Data-set of CO2, CH4, N2O dissolved concentrations and ancillary data in 15 Ecuadorian high-altitude lakes

Data-set of the dissolved concentrations of CO2, CH4 and N2O and ancillary data in 15 lakes located in the northern region of Ecuadorian Andes along an elevational gradient from 2,213 to 4,361 m above sea level, as well as a gradient of lake surface area (0.003 to 6.1 km2) and depth (0.9 to 74 m) (Fig. 1). Most lakes were located in the páramos of Salve Facha and Antisana y Mojanda.

Sampling was carried out over a period from April 2019 to March 2022, with an inflatable boat approximately in the center of the lake, during day-time only (early morning to late afternoon). Water temperature, specific conductivity, pH, and %O2 were measured in surface water with a YSI multi-parameter probe (ProPlus). Water for CH4 and N2O samples was collected with a sampling devise consisting of a 2L polyethylene bottle with the bottom cut and fitted with a silicone tubing at the stopper (Abril et al. 2007). Two borosilicate serum bottles (Weathon) with a volume of 40 ml were filled with the silicone tubing, poisoned with 100 µl of a saturated solution of HgCl2 and sealed with a butyl stopper and crimped with an aluminium cap. Measurements were made, after over-night equilibration, on an headspace (Weiss 1981) (created by injecting 15 ml of high-purity N2 into the 40 ml sample bottles), with a gas chromatograph (SRI 8610C) with a flame ionisation detector for CH4 and electron capture detector for N2O calibrated with CH4:N2O:N2 gas mixtures (Air Liquide Belgium) with mixing ratios of 1, 10 and 30 ppm for CH4, and 0.2, 2.0 and 6.0 ppm for N2O. The precision of measurement based on duplicate samples was ±10.9% for CH4 and ±5.8% for N2O.

The partial pressure of CO2 (pCO2) was measured in the field with a Li-Cor Li-820 infra-red gas analyser based on the headspace technique with four 60 ml polypropylene syringes that were filled directly with surface water. The pCO2 in the atmosphere was measured by injecting ambient air sampled with an additional polypropylene syringe. The Li-Cor Li-820 was calibrated with pure N2 and CO2:N2 gas mixtures (Air Liquide Belgium) of 388, 804, 3,707 and 8,146 ppm. The final pCO2 value was computed taking into account the partitioning of CO2 between water and the headspace, as well as equilibrium with HCO3- (Dickson et al. 2007) using water temperature measured in-situ and after equilibration, and total alkalinity (TA). The precision of pCO2 measurement was ±5.2%.

The CO2 concentration is expressed as partial pressure in parts per million (ppm) and as dissolved concentration for CH4 (nmol L-1), in accordance with convention in existing topical literature. Variations of N2O were modest and concentrations fluctuated around atmospheric equilibrium, so data are presented as percent of saturation level (%N2O, where atmospheric equilibrium corresponds to 100%), computed from the global mean N2O air mixing ratios given by the Global Monitoring Division (GMD) of the Earth System Research Laboratory (ESRL) of the National Oceanic and Atmospheric Administration (NOAA) (https://www.esrl.noaa.gov/gmd/hats/combined/N2O.html), using the Henry’s constant (Weiss and Price 1980).

Samples for the stable isotope composition of DIC (δ13C-DIC) were collected in 12 ml Exetainer vials (Labco) and poisoned with 50 µL of a saturated solution of HgCl2. Prior to the analysis of δ13C-DIC, a 2 ml helium headspace was created and 100 µL of phosphoric acid (H3PO4, 99%) was added in the vial in order to convert CO32- and HCO3- to CO2. After overnight equilibration, up to 1 mL of the headspace was injected with a gastight syringe into a coupled elemental analyser - IRMS (EA-IRMS, Thermo FlashHT or Carlo Erba EA1110 with DeltaV Advantage). The obtained data were corrected for isotopic equilibration between dissolved and gaseous CO2 as described by Gillikin and Bouillon (2007). Calibration was performed with certified standards (NBS-19 or IAEA-CO-1, and LSVEC). Reproducibility of measurement based on duplicate injections of samples was typically better than ±0.2 ‰.

Water was collected in surface water with a 2L polyethylene bottle. The water filtered through 47 mm diameter GF/F Whatman glass fibber filters was collected and further filtered through polyethersulfone syringe encapsulated filters (0.2 µm porosity) for nitrate (NO3-), nitrite (NO2-), ammonium (NH4+), TA, major elements (Na+, Mg2+, Ca2+, K+), as well as dissolved silicate (DSi) and Fe, stable isotope composition of O and H of H2O (δ18O-H2O and δ2H-H2O) and dissolved organic carbon (DOC). An additional water filtration was made on 25 mm diameter GF/F Whatman glass fibber filters for particulate organic carbon (POC) analysis.

Samples for NO3-, NO2-, and NH4+ were stored frozen (-20°C) in 50 ml polypropylene vials. NO3- and NO2- were determined with the sulfanilamide colorimetric with the vanadium reduction method (American Public Health Association, 1998), and NH4+ with the dichloroisocyanurate-salicylate-nitroprussiate colorimetric method (Standing committee of Analysts, 1981). Detection limits were 0.3, 0.01, and 0.15 µmol L-1 for NH4+, NO2- and NO3-, respectively. Precisions were ±0.02 µmol L-1, ±0.02 µmol L-1, and ±0.1 µmol L-1 for NH4+, NO2- and NO3-, respectively.

Samples for TA were stored at ambient temperature in polyethylene 55 ml vials and measurements were carried out by open-cell titration with HCl 0.1 mol L-1 according to Gran (1952), and data quality checked with certified reference material obtained from Andrew Dickson (Scripps Institution of Oceanography, University of California, San Diego, USA), with a typical reproducibility better than ±3 µmol kg-1.

Samples for δ18O-H2O and were δ2H-H2O stored at ambient temperature in polypropylene 8 ml vials. δ2H-H2O was measured on H2 gas derived from a high‐temperature (1,030°C) Cr‐based reactor by automated injections of water using a TriPlus autosampler on an elemental analyzer (Thermo Flash HT/EA; Thermo Finnigan) coupled to a continuous‐flow isotope‐ratio mass spectrometer (Delta V Advantage; Thermo Finnigan). δ18O-H2O values were measured on a Thermo GasBench II coupled to a Thermo Delta XP IRMS after equilibration with CO2. The long-term uncertainty for standard δ18O values was ±0.1‰.

Samples for major elements were stored at ambient temperature in 20 ml scintillation vials and preserved with 50 μl of HNO3 (65%). Major elements were measured with inductively coupled plasma MS (ICP-MS; Agilent 7700x) calibrated with the following standards: SRM1640a from National Institute of Standards and Technology, TM-27.3 (lot 0412) and TMRain-04 (lot 0913) from Environment Canada, and SPS-SW2 Batch 130 from Spectrapure Standard. Limit of quantification was 0.5 µmol L-1 for Na+, Mg2+ and Ca2+, 1.0 µmol L-1 for K+ and 8 µmol L-1 for DSi.

Samples to determine DOC were stored at ambient temperature and in the dark in 40 ml brown borosilicate vials with polytetrafluoroethylene (PTFE) coated septa and poisoned with 50 µL of H3PO4 (85%), and DOC concentration was determined with a wet oxidation total organic carbon analyzer (IO Analytical Aurora 1030W), with a typical reproducibility better than ±5%.

Filters for POC analysis were decarbonated with HCl fumes for 4h and dried before encapsulation into silver cups; POC concentration was analysed on an EA-IRMS (Thermo FlashHT with DeltaV Advantage), with a reproducibility better than ±5%. Data were calibrated with certified (IAEA-600: caffeine) and in-house standards (leucine, tuna muscle tissue) that were previously calibrated versus certified standards.

References

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