were sampled and analyzed for physicochemical properties, for oxygen penetration depth, and for dissolved methane in the sediment porewater. Details of methods are published in our recent study on lake sediments [Sobek et al., 2009]. Additional information on methods can be found in the auxiliary material.

3. Results and Discussion

3.1. Sediment Characteristics and Burial Efficiency

[6] The sediments were relatively similar at all coring sites (Table S1), with low OC (range 0.27–2.8%) and water (range 23–60%) content, and low porosity (range 0.43–0.79). Mineral particles were mainly silt-sized (range of median grain size, 8–80 μm), apart from a sand layer in core H. The organic matter in Lake Wohlen sediments originates primarily from terrestrial sources, as suggested by independent indicators (see auxiliary material). Sedimentation rates were very high and variable (range 5.2–11.5 cm yr−1), resulting in substantial but variable OC burial rates (range 536–1950 g C m−2 yr−1; Table 1). OC burial in Lake Wohlen was at the high end compared to other reservoirs (mean 500, range 14–3300 g C m−2 yr−1) [Mulholland and Elwood, 1982], beyond the observed range for natural lakes (maximum 300 g C m−2 yr−1) [Mulholland and Elwood, 1982; Sobek et al., 2009], and only surpassed by eutrophic farm ponds (mean 3200, range 148–17,392 g C m−2 yr−1) [Downing et al., 2008]. OC mineralization in Lake Wohlen sediment (range 86–229 g C m−2 yr−1) was within the range usually observed in freshwater sediments (16–740 g C m−2 yr−1) [Gudasz et al., 2010] and much lower than OC burial at all sites (Table 1), thus returning the highest OC burial efficiencies (buried OC : deposited OC; range 83–94%) in freshwater sediments so far reported in the literature [Sobek et al., 2009]. Clearly, the sediments of Lake Wohlen constitute a strong and efficient OC sink.

Table 1. Sedimentation Rates, Oxygen Exposure Times and Carbon Fluxes in Lake Wohlen Sedimentsa
SiteSedimentation Rate (cm yr−1)O2 Exposure Time (d)Mineralization (g C m−2 yr−1)OC Burial (g C m−2 yr−1)OC Burial Efficiency (%)
  • a

    n.d. = not determined.

A5.2229553685
B5.3508665688
C7.47173104086
D7.94229111083
E6.8n.d.n.d.1120n.d.
F11.55108166094
G11.3n.d.n.d.1950n.d.
H7.1912383787
Mean ± sd7.8 ± 2.416 ± 18136 ± 551113 ± 48287 ± 4

[7] It is likely that the very high OC burial efficiencies in Lake Wohlen are linked to the very short oxygen exposure times. The rapid sedimentation rates of about 5–11 cm yr−1 in conjunction with oxygen penetration depths of about 1–7 mm limits oxygen exposure time to a few days or weeks (Table 1). The fast transfer to anoxic sediments can be expected to limit the mineralization of the sediment OC as the OC burial efficiency of both marine and freshwater sediments has been shown to be negatively related to oxygen exposure time [Hartnett et al., 1998; Sobek et al., 2009].

3.2. Methane Emission in Relation to Sediment Characteristics

[8] CH4 emission surveys showed that ebullition from the sediments released about 33 g C m−2 yr−1 of CH4 to the water column, of which 24 g C m−2 yr−1 directly reached the atmosphere [DelSontro et al., 2010]. The proportion of gas bubbles that dissolved during ascent (9 g C m−2 yr−1) contributed to CH4 emission after turbine passage (16 g C m−2 yr−1). Adding CH4emission via diffusion over the water-air interface, the estimated total CH4 emission from Lake Wohlen to the atmosphere was 43 g C m−2 yr−1 [DelSontro et al., 2010]. This is roughly one order of magnitude higher than the range of average CH4 emission reported from temperate reservoirs (4.6–5.5 g C m−2 yr−1) [Bastviken et al., 2011; Soumis et al., 2005; St. Louis et al., 2000] or lakes (3.2 g C m−2 yr−1) [Bastviken et al., 2011], and within the range of the reported average CH4 emission from tropical reservoirs (37–82 g C m−2 yr−1) [Bastviken et al., 2011; Soumis et al., 2005; St. Louis et al., 2000].

[9] We propose that the extreme CH4ebullition in Lake Wohlen is ultimately attributable to very high sedimentation rates that result in limited oxic degradation of organic matter and rapid transfer of OC to deeper sediment layers. Profiles of the amino acid-derived degradation index (DI) [Dauwe et al., 1999], which links the amino acid composition of organic matter to its reactivity, remained generally well above zero throughout the entire sediment column (Figure 1), indicating the presence of reactive organic matter in relatively deep sediment layers. This is unusual since the reactivity of organic matter rapidly declines with age [Middelburg et al., 1993], thus DI tends to decrease with depth in sediments [Dauwe et al., 1999; Meckler et al., 2004]. It is likely that the high DI at depth is due to the rapid sediment accumulation, leaving limited time for diagenesis. Substantial OC reactivity was reflected in dissolved CH4 concentration in sediment porewater being close to or above saturation below ∼10 cm depth at most sites (Figure 2, cores C, D, E, G). Contrarily, site B had a significantly lower DI than all other sites (Anova, Tukey post-hoc test,F4,69 = 7.27, p = 0.001) and porewater dissolved CH4 was far below saturation throughout the core (Figure 2). This strongly suggests that high sedimentation rates rapidly shunt organic matter to deep sediment layers, where its reactive fraction fuels methanogenesis. As diffusion over tens of cm in low-porosity sediment (mean 0.68) is very limited, deeply formed CH4 is bound to accumulate at depth, leading to supersaturation and consequent bubble formation and release. It is likely that this mechanism (Figure 3a) is valid in freshwater sediments so long as sedimentation rate is high, and organic matter reactivity is high enough to sustain methanogenesis rates higher than CH4 diffusion rates (see also auxiliary material). After release from the sediment, the fate of the CH4 contained in a bubble depends on the depth of the water column. The majority of bubbles released in deep water (>∼40 m) will substantially dissolve during rise, subjecting the dissolving CH4 to oxidation by aquatic methanotrophs. If bubbles are released from shallow sediments (<∼10 m water depth), such as in Lake Wohlen, most of the bubble gas will reach the atmosphere, thus bypassing the aquatic methane oxidizers, and resulting in high atmospheric CH4 emission [McGinnis et al., 2006].

Figure 1.

Degradation indices (DI) of selected cores, calculated from the amino acid composition [Dauwe et al., 1999]. (a) DI profiles at site B (filled circles) and site C (open circles). (b) DI profiles at site D (filled circles), site E (open circles) and site G (grey triangles). The lower the DI, the more degraded is the organic matter. For comparison, the DI of highly reactive plankton debris is in the range of 1–1.5, while highly degraded deep-sea sediments have DIs as low as −2.

Figure 2.

Examples of profiles of dissolved CH4 in the porewater of Lake Wohlen sediments. (a) CH4 profiles at site B (filled circles) and site C (open circles); water depth at these sites is 13 m. (b) CH4 profiles at site D (filled circles) and site E (open circles); water depth at these sites is 8 m. Dotted lines indicate the saturation concentration of CH4.

Figure 3.

Conceptual graph of methane production and emission in freshwater sediments. (a) Lake Wohlen sediments have very high sedimentation rates (several cm yr−1), resulting in minimal oxygen exposure times and reactive organic matter (as indicated by positive degradation indices) rapidly being transferred to deep sediment layers. This fuels substantial methanogenesis over the entire sediment profile (shaded area). Deeply formed methane accumulates at depth due to limited diffusion, until supersaturation leads to the formation and rise of gas bubbles. Values were taken from [DelSontro et al., 2010]. (b) Lake Zug in Switzerland, serving as an example of a typical lake sediment with moderate sedimentation rate (∼4 mm yr−1) and reactive organic matter being confined to the top 10 cm of sediment [Meckler et al., 2004]. Methane production is confined to surficial sediments, from where the methane can diffuse across the sediment-water interface (value is based on own data); hence accumulation of CH4 in the sediment, followed by ebullition, is unlikely (see auxiliary material).

3.3. Effects of River Damming on the Carbon Budget

[10] In terms of carbon units, OC burial (1110 g C m−2 yr−1) outweighs the sum of CH4 emission (43 g C m−2 yr−1) and CO2 emission (24 g C m−2 yr−1; calculated from measured dissolved inorganic carbon concentration, temperature, pH, and wind speed). Accounting for the 25 times higher global warming potential of CH4 compared to CO2 returns a total greenhouse gas emission of 1520 g CO2-equivalents m−2 yr−1. For comparison, OC burial corresponds to 4070 g CO2-equivalents m−2 yr−1, or more than twice the greenhouse gas emission. However, such a budgeting would describe the effect of damming inappropriately; in the absence of the Lake Wohlen dam, a large fraction of the suspended sediment load currently deposited in the reservoir would be deposited in the natural downstream lake (Lake Biel) and thus replace Lake Wohlen as a strong OC sink. Reports of high OC burial efficiencies in the river mouths of natural lakes (60–80%) [Sobek et al., 2009] support this assumption. On the other hand, assuming that methanogenesis was as intense in Lake Biel (in the absence of the dam) as it presently is in Lake Wohlen, only ∼30% of the methane in a 6 mm bubble released from the mean depth of Lake Biel (31 m) would reach the atmosphere [McGinnis et al., 2006]. The corresponding number for shallow Lake Wohlen is ∼75%, suggesting the key effect of damming was the creation of highly methanogenic sediments overlain by only a shallow water column, resulting in a substantial increase in the transformation of fixed CO2 to atmospheric CH4.

4. Implications

[11] These results from Lake Wohlen suggest that in general, shallow inland waters with high sedimentation rates and significant organic matter reactivity ought to be considered potential CH4ebullition hot spots. Such conditions apply for river deltas in natural lakes but they are particularly prevalent in man-made impoundments, such as reservoirs and farm ponds [Downing et al., 2008; Mulholland and Elwood, 1982; Sobek et al., 2009]. A recent study of a large tropical reservoir found that methane ebullition was at least one order of magnitude higher in bays with river inputs as compared to bays without river inputs [DelSontro et al., 2011], strongly supporting the concept of high sedimentation areas as CH4 ebullition hot spots. Therefore, we expect a significant and presently unaccounted CH4 ebullition flux to the atmosphere from a variety of different freshwater systems. Consequently, inland water CH4 emission to the atmosphere most likely offsets even more than the currently estimated 25% of the terrestrial carbon sink [Bastviken et al., 2011], especially since the globally impounded water surface area is predicted to double within the next 50 years if current growth rates are maintained [Downing et al., 2006; Tranvik et al., 2009].

Acknowledgments

[12] We thank Arnaud Jullian for amino acid analyses, Edith Durisch-Kaiser, Dan McGinnis, Ilia Ostrovsky, Carsten Schubert and Alfred Wüest for support, Andreas Brand, Michael Schurter, Roland Zurbrügg and Alois Zwyssig for assistance in the field and in the lab, and BKW (the hydropower company) for access to the dam and sharing of data. This study was supported by the Swiss National Science Foundation (grants 200021-112274 and 200020-120112). Additional support to S.S. by Formas (the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning) is acknowledged.

[13] The Editor thanks Jack J. Middelburg and an anonymous reviewer for their assistance in evaluating this paper.

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