Methane emission offsets carbon dioxide uptake in a small productive lake

Here we investigate the importance of net CH4 production and emissions in the carbon (C) budget of a small eutrophic lake by monitoring CH4, CO2 and O2 during two consecutive years. During the study period, the lake was mostly a net emitter of both CH4 and CO2, while having a net autotrophic metabolism. The analyses suggest that during the whole study period 29 ± 23 % of C produced by net ecosystem production was ultimately converted to CH4 and emitted to the atmosphere. When converted to global warming potential, CH4 emission (in CO2 equivalents) was about 3 to 9 times higher than CO2 uptake from in-lake net ecosystem production over 100-years and 20-year timeframes, respectively. Our results suggest that the continuing eutrophication of inland waters will increase the amount of C released from lakes as CH4 instead of CO2> and may exacerbate the lake’s role in climate regulating processes.

Methods

Geographic coverage bounding coordinates

47.09
N, 8.08
E

Time frame - Begin date

2016, April 26th

Time frame - End date

2018, January 8th

General study design

The study was conducted in the small lake Soppensee situated in the Canton Lucerne, Switzerland (lake area: 23 ha, maximum depth: 26 m, mean depth: 12.2 m). Between April 2016 and January 2018, water column was sampled at several depth for dissolved gas concentration and water temperature on an approximately monthly basis. Moored instruments were installed to monitor water temperature at various depths and surface dissolved O2 and pCO2. Inverted funnels were deployed to estimate CH4 ebullition fluxes.

Methods description

Dissolved O2, specific conductivity and water temperature profiles were measured using a multi-parameter sonde (Yellow Spring Instrument EXO2, after May 2017 a Seabird CTD profiler, SBE19) and 1-meter resolution is reported here. Dissolved CH4 and CO2 in the water column was measured using the headspace method (Vachon et al. 2019). Lake water was sampled at defined depth using a 5 L Niskin water sampler. Water from the Niskin bottle was gently poured into a 1-L glass bottle, letting overflow to replace 1-L bottle volume several times. A volume of 400 mL of water was carefully removed and replaced with ambient air without creating turbulence. The bottle was capped and shaken vigorously for at least 2 minutes. The headspace was transferred into a gas-tight atmospheric sampling bag (SupelTM Inert Multi-Layer Foil) for further gas analysis on a Cavity Ring-Down Spectrometer (Picarro G2201-i). In case of high CH4 concentration (e.g. samples below 10 m in Soppensee), an additional dilution was applied before the analysis by subsampling the bag and diluting a known volume into a 50 ml glass gas-tight syringe (SGE Analytical Science, Australia) filled with synthetic air (Carbagas: 80% N2, 20% O2; ± 1%). In these cases, the gas was subsampled two or three times and the average was taken. The average coefficient of variation (C.V.) for all these diluted replicates was 5% (n = 67). Dissolved CO2 and CH4 in the lake water was back-calculated using the air-water volume ratio in the 1-L bottle, using atmospheric concentrations of pCH4 and pCO2 (2 and 400 matm, respectively), water temperature for the Henry’s constant and the local atmospheric pressure. For CO2, we additionally applied a simple DIC mass balance in the bottle using alkalinity and carbonate system dissociation constants (Stumm and Morgan 1995) to account for carbonate system re-equilibration effect on the CO2 balance in the bottle.

Dissolved O2 and CO2 sensors were deployed at about 1.5 m depth on a mooring situated at the deepest point of the lake. The dissolved O2 probe (miniDOT, Precision Measurements Engineering) logged temperature, dissolved O2 concentration (mg O2 L-1) and saturation (%) every minute. CO2 partial pressure (pCO2; matm) was measured every hour by a GasPro probe (detection limit between 10-20 µatm) (Graziani et al. 2014) fitted with a copper mesh. Measurement of pCO2 is based on equilibration of the surrounding water with a small-volume headspace containing a miniature nondispersive infrared (NDIR) detector (model IRC-A1; Alphasense) via diffusion through a gas permeable membrane (Teflon AF 2400; Biogeneral Inc.). Continuous pCO2 measurements were not available during April 2016 to July 2016 and from September to November 2016.

We measured gas ebullition fluxes using five bubble traps made of inverted funnels with collectors deployed over various sediment depths (Vachon et al. 2019). In May 2016, two funnels (opening of 0.79 m2) were deployed 4 m below the water surface over 10 m and 21 m deep sediments. Another bubble trap (also 0.79 m2) was installed in June 2017 over 15 m deep sediments, also moored at 4 m below surface. Two smaller funnels (0.25 m2) (Delwiche et al. 2015) were installed in April 2017 at shallower depths (about 10 cm above sediments at 3 and 6 m) to allow better spatial coverage. All bubble traps were moored with an inverted-V configuration to avoid disturbing the sediments below them. For sampling gas accumulation, the traps were gently brought to the surface (except for the 2 shallow funnels that could be sampled from a tube at the surface) and the gas volume was sampled using a 60-ml plastic syringe. Total gas flux (mmol m-2 d-1) received by each funnel was calculated using the Law of Ideal gases using in situ water temperature and atmospheric pressure, and by accounting for funnel area and deployment duration. Total gas flux was multiplied by the fraction of CH4 found in the bubbles (0.596 at 3m, 0.675 at 6m, 0.61 at 10m, 0.561 at 15m and 0.555 at 21m). These proportion were estimated by first measuring the CH4 fraction in the sediment bubbles at various depths (Langenegger et al. 2019) and by correcting for gas exchange with water during the ascend using a discrete bubble model (SiBu-GUI: Single Bubble – Dissolution Model / Version 1.2.6a)  (McGinnis et al. 2006). The simulations were performed using an initial bubble diameter of 5 mm (McGinnis et al. 2006; Delsontro et al. 2015).

Laboratory, field, or other analytical methods

DIC concentrations were derived from dissolved CO2 and total alkalinity (Alk) using water temperature and the carbonate system dissociation constants (Stumm and Morgan 1995). Measured Alk in 2016 varied between around 3 meq L-1 in surface waters and 4 meq L-1 in bottom waters (Canton of Lucerne, pers. comm.). A significant linear relationship was found between specific conductivity at 20°C (cond) and Alk (meq L-1) measurements performed twice a year between 2008 and 2016 by the Canton of Lucerne monitoring program (Alk = 0.012 × cond - 0.52, r2 = 0.98, n=96). From this relationship, we estimated Alk for each profile of the whole study using specific conductivity measured by the multi-parameter sonde.

Quality control

For water column dissolved gases a second sample was occasionally taken for a given depth to test for replicability of the method (n = 8). The average coefficient of variation (C.V.) for all the replicates was 9.7% and 10.6% for CH4 and CO2, respectively. Both O2 and pCO2 probes were frequently deployed in bubbled lake water (in a bucket with aquarium pumps) for several hours and check for gas atmospheric equilibrium values. Continuous pCO2 and O2 measurements were in excellent agreement with manual sampling.

References

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Delwiche, K., S. Senft-Grupp, and H. Hemond. 2015. A novel optical sensor designed to measure methane bubble sizes in situ. Limnol. Oceanogr. 13: 712–721. doi:10.1002/lom3.10060

Graziani, S., S. E. Beaubien, S. Bigi, and S. Lombardi. 2014. Spatial and temporal pCO2 marine monitoring near Panarea Island (Italy) using multiple low-cost gaspro sensors. Environ. Sci. Technol. 48: 12126–12133. doi:10.1021/es500666u

Langenegger, T., D. Vachon, D. Donis, and D. F. McGinnis. 2019. What the bubble knows: Lake methane dynamics revealed by sediment gas bubble composition. Limnol. Oceanogr. 64: 1526–1544. doi:10.1002/lno.11133

McGinnis, D. F., J. Greinert, Y. Artemov, S. E. Beaubien, and  a. Wüest. 2006. Fate of rising methane bubbles in stratified waters: How much methane reaches the atmosphere? J. Geophys. Res. 111: C09007. doi:10.1029/2005JC003183

Stumm, W., and J. J. Morgan. 1995. Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters, 3rd Editio. Wiley-Interscience.

Vachon, D., T. Langenegger, D. Donis, and D. F. McGinnis. 2019. Influence of water column stratification and mixing patterns on the fate of methane produced in deep sediments of a small eutrophic lake. Limnol. Oceanogr. 1–15. doi:10.1002/lno.11172