Interannual, summer, and diel variability of CH4 and CO2 effluxes from Toolik Lake, Alaska, during the ice-free periods 2010-2015

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Eugster, W., DelSontro, T., Shaver, G. R., & Kling, G. W. Interannual, summer, and diel variability of CH4 and CO2 effluxes from Toolik Lake, Alaska, during the ice-free periods 2010-2015. Environmental Science: Processes & Impacts, 22(11), (2020): 2181-2198, doi: 10.1039/D0EM00125B.Accelerated warming in the Arctic has led to concern regarding the amount of carbon emission potential from Arctic water bodies. Yet, aquatic carbon dioxide (CO2) and methane (CH4) flux measurements remain scarce, particularly at high resolution and over long periods of time. Effluxes of methane (CH4) and carbon dioxide (CO2) from Toolik Lake, a deep glacial lake in northern Alaska, were measured for the first time with the direct eddy covariance (EC) flux technique during six ice-free lake periods (2010–2015). CO2 flux estimates from the lake (daily average efflux of 16.7 ± 5.3 mmol m−2 d−1) were in good agreement with earlier estimates from 1975–1989 using different methods. CH4 effluxes in 2010–2015 (averaging 0.13 ± 0.06 mmol m−2 d−1) showed an interannual variation that was 4.1 times greater than median diel variations, but mean fluxes were almost one order of magnitude lower than earlier estimates obtained from single water samples in 1990 and 2011–2012. The overall global warming potential (GWP) of Toolik Lake is thus governed mostly by CO2 effluxes, contributing 86–93% of the ice-free period GWP of 26–90 g CO2,eq m−2. Diel variation in fluxes was also important, with up to a 2-fold (CH4) to 4-fold (CO2) difference between the highest nighttime and lowest daytime effluxes. Within the summer ice-free period, on average, CH4 fluxes increased 2-fold during the first half of the summer, then remained almost constant, whereas CO2 effluxes remained almost constant over the entire summer, ending with a linear increase during the last 1–2 weeks of measurements. Due to the cold bottom temperatures of this 26 m deep lake, and the absence of ebullition and episodic flux events, Toolik Lake and other deep glacial lakes are likely not hot spots for greenhouse gas emissions, but they still contribute to the overall GWP of the Arctic.We acknowledge support received from the Arctic LTER grants NSF-DEB-1637459, 1026843, 1754835, NSF-PLR 1504006, and supplemental funding from the NSF-NEON and OPP-AON programs. W. E. acknowledges additional funding received from ETH Zurich scientific equipment grants 0-43350-07 and 0-43683-11. James Laundre is thanked for technical support, Jason Dobkowski for supervising deployment and removal of the float to and from the lake, and Randy Fulweber for his GIS support. Many thanks also go to Toolik Field Station staff members for support

Discovery of a natural CO2 seep in the German North Sea: implications for shallow dissolved gas and seep detection

A natural carbon dioxide (CO2) seep was discovered during an expedition to the southern German North Sea (October 2008). Elevated CO2 levels of ∼10–20 times above background were detected in seawater above a natural salt dome ∼30 km north of the East-Frisian Island Juist. A single elevated value 53 times higher than background was measured, indicating a possible CO2 point source from the seafloor. Measured pH values of around 6.8 support modeled pH values for the observed high CO2 concentration. These results are presented in the context of CO2 seepage detection, in light of proposed subsurface CO2 sequestering and growing concern of ocean acidification. We explore the boundary conditions of CO2 bubble and plume seepage and potential flux paths to the atmosphere. Shallow bubble release experiments conducted in a lake combined with discrete-bubble modeling suggest that shallow CO2 outgassing will be difficult to detect as bubbles dissolve very rapidly (within meters). Bubble-plume modeling further shows that a CO2 plume will lose buoyancy quickly because of rapid bubble dissolution while the newly CO2-enriched water tends to sink toward the seabed. Results suggest that released CO2 will tend to stay near the bottom in shallow systems (<200 m) and will vent to the atmosphere only during deep water convection (water column turnover). While isotope signatures point to a biogenic source, the exact origin is inconclusive because of dilution. This site could serve as a natural laboratory to further study the effects of carbon sequestration below the seafloor

Eddy covariance flux measurements confirm extreme CH(4) emissions from a Swiss hydropower reservoir and resolve their short-term variability

Greenhouse gas budgets quantified via land-surface 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 (CH4) 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 annual average of 3% of the particulate organic matter (POM) input via the river is sufficient to sustain these large CH4 fluxes. To compensate the global warming potential associated with the CH4 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.ISSN:1726-4170ISSN:1726-417

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

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 (CH(4)) at high rates. Here we show that an old, temperate reservoir acts simultaneously as a strong OC sink and CH(4) source, because the high sedimentation rate supplies reactive organic matter to deep, anoxic sediment strata, fuelling methanogenesis and gas bubble emission (ebullition) of CH(4) from 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 CO(2) to atmospheric CH(4). Similar high OC burial and CH(4) ebullition is expected in other reservoirs and natural river deltas

Interannual, summer, and diel variability of CH4 and CO2 effluxes from Toolik Lake, Alaska, during the ice-free periods 2010–2015

ISSN:2050-7887ISSN:2050-789

Minor methane emissions from an Alpine hydropower reservoir based on monitoring of diel and seasonal variability

ISSN:2050-7887ISSN:2050-789

Spatial Heterogeneity of Methane Ebullition in a Large Tropical Reservoir

Tropical reservoirs have been identified as important methane (CH4) sources to the atmosphere, primarily through turbine and downstream degassing. However, the importance of ebullition (gas bubbling) remains unclear. We hypothesized that ebullition is a disproportionately large CH4 source from reservoirs with dendritic littoral zones because of ebullition hot spots occurring where rivers supply allochthonous organic material. We explored this hypothesis in Lake Kariba (Zambia/Zimbabwe; surface area >5000 km(2)) by surveying ebullition in bays with and without river inputs using an echosounder and traditional surface chambers. The two techniques yielded similar results, and revealed substantially higher fluxes in river deltas (similar to 10(3) mg CH4 m(-2) d(-1)) compared to nonriver bays (<100 mg CH4 m(-2) d(-1)) Hydroacoustic measurements resolved at 5 m intervals showed that flux events varied over several orders of magnitude (up to 10(5) mg CH4 m(-2) d(-1)), and also identified strong differences in ebullition frequency. Both factors contributed to emission differences between all sites. A CH4 mass balance for the deepest basin of Lake Kariba indicated that hot spot ebullition was the largest atmospheric emission pathway, suggesting that future greenhouse gas budgets for tropical reservoirs should include a spatially well-resolved analysis of ebullition hot spots

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

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