Continuous Seasonal River Ebullition ... - ACS Publications

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Continuous Seasonal River Ebullition Measurements Linked to Sediment Methane Formation Jeremy Wilkinson,*,† Andreas Maeck,‡ Zeyad Alshboul,† and Andreas Lorke† †

University of Koblenz-Landau, Institute for Environmental Sciences, Fortstr. 7, 76829 Landau, Germany Senect GmbH & Co. KG, An 44 - No. 11, 76829 Landau, Germany

‡

S Supporting Information *

ABSTRACT: Laboratory sediment incubations and continuous ebullition monitoring over an annual cycle in the temperate Saar River, Germany confirm that impounded river zones can produce and emit methane at high rates (7 to 30 (g CH4 m−3 d−1) at 25 °C and 270 to 700 (g CH4 m−2 yr−1), respectively). Summer methane ebullition (ME) peaks were a factor of 4 to 10 times the winter minima, and sediment methane formation was dominated by the upper sediment (depths of 0.14 to 0.2 m). The key driver of the seasonal ME dynamics was temperature. An empirical model relating methane formation to temperature and sediment depth, derived from the laboratory incubations, reproduced the measured daily ebullition from winter to midsummer, although late summer and autumn simulated ME exceeded the observed ME. A possible explanation for this was substrate limitation. We recommend measurements of methanogenically available carbon sources to identify substrate limitation and help characterize variation in methane formation with depth and from site to site.

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INTRODUCTION After CO2, methane (CH4) is regarded as the second-mostimportant anthropogenic greenhouse gas. Global methane emission rates of between 500 and 600 Tg yr−1,1 account for 20% of the observed warming since the beginning of the industrial era.2 The global boom in major dam construction3 and response of CH4 to increasing temperature (greater than for CO2)4−6 means that CH4 will become increasingly important in global warming as temperatures rise. Of total global emissions, freshwaters contribute around 103 Tg CH4 yr−1, of which over 53% is emitted via gas bubbles.7 Lakes and reservoirs are a well-known methane source due to flooded organic matter decay and accumulated carbon rich sediments.8−10 Until recently, longer-term and spatially distributed riverine methane measurements were lacking;11 now, larger studies are yielding data from around the world.12−18 Methane emissions are highly variable temporally and spatially11,19−22 and vary strongly with temperature.4,19 Hotspot methane ebullition is well-known in Arctic thaw lakes22 and tropical reservoirs and lakes,15,23 but temperate freshwater sources were only recently highlighted.16,17,24 Most large rivers are impounded,24,25 thereby creating zones of preferential sedimentation and methane generation upstream of dam and weir structures. A large proportion of the methane is released by ebullition, and methane in solution is degassed in the turbulent zone downstream of the dams,16 diffusion is only a small component of emissions in such areas. Similar patterns have been observed for river delta bays in lakes.26 If replicated © XXXX American Chemical Society

globally, impounded river zones could add between 0.8 and 7% to total global freshwater−methane emissions.16 Existing estimates of methane emissions from hot-spot ebullition zones are restricted to rather short and episodic measurements.16,26 The observed dynamics of ebullition have mainly been discussed in respect to the short-term physical forcing (e.g., changes in hydrostatic pressure)20 but not in regard to the temperature- and substrate-dependent methane formation rate within the sediments. Between August 2012 and August 2013, we used automated bubble traps to collect continuous time-series of methane ebullition in river impoundments, with the aim of answering the following questions: 1. How does ebullition vary over an annual cycle, and what are the key drivers of this? 2. Can seasonal ebullition variation be simulated from drivers and simple measures of sediment methane production rates alone?

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METHODS AND MATERIALS Study Site. We studied impoundments of the temperate Saar River, which flows 246 km from the French Vosges Mountains to the Moselle in Germany (Figure 1a). The 7431

Received: June 29, 2015 Revised: October 12, 2015 Accepted: October 19, 2015

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DOI: 10.1021/acs.est.5b01525 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

Figure 1. Location of (A) the study basin, (B) the main study area, and (C) the measurement stations in the impoundment Serrig. Note that the river km notations in (B) are the distance to the confluence with the Moselle.

km2 catchment consists of forest, agricultural, and urban areas. The mean discharge from August 2012 to August 2013 was 71.9 m3 s−1, consistent with the long-term mean annual discharge (1953−2005; 73.4 m3 s−1) in the studied section. Flows ranged from 13 m3 s−1 to a peak daily flow of 405 m3 s−1. Impoundment (between the years of 1976 and 2000) of the lowermost 96 km with six dams with ship-locks and hydropower plants has increased the minimum depth of the main channel from 1.9 m to at least 4 m.27 The impounded reaches are more reservoir-like than riverine in character. Damming reduced the typical flow velocities from 0.34 to 0.09 m s−1 and increased the water residence times from 5 to 13 days at low discharge and 1.3 to 4.1 days at moderate discharge (80 m3.s−1).27 This has resulted in sedimentation upstream of dams, with accumulations of up to 5 m depth away from shipping channels,16 and caused high methane emission rates.28 Upstream of the impounded zones, the riverbed is characterized by stones and gravel. During summer and periods of low discharge, partial thermal stratification and oxygen deficits (3 m), respectively. Midsummer stratification in the water column,29 was weak and broke down at night, and it is assumed not to influence sediment temperature. Sediment porosity profiles at various sites along the Saar were consistent, ranging from around 0.78 at depths of 0.4 m to 0.86 at a few cm below the sediment−water interface (SWI). We fitted an exponential relationship35 to these and used estimated porosity in the temperature model. Sediment Methane Formation. Sediment cores collected on July 31, 2013 at each ABT in the Serrig impoundment only and were subsampled at up to 4 depths for methane formation

(2)

where yk is the smoothed output variable, xk the input variable (observed ME), and k is the time step. The smoothing parameter α has time constant τ = 1/(1 − α); the time taken for y to reach 63.2% of a step-change in x because y at t = τ is x (1−e −t/τ). For α → 1, smoothing is maximized; for α → 0, smoothing is minimized. We took the mean of the hourly values in the following 24 h for the initial condition for yk. Annual Sediment Temperature Depth Profile. To measure monthly sediment-temperature profiles (from March to September in 2013) at each ABT, we pushed a rod with a calibrated thermistor (TR-1060, RBR) into the sediment and measured the temperature at 0.2 to 0.5 m depth intervals from the sediment surface. The probe was held at each depth for 5 min to ensure thermal equilibration. Physical resistance to C


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Figure 3. (A) Sediment incubation MF (g CH4 m−3 d−1) versus temperature (°C) for shallow subsamples (Z < 0.25 m (p = 0.0001)) and deeper (Z > 0.25 m, subsamples (p < 0.00001)) (mean of data for duplicates from core samples at ABT1 and 3; see Figure S1). (B) MF (g CH4 m−3 d−1) at 25 °C for all cores and depths, overlaid with simulated MF depth profiles (also at 25 °C) for each ebullition monitoring site.

m). At four times each characteristic depth, MF from that component approaches 0. Assuming that sediment depth controls MF rate,16 the area under the MF-depth profile curve was set proportional to sedimentation rate at each ABT site (Table S1). Maximum sediment depth was taken from observed sedimentation rate (over 17 years), which provided satisfactory results (Table S1). Combined SWI effects, simply termed “methane loss”, were simulated using a sigmoid function, adjusted by b3 (slope) and c1 to raise or lower the depth of the y-axis intercept. Sediment layer fluxes were summed to the SWI and adjusted for bubble dissolution with rise to each ABT using the SiBu model.37 Methane loss is expressed as the equivalent amount of oxygen required to sufficiently consume methane (and hence match simulated ME from MF to observed ME). Carbon Burial. The carbon burial rate was estimated from the rate of sedimentation, the mean C content of the sediment (from loss on ignition testing of sediment subsamples), and ME plus methane loss (assuming this includes the portion that would be emitted as CO2) (see the Supporting Information).

(MF) incubation (potential methane production) at 5 temperatures (4, 10, 15, 20, and 25 °C). To minimize sediment disturbance at each site, we took cores 5 to 10 m away from each ABT (Table 1). The core from near ABT1 was sampled from shallower water to avoid operating in the shipping channel. The incubation methodology is based on the Hungate method36 (see the Supporting Information). Incubation flasks were sampled weekly over a 21 day period. Extracted headspace gas (100 μL by glass syringe) was analyzed in closed-loop by cavity-enhanced laser absorption spectrometry (UGGA, model 915−0011, Los Gatos Research Inc., Mountain View, CA). Adjustment for closed-loop volume Vl of the equilibration concentration Ce gives the flask headspace concentration (in ppmv), Ch =

(Ce − C0)(Vi + Vl ) Vi

(3)

where C0 is the purged loop (background) concentration and Vi is the injected volume. The mass of CH4 per unit wet sediment volume is M ws =

C h pVh · Vsed R sTK

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RESULTS Seasonal Variations of ME. Before monitoring on the Saar had been completed, the initial part of the ebullition data from autumn 2012 to late winter 2013 were reported in relation to physical forcing mechanisms.20 Here, we present previously unreported data that extends the record to the end of August 2013. In general, ME at all sites was low in autumn and through the winter period (Figure 2). ME increased in spring (late March 2013) and into summer. Peak ebullition was around mid- to late June 2013, after which it declined. At ABT1, latesummer ME continued to increase in July and August 2013, in contrast to the results from other sites. The peak ME at ABT2 was also inconsistent with the other sites due a major storm event that caused a large ebullition event in May. Without this event, it appears that the seasonal pattern at ABT2 would have been consistent with the other sites. Applying the observed 11 month mean to the whole year gives total annual ME values of 270 to 700 g CH4 (m−2 yr−1) (Table 1). ABT1, at the deepest midchannel location, collected the greatest ME flux, with a mean of 1.90 g CH4 (m−2 d−1); this was almost three times greater than at the shallower channel margin site ABT2 (mean 0.75 (g CH4 m−2 d−1)). ABT3, also off the mid-channel and with a depth of 2.7 m, had a mean flux

(4)

−3

(g m ), where p is flask pressure (kPa), Vh is the headspace volume (ml), and Vsed flask sediment volume (ml). A units conversion factor of 10−6 was applied. MF (g CH4 m−3 d−1) was estimated from the slope of the linear regression of Mws(t) − Mws(0) (the starting sediment methane mass) against time, grouped by treatment temperature and subsample depth (Figure S1). Negative results, assumed to indicate leakage or contamination with ambient air, were censored. Depth Profile Function. We applied an empirical relationship to simulate MF (g CH4 m−3 d−1) at depth z (m) below the SWI and temperature T (°C), ⎛ ⎛ ⎞⎞ 1 MFz , T = ⎜a1e−z / z1 + a 2e−z / z 2 + a3⎜ − 1⎟⎟a4 − b z + c 3 3 1 ⎝1 + e ⎠⎠ ⎝ T b4

(5)

where a1 to a3 scale the MF-depth decline curve, which comprises a rapid (shallow) and slower (deep) decay curve (characteristic depth values z1 and z2 are the depth at which each MF component is approximately 37% of its value at z = 0 D


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Environmental Science & Technology of 1.08 (g CH4 m−2 d−1). ABT4 was comparable to ABT2, with 0.75 (g CH4 (m−2 d−1)). The peak summer ME was between 4 and 10 times greater than autumn and winter ME (Figure 2), and the ratio of mean winter to summer ME at each site is between 2.7 and 4.5 for a twofold increase in mean temperature (Table 1). The two sites with higher flux (ABT1 and ABT3) consistently produced gas throughout winter, whereas ABT2 had frequent occurrences of zero ME, totaling approximately 28 days from October to February (Figure S2). Zero emission showed no strong daily pattern; over winter there were between 25 and 35 zero ME hours recorded for each hour of the day. Zero ME was most frequent at 9 and 10 pm and least frequent at 1 pm (Figure S3). ABT4 at Mettlach clogged regularly, causing frequent breaks in record. The May 2013 high flow caused an ME peak at ABT2 of 11.4 (g CH4 m−2 d−1) and a total of 36 (g CH4 m−2) (mean 4 g CH4 m−2 d−1) for the 9 day duration of the episode (13% of the annual total). ME at all other sites during this event was suppressed. Sediment-Temperature Profiles. Sediment- temperature depth profiles (Figure S4a) showed colder surface temperatures in early summer and warm surface temperatures in late summer, converging with depth to a constant ∼11 °C at 3 m below the SWI. Simulated sediment temperature showed a good agreement with the measurements (Figure S4b). Sediment Incubations. Incubated sediment CH4 volume increased for all temperatures (Figure S1). A clear relationship between MF and the depth below the SWI was observed (Figure 3). MF variation between core samples was small at depths >0.2 m below the SWI (ranging between 1 and 4 (g CH4 m−3 d−1)). Differences in MF in the uppermost subsamples were greater (Figure S5); the cores from near ABT1 and 3 gave similar MF rates (about 10 and 7 (g CH4 m−3 d−1) at 25 °C, respectively; Figure 3b). MF from the core sampled near ABT2 was much higher (>25 (g CH4 m−3 d−1) at 25 °C, Figure 3b). The MF results suggested strong spatial heterogeneity and that a greater number of cores would be needed to better characterize local MF. Individual-core incubation MF was not proportional to ME at the adjacent ABT sites; cores were not sampled at ABT-deployment water depths (Table 1). Consequently, no site-specific MF−core depth relationships could be discerned. MF from the ABT2 core was too high to give a sensible relationship with ME at any of the three sites and was not used in subsequent calculations. Thus, we used pooled MF results for the ABT1 and three cores. These showed a highly significant relationship with temperature (Figure 3a, p < 0.0001 from power-law fit of estimated MF to observed MF). MF at 25 °C was more than tenfold greater than at 4 °C. Applying the winter and summer mean sediment temperatures (8.5 and 16.5 °C, respectively) to the MF temperature relationship (Figure 3a) gives MF values of 1.8 and 5.0 (g CH4 m−3 d−1), with a summer-to-winter ratio of 2.7, identical to the summer-to-winter ME ratios of ABT1 and ABT3 (Table 2). Simulating ME from MF. Simulated MF at 0.05 m depth intervals (Figure 4) demonstrates the summer MF maximum below the gradient from the oxygenated surface layer for ABT3. Deeper-water sites ABT1 and ABT3 required less methane loss (equivalent to 0.576 and 0.321 (g O2 m−2 d−1), respectively) than the shallower sites ABT2 (0.741 (g O2 m−2 d−1) and ABT4 (0.806 g O2 m−2 d−1) to match the observed ME. Water Depth, Sedimentation Rate, Carbon Burial Rate, and Methane loss. An apparent correlation of observed

Table 2. Mean MF (g CH4 m−3 d−1) of Sediment from Core Samples At ABT1 and ABT3a with Duplicate Subsamples from Four Depths (Z) below the SWI Incubated at Five Temperatures (T) depth, Z (m)

0.16

0.41

0.66

0.89

0.76 2.64 4.82 8.25 10.70

0.20 0.75 1.19 1.55 3.00

0.12 0.71 1.03 1.83 2.69

0.16 0.61 1.91 1.66 2.15

T (°C) 4 10 15 20 25 a

See Figure S5.

mean ME to water depth (p = 0.005) was an artifact the ABT deployment locations and sedimentation rate (Figure S6a). Carbon burial rates (Table 1) at deeper water sites (Figure S6b) with higher sedimentation rates were greater than at shallow water sites with more hydrodynamic disturbance. Total carbon and nitrogen in the core subsamples correlated positively with MF in the sediment upper layer (R2 > 0.97, n = 3, Figure S7) but had no relationship in deeper subsamples. The C/N ratio was negatively correlated with MF (R2 = 0.905) in the upper layer and also had no relationship to MF at greater depths. Total carbon and nitrogen in all sediment subsamples was also closely correlated (R2 = 0.854). Core subsample sediment age (determined on the basis of the annual sedimentation rate 0.07 to 0.29 m yr−1 and subsample depth) showed no relationship to MF. The “oldest” subsample (ABT2, 20 cm, 3 years old) produced three times more methane than the youngest (ABT1, 14 cm, 6 months old). This subsample (ABT2) had slightly elevated total carbon, perhaps due to the mixing-in of fresh carbon by the late May fluvial disturbance.

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DISCUSSION Overall ME. Overall mean ME from the Saar River sites was consistent with high emissions from other systems.15,22,23 The greatest mean daily flux, 1.90 (g CH4 m−2 d−1) at ABT1, was almost three times that at ABT2 (mean 0.75 (g CH4 m−2 d−1)), and the mean daily ME correlated positively with sedimentation rate. Compared to our September 2010 data for echosounder ME and sedimentation rate,16 the ABT ME at low sedimentation sites is greater, and the ME at ABT1 (high sedimentation rate) was less than predicted by the echosounder relationship. The echo-sounder survey provides a high spatial resolution snapshot of emissions,17 whereas ABT measurements provide a temporally detailed record for a spatially limited area that, when combined at a wide range of sites, offer the potential to reduce the uncertainty of global riverine methane emissions. Backward Filtering. Backward smoothing removes shortterm variations from the data, aiding the visualization of longer time-scale features in relation to river flow and temperature. The modified data provide inferences into the storage and formation of methane in sediments. The filter time-constant of 8.3 days reduced the time-series of CoV from 142−213% to 56.7−87% (Table S1), revealing the relationship with water temperature and the influence of discrete-flow-derived perturbations. Backward smoothing helps to overcome the problem of decoupling of ebullition and environmental forcing due to gas trapping and build-up.38 Actual gas build-up should, however, be more gradual and flatter prior to each release. At ABT2, the May 2013 storm flow must have been sufficient to E


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Figure 4. Simulated seasonal sediment temperature (°C) over the period of October 2012 to November 2013 (upper panel) used to derive MF (g CH4 m−3 d−1) estimates (lower panel) at site ABT3; fit parameters given in Table S2.

Figure 5. Sediment temperature at 10 cm below SWI (T10cm) over the period of October 2012 to November 2013 and observed and simulated ME (g CH4 m−2 d−1) for ABT1. The vertical dotted line shows the sediment core sampling date. (Further simulated ME shown in Figure S7.).

substrates usable by methanogens.44,46 If surface sediments have a greater proportion of fresh organic matter (and thus a greater diversity of hydrolyzers, fermenters, and acetogenic and syntrophic bacteria), then the supply of methanogen substrate is likely to be greater, and so is methane production; older and deeper sediment may be substrate-depleted and produce less CH4. Simulating ME from MF. In our model, temperature was the only driving variable. Other forcings (such as river flow, shipping, lock operations, and internal processes), microbial mechanisms, upward-migrating methane, gas expansion, solubility, or chemical interactions are ignored. To adequately account for the processes and mechanisms within the system, a more comprehensive model would be required, such as those used for wetland systems.47 Despite this, we achieved a good agreement between the magnitudes and ranges of variation over an annual temperature cycle (Figures 5 and S8) by making the active methane producing zone for each ABT site proportional to the sedimentation rate and adjusting methane loss in the upper sediment. Measured sediment oxygen flux in the vicinity of our ABT sites is around 0.5 to 1 (g O2 m−2 d−1).48 V. Kirchesch (unpublished data) found values ranging from 0.6 to 4.9 (g O2 m−2 d−1) in the same river reach. Our methane loss values ranged from 0.46 to 2.7 (g O2 m−2 d−1); if all of the oxygen were consumed by CH4 oxidation, this would imply an oxidation rate between 0.23 and ∼1.35 (g CH4 m−2 d−1). The shallow water sites (ABT2/4) required greater methane loss to match observed ME. This may be plausible if such sites encounter greater hydrodynamic disturbance20,49 and, hence, greater gas exchange in the upper sediment than in deeper water. Divergence of ME from Simple Temperature Dependency. ME measured in late summer by our ABTs below that

scour the sediment surface, releasing the stored gas, a total of 13% (ca. 45 L, 4.5% of the sediment volume) of the total annual gas volume over 9 days. MF and Temperature. Our sediment MF incubations produced consistent relationships with temperature (p < 0.0001). Estimated Q1039 for methane flux and methane formation for a selection of studies include high values: 8.05−4.56 from lake sediments (MF, 10−20 °C),39 5.66 from high-latitude lakes (9−17 °C),19 and 3.85 for systems ranging from pure methanogen cultures to ecosystem level responses (0−30 °C).4 Lower Q10 values (1.22−2.72 for a midlatitude lake (MF, 4−30 °C)40 and 1.72 and 2.96 (as above))39 were consistent with our estimates of 2.16−3.03 (MF, 10−25 °C) and 1.75−2.23 (ME, 10−25 °C). MF and Depth. MF from 0.14 to 0.20 m was consistently (threefold) greater than that from deeper in the sediments (Table 2), as was found elsewhere.41 Peak MF generally occurs closer to the sediment surface, between 0.02 to 0.05 m,40 with a sharp reduction in methanogenesis with increasing depth.42 Thus, a shallower subsampling of our sediments would most likely have found greater MF. In littoral sediments of Lake Constance, peak CH4 production occurred at 0.02−0.05 m, with acetate as the main substrate for methanogenesis.43 Reduced microbial density with depth has been observed, as well as hydrogen-ion depletion and declining MF with increasing depth.44 The reduction of free energy for methanogenesis with depth has been observed in marine sediments,45 although methanogenic archaea were found to be supported by very low energy yields. Methanogens require other anaerobes to break down complex organic compounds into fatty acids and simple sugars, which in turn must be degraded by syntrophs, fermenters, and acetogens into hydrogen, carbon dioxide, formate, acetate, and methyl group compounds that are F


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99.7%. Thus, with greater settlement rate, the carbon is buried before it can be mineralized. Conversely, the hydrodynamic disturbance of shallow sediments may result in the greater mobilization of available carbon and a lower carbon-burial rate. This is consistent with broad findings for lake sediments.54,55 Monitoring Period and Duration. In early August and September 2011, manual bubble traps were deployed, which only capture a volume of gas and can’t resolve the temporal variability. They had to be emptied manually so they could be checked over a period of hours or days. The manual traps collected both low and high volumes of methane (Figure S9), and their deployment was too brief to capture a representative ME value.20 Over the reported monitoring period, the mean ME for April was the same as the mean for the entire record, and the mean for the 14 days of mid-April was approximately equal to the monthly mean. In a less disturbed system, where ebullition is not so heavily driven by hydraulic and hydrostatic influences, a shorter observation period might suffice. However, we recommend a month of observation to indicate a “true” monthly mean. We have no data with which to comment on interyear variability, although it seems likely that the magnitude and duration of hot and cold seasons, as well as the timing and regularity of fluvial events, may contribute to interannual variability. Short-term variability in ME in relation to physical forcing20 and the observed seasonal variation make short periods of observation inadequate for obtaining representative data on longer-term emission rates, which would be appropriate for larger-scale budgets.7 ABT deployments are capable of providing high-temporal-resolution flux estimates over seasonal cycles. They are relatively inexpensive and, once deployed, can be left for 4 to 6 weeks between maintenance, a major improvement on manually serviced bubble traps. Implications for Estimating Larger-Scale Emissions. The seasonal and shorter-term variation in methane ebullition in an impounded zone of the Saar River in Germany reported here highlights the importance of monitoring impounded zones of temperate river systems over sufficiently long (i.e., 1 or more years) periods to adequately characterize emissions. While it was possible to satisfactorily estimate the overall annual ME at Serrig in the Saar on the basis of simple observations such as temperature, local MF rates (accounting for estimates of oxidative loss), and sedimentation rate, there is currently insufficient confidence in such simple relationships to reliably estimate ME from other temperate impounded systems. To better explain the spatial variability in ME at a location or between different impoundments, future investigations should seek to characterize the dependence of MF on sediment carbon bulk properties, including organic carbon quantity and quality, in greater detail.

predicted by simple temperature dependency may be due to substrate limitation. Methanogenesis in temperate freshwaters shows strong seasonal dependency,42 and methanogen communities in lake sediments can increase their metabolic activity and density in summer;40 during periods of steady temperature, MF may adjust to carbon supply, such that abrupt temperature changes are superimposed upon the organic input response.50 Archaeal abundance can remain relatively constant compared to that of bacteria, which have been seen to increase in summer, suggesting that bacterial activity limits methanogen precursor availability and hence controlling seasonal methanogenesis.42 Competition by other organisms for acetate, H2, and CO2 as well as seasonal chemical variations in sediment and water composition can also limit methane production.51 We saw low ME in autumn 2012 and late summer 2013 relative to values predicted by simple temperature dependency. There were low flows during both periods, suggesting similar conditions. In a small English lake with frequently late summers, the limitation of anaerobic decomposition processes coincided with sedimentation of the spring diatom bloom and a rapid decline in sediment acetate levels.52 Carbon sources in the Saar include wastewater treatment plant inflows, phytoplankton production, and leaf-fall (which can be a key source of acetate and methanogenesis limiting factor).53 At Serrig, the algal productivity maximum occurs from June to end September,29 and it seems plausible that the late-summer divergence from temperature dependency may be due to substrate (e.g., acetate) limitation. We examined total carbon in the sediment cores collected but not DOC or methanogenesis precursors such as acetate. While MF in the sediment subsamples from between 0.14 and 0.20 m was correlated with total carbon, there were only three data points (Figure S7). MF from the deeper sediment subsamples was much less, and there was no relationship to total carbon. A comprehensive study of carbon burial in lake sediments found that the availability of oxygen was a key factor in organic carbon mineralization;54 deeper and rapidly buried sediments no longer receive oxygen, and hence, mineralization is shut down because methanogen precursors are no longer being generated. At ABT1, ME continued to rise in late summer (2013), when the other sites had declining values. This period was characterized by many large ME spikes, and it is possible that shipping operations were causing sufficient disturbance to maintain high emissions. Timing of Sediment Sampling. Sediment cores and laboratory incubation used to infer ME over a range of seasonal temperature variation may only be conditionally valid. The sediment from cores taken at the end of July 2013 appears to have been suitable for representing the MF and ME in the period of winter through summer 2013 but not for the late summer and autumn period of 2012 or 2013. Given the possibility of seasonal substrate limitation of MF, it might be appropriate to collect material for MF incubation experiments at the same time of year that ME observations are made. Carbon Burial. Estimates of carbon-burial rate were significantly correlated with log10 sedimentation rate (p = 0.019), which was consistent with and greater than upper values reported for lakes55 (Figure S7). For our simple carbon-burial estimate, DOC was not measured, and a more complete carbon budget should be considered. Although deeper water sites (ABT1/3) produced more methane per meter squared than the shallower water sites, they also had greater sedimentation and, consequently, carbon burial; values ranged from 94.9% to

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b01525. Additional information on the experimental background, automated bubble traps, model adjustment, carbon burial, sediment incubation for MF, periods of zero ME in winter, ebullitive contribution to overall emissions, seasonal gaseous expansion, microbial controls on MF, and acetate substrate dominance. Tables showing summary statistics for hourly mean ME data, parameters used to match simulated ME to measured values, and G


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carbon burial estimates. Figures showing the results of incubation experiments, summed hours with zero ebullition, occurrence of periods of zero-emission hours by hour of the day, sediment temperature profiles and comparisons, MF by temperature and depth for individual cores, incubation MF for three cores, simulated ME for each ABT location, and ABT flux periods. (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: +49 (6341) 280-31825; fax: +49 (6341) 280-31-577; email: [email protected]. Author Contributions

Manuscript writing and analysis was mainly carried out by R.J.W. A.L. implemented the sediment temperature model and wrote the relevant section. All authors have given approval to the final version of the manuscript. A.M. and Z.A. carried out the field deployment of ABTs, temperature profile surveys, and sediment core sampling. A.M. completed the data preprocessing. Z.A. undertook the sediment incubation work and preliminary results calculation. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Water and Shipping Agency of Saarbrücken (WSV) for support during the deployment of ABTs and the Helmut Fischer (Federal Institute of Hydrology (BfG)) for infrastructure and administrative support. Special thanks to Florian Maeck for the development of electronic ABT interface. Sebastian Geissler constructed the housing and supported the field campaigns. Thanks also to Emily G. Yang for her careful proofreading of the original manuscript. The study was financially supported by the German Research Foundation (grant LO 1150/5).

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ABBREVIATIONS ABT, automated bubble trap; ME, methane ebullition; MF, methane formation in sediments REFERENCES

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