DOI: 10.5061/dryad.3bk3j9kf4
title: Data from: Methane emission offsets carbon dioxide uptake in a small productive lake

description: 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&gt; and may exacerbate the lake’s
 role in climate regulating processes.
description: <b>Geographic coverage bounding
 coordinates</b> 
 <i>47.09<br clear="all"> N, 8.08<br clear="all"> E</i> 
 <b>Time
 frame - Begin date</b> <i>2016, April
 26th</i> 
 <b>Time frame - End date</b> 
 <i>2018, January
 8th</i> 
 <b>General study design</b> 
 <i>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 pCO<sub>2</sub>. Inverted
 funnels were deployed to estimate CH<sub>4</sub> ebullition
 fluxes.</i> 
 <b>Methods description</b> 
 <i>Dissolved
 O<sub>2</sub>, 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 CH<sub>4</sub> and
 CO<sub>2</sub> in the water column was measured using the
 headspace method </i>(Vachon et al. 2019)<i>. 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
 CH<sub>4</sub> 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.</i>
 <i>The average coefficient of variation (C.V.) for all these diluted
 replicates was 5% (n = 67). </i><i>Dissolved
 CO<sub>2</sub> and CH<sub>4</sub> in the lake
 water was back-calculated using the air-water volume ratio in the 1-L
 bottle, using atmospheric concentrations of pCH<sub>4</sub>
 and pCO<sub>2</sub> (2 and 400
 </i><i>m</i><i>atm, respectively), water
 temperature for the Henry’s constant and the local atmospheric pressure.
 For CO<sub>2</sub>, we additionally applied a simple DIC mass
 balance in the bottle using alkalinity and carbonate system dissociation
 constants </i>(Stumm and Morgan 1995)<i> to account for
 carbonate system re-equilibration effect on the CO<sub>2</sub>
 balance in the bottle. </i> <i>Dissolved
 O<sub>2</sub> and CO<sub>2</sub> sensors were
 deployed at about 1.5 m depth on a mooring situated at the deepest point
 of the lake. The dissolved O<sub>2</sub> probe (miniDOT,
 Precision Measurements Engineering) logged temperature, dissolved
 O<sub>2</sub> concentration (mg O<sub>2</sub>
 L<sup>-1</sup>) and saturation (%) every minute.
 CO<sub>2</sub> partial pressure (pCO<sub>2</sub>;
 </i><i>m</i><i>atm) was measured every hour by a
 GasPro probe (detection limit between 10-20 µatm) </i>(Graziani et
 al. 2014)<i> fitted with a copper mesh. Measurement of
 pCO<sub>2</sub> 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
 pCO<sub>2</sub> measurements were not available during April
 2016 to July 2016 and from September to November 2016.
 </i> <i>We measured gas
 ebullition fluxes using five bubble traps made of inverted funnels with
 collectors deployed over various sediment depths </i>(Vachon et al.
 2019)<i>. 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) </i>(Delwiche et al. 2015)<i> 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<sup>-2</sup>
 d<sup>-1</sup>) 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 CH<sub>4</sub>
 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
 CH<sub>4</sub> fraction in the sediment bubbles at various
 depths </i>(Langenegger et al. 2019)<i> 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)
  </i>(McGinnis et al. 2006)<i>. The simulations were performed
 using an initial bubble diameter of 5 mm </i>(McGinnis et al. 2006;
 Delsontro et al. 2015)<i>.</i> 
 
 <b>Laboratory, field, or other analytical
 methods</b> <i>DIC concentrations were
 derived from dissolved CO<sub>2</sub> and total alkalinity
 (Alk) using water temperature and the carbonate system dissociation
 constants </i>(Stumm and Morgan 1995)<i>. Measured Alk in 2016
 varied between around 3 meq L<sup>-1</sup> in surface waters
 and 4 meq L<sup>-1</sup> in bottom waters (Canton of Lucerne,
 pers. comm.). A significant linear relationship was found between specific
 conductivity at 20</i><i>°</i><i>C (cond) and Alk
 (meq L<sup>-1</sup>) measurements performed twice a year
 between 2008 and 2016 by the Canton of Lucerne monitoring program (Alk =
 0.012 × cond - 0.52, r<sup>2</sup> = 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.</i> 
 <b>Quality
 control</b> <i>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
 CH<sub>4</sub> and CO<sub>2</sub>, respectively.
 Both O<sub>2</sub> and pCO<sub>2</sub> 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 pCO<sub>2</sub> and O<sub>2</sub>
 measurements were in excellent agreement with manual
 sampling.</i>   
 <b>References</b>
 Delsontro, T., D. F. McGinnis, B. Wehrli, and I. Ostrovsky. 2015. Size does matter: Importance of large bubbles and small-scale hot spots for methane transport. Environ. Sci. Technol. <b>49</b>: 1268–1276. doi:10.1021/es5054286 Delwiche, K., S. Senft-Grupp, and H. Hemond. 2015. A novel optical sensor designed to measure methane bubble sizes in situ. Limnol. Oceanogr. <b>13</b>: 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. <b>48</b>: 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. <b>64</b>: 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. <b>111</b>: 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    
description: <strong>Water column profiles and high-frequency
 measurements of dissolved O2, CO2 and CH4 in a small eutrophic lake
 (Soppensee, Switzerland) </strong> This dataset
 consists of vertical profiles of dissolved CH<sub>4</sub>,
 CO<sub>2</sub> and O<sub>2</sub> concentrations
 measured approximately every month (less often in winter) and surface
 water daily measurement of O<sub>2</sub> and
 CO<sub>2</sub> concentrations, which is used to complete a
 whole lake mass balance in a small eutrophic lake in Switzerland. The
 dataset is from April 2016 to January 2018. All the samples were taken in
 the center of the lake at the deepest point (26 m). Additional data are
 water temperature (profiles and surface high-frequency) and
 CH<sub>4</sub> ebullition rates from a set of 5 inverted
 funnels. Weather data of the nearest meteorological station Egolzwil is
 available through the MeteoSwiss online download platform IDAweb at https://gate.meteoswiss.ch/idaweb/login.do. Dataset filename: <i>Water_column_temperature_oxygen_profiles.txt</i> Dataset description: <i>This table reports 20 profiles of water temperature and dissolved oxygen concentration for each meter depth (down to 25 meters) in the center of Soppensee between April 2016 and January 2018. Volume of each 1-meter lake stratum is also displayed. Tab delimited.</i> <b>Column name</b> <b>Description</b> <b>Units</b> <b>Code explanation</b> <b>Data format</b> <b>Missing data code</b> <i>Date</i> <i>Date of the measured profile</i>     <i>yyyy-mm-dd</i> <i>blank cell</i> <i>Depth</i> <i>Depth of measurement</i> <i>meter below surface, m</i>     <i>blank cell</i> <i>Vol</i> <i>Volume of the lake stratum at the corresponding depth</i> <i>Cubic meter, m3</i>     <i>blank cell</i> <i>Temp</i> <i>Water temperature at corresponding depth</i> <i>Celcius, </i><i>°</i><i>C</i>     <i>blank cell</i> <i>O2conc</i> <i>Concentration of dissolved oxygen</i> <i>mmol/m3</i>     <i>blank cell</i>   Dataset filename: <i>Water_column_gas_concentration_profiles.txt</i> Dataset description: <i>This table reports 20 profiles of dissolved methane concentration, dissolved carbon dioxide and dissolved inorganic carbon at sampling depth (down to 25 meters) in the center of Soppensee between April 2016 and January 2018. Tab delimited.</i> <b>Column name</b> <b>Description</b> <b>Units</b> <b>Code explanation</b> <b>Data format</b> <b>Missing data code</b> <i>Date</i> <i>Date of the measured profile</i>     <i>yyyy-mm-dd</i> <i>blank cell</i> <i>Depth</i> <i>Depth of measurement</i> <i>meter below surface, m</i>     <i>blank cell</i> <i>CH4conc</i> <i>Concentration of dissolved methane</i> <i>mmol/m3</i>     <i>blank cell</i> <i>CO2conc</i> <i>Concentration of dissolved carbon dioxide</i> <i>mmol/m3</i>     <i>blank cell</i> <i>DICconc</i> <i>Concentration of dissolved inorganic carbon</i> <i>mmol/m3</i>     <i>blank cell</i>   Dataset filename: <i>daily_surface_temperature_amd_gas_concentration.txt</i> Dataset description: <i>Daily averaged surface water temperature, dissolved oxygen, dissolved methane and carbon dioxide partial pressure in Soppensee between April 2016 and January 2018. Ice cover and whole lake daily CH<sub>4</sub> ebullition is also given. Tab delimited.</i> <b><i>Column name</i></b> <b><i>Description</i></b> <b><i>Units</i></b> <b><i>Code explanation</i></b> <b><i>Data format</i></b> <b><i>Missing data code</i></b> <i>date</i> <i>Date of corresponding measurement</i>     <i>yyyy-mm-dd</i> <i>blank cell</i> <i>ice</i> <i>Ice cover on the lake</i>   <i>0 = ice free, 1 = ice covered</i>   <i>blank cell</i> <i>waterT</i> <i>Water temperature measured every minute at surface of lake (</i><i>~</i><i>1m) and daily averaged.</i> <i>Celcius, </i><i>°</i><i>C</i>     <i>blank cell</i> <i>DO</i> <i>Dissolved oxygen measured at surface of lake (</i><i>~</i><i>1.5m). Measured every minute and daily averaged.</i> <i>mg O<sub>2</sub> / L</i>     <i>blank cell</i> <i>CH4</i> <i>Point measurement of dissolved methane measured at surface of lake (0.5 m). Measured once during the day between 9AM and 4PM.</i> <i>mmol/m3</i>     <i>blank cell</i> <i>pCO2</i> <i>Carbon dioxide partial pressure measured at surface of lake (</i><i>~</i><i>1.5m). Measured every hour and daily averaged.</i> <i>m</i><i>atm</i>     <i>blank cell</i> <i>ebu</i> <i>Daily whole lake average CH<sub>4</sub> ebullition estimated from 5 inverted funnels</i> <i>mmol CH<sub>4</sub>/m<sup>2</sup>/d</i>     <i>blank cell</i> Dataset filename: <i>methane_ebullition_Soppensee.txt</i> Dataset description: <i>Methane ebullition reaching the atmosphere averaged over the time between two visits of the lake at five different depths in Soppensee between April 2016 and January 2018. Tab delimited.</i> <b>Column name</b> <b>Description</b> <b>Units</b> <b>Code explanation</b> <b>Data format</b> <b>Missing data code</b> <i>date_start</i> <i>Date of the start of the funnel deployment</i>     <i>yyyy-mm-dd</i> <i>blank cell</i> <i>date_stop</i> <i>Date of the funnel measurement</i>     <i>yyyy-mm-dd</i> <i>blank cell</i> <i>days</i> <i>Number of days of the deployment</i> <i>days</i>     <i>blank cell</i> <i>depth</i> <i>Water column depth at the funnel location, i.e. distance between sediments and air-water interface.</i> <i>Meters, m</i>     <i>blank cell</i> <i>ebu_total</i> <i>Volume of gas measured in the gas collector of the funnel during the deployment per m<sup>2</sup> per day</i> <i>ml / m<sup>2</sup> / d</i>     <i>blank cell</i> <i>CH4_rate</i> <i>Daily averaged methane ebullition rate during the deployment.</i> <i>mmol / m<sup>2</sup> / d</i>     <i>blank cell</i>