Technical description :
The geo-referenced and timestamped data-set consists of 7 files:
- “db_cruise_CTD” contains the CTD profiles obtained during the cruises
- “db_cruise_GHGs” contains CO2, CH4, N2O dissolved concentrations, chlorophyll-a concentrations, inorganic nutrients (NO3-, N2O-, NH4+, PO43-) and d13C-CH4 from the 4 cruises
- “db_monitoring” contains CO2, CH4, N2O dissolved concentrations, chlorophyll-a concentrations, and POC from the monitoring at two stations (January 2017 to December 2019)
- “db_uw” contains the continuous of CO2 and CH4 (plus EXO-II data) on 19/03/2019
- “meteo_Mweya” contains the meteorological data acquired from June 2016 to March 2019
- “db_monitoring_CTD” contains the CTD profiles from the deep station of the monitoring.
- “mooring” contains the temperature data from a mooring at a station 10 m deep (March 2019)
Data were acquired in Lake Edward, Kazinga Channel and Lake George on four occasions (20/10-07/11/2016, 23/03-08/04/2017, 18/01-02/02/2018, 21/03-30/03/2019). From January 2017 to December 2019, a shallow station (3 m bottom depth) and a deeper station (22 m bottom depth) were regularly sampled, every 21 d in 2017 and 2018, and every 30 d in 2019. A mooring was deployed at a station at 10m bottom depth in Lake Edward (-0.2459°N 29.8635°E) equipped with RBR Solo temperature sensors at 6 depths from surface to 1m above the sediment (0.2, 1.0, 2.0, 5.0, 7.5 and 9.0 m depth) from 21/03/2019 (13:00 local time (LT)) to 23/03/2019 (13:50 LT).
Solar radiation, ultraviolet radiation, wind speed (cup anemometer), wind direction (wind vane), rain (mechanical rain collector), air temperature, barometric pressure data were acquired with a Davis Instruments weather station (Vantage Pro2 fitted with standard manufacturer sensors) in Mweya on top of a building of the Uganda Wildlife Authority, 4m above ground (-0.190384°N 29.899103°E) . Data were measured every 5 seconds, averaged and logged every 10 minutes.
During the March 2019 cruise, continuous measurements (1 min interval) of partial pressure of CO2 (pCO2) and of partial pressure of CH4 (pCH4) were made with an equilibrator designed for turbid waters consisting of a tube filled with glass marbles (Frankignoulle et al. 2001) coupled to a Los Gatos Research off-axis integrated cavity output spectroscopy analyzer (Ultraportable Greenhouse Gas Analyzer with extended range for CH4). In parallel water temperature, specific conductivity, pH, dissolved oxygen saturation level (%O2), turbidity, chlorophyll-a (Chl-a), and fluorescent dissolved organic matter (FDOM) were measured with an YSI EXO-II multi-parameter probe, position with a Garmin geographical position system (Map 60S) portable probe, and depth with a Humminbird Helix 5 echo-sounder. Surface water was pumped to the equilibrator and the multi-parameter probe (on deck) with a 12V-powered water pump (LVM105) attached to the side of the boat at a fixed depth of about 0.5 m depth.
Discrete sampling was done from the side of the boat with a 5.0 L Niskin bottle (General Oceanics). During the first cruise, vertical profiles of water temperature, specific conductivity, pH, %O2 and Chl-a were measured with a Hydrolab DS5 multi-parameter probe, while during the other three cruises and also during the monitoring, turbidity and FDOM were measured additionally with a YSI EXO-II multi-parameter probe. Both multi-parameter probes were calibrated according to manufacturer’s specifications, in air for %O2 and with standard solutions for other variables: commercial pH buffers (4.00, 7.00, 10.00), a 1000 µS cm-1 standard for conductivity. pCO2 was measured directly after water sampling with a Li-Cor Li-840 infra-red gas analyser (IRGA) based on the headspace technique with 4 polypropylene 60 ml syringes (Borges et al. 2015). The Li-Cor 840 IRGA was calibrated before and after each cruise with ultrapure N2 and a suite of gas standards (Air Liquide Belgium) with CO2 mixing ratios of 388, 813, 3788 and 8300 ppm. The overall precision of pCO2 measurements was ±2.0%.
Samples for CH4 and N2O were collected from the Niskin bottle with a silicone tube in 60 ml borosilicate serum bottles (Wheaton), poisoned with 200 µL of a saturated solution of HgCl2 and sealed with a butyl stopper and crimped with an aluminium cap. Measurements were made with the headspace technique (Weiss 1981) and a gas chromatograph (GC) (SRI 8610C) with a flame ionisation detector for CH4 and electron capture detector for N2O calibrated with CO2:CH4:N2O:N2 gas mixtures (Air Liquide Belgium) with mixing ratios of 1, 10 and 30 ppm for CH4, 404, 1018, 3961 ppm for CO2, and 0.2, 2.0 and 6.0 ppm for N2O. The precision of measurement based on duplicate samples was ±3.9% for CH4 and ±3.2% for N2O.
Samples for the stable isotope composition of CH4 (δ13C-CH4) were collected and preserved as described above for the CH4 concentration. The δ13C-CH4 was determined with a custom developed interface, whereby a 20 ml He headspace was first created, and CH4 was flushed out through a double-hole needle, non-CH4 volatile organic compounds were trapped in liquid N2, CO2 was removed with a soda lime trap, H2O was removed with a magnesium perchlorate trap, and the CH4 was quantitatively oxidized to CO2 in an online combustion column similar to that of an elemental analyzer. The resulting CO2 was subsequently pre-concentrated by immersion of a stainless steel loop in liquid N2, passed through a micropacked GC column (Restek HayeSep Q, 2m length, 0.75mm internal diameter), and finally measured on a Thermo DeltaV Advantage isotope ratio mass spectrometer (IRMS). Calibration was performed with CO2 generated from certified reference standards (IAEA-CO-1 or NBS-19, and LSVEC) and injected in the line after the CO2 trap. Reproducibility of measurement based on duplicate injections of samples was typically better than ±0.5 ‰.
Water was filtered on Whatman glass fibre filters (GF/F grade, 0.7 µm porosity) for particulate organic carbon (POC) and Chl-a (47 mm diameter). Filters for POC were stored dry and filters for Chl-a were stored frozen at -20°C. 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 and muscle tissue of Pacific tuna) that were previously calibrated versus certified standards. The Chl-a samples were analysed by HPLC according to Descy et al. (2005), with a reproducibility of ±0.5% and a detection limit of 0.01 µg L-1.
The water filtered through GF/F Whatman glass fibre filters was collected and further filtered through polyethersulfone syringe encapsulated filters (0.2 µm porosity) for nitrate (NO3-), nitrite (NO2-) and ammonium (NH4+) and were stored frozen (-20°C) in 50 mL polypropylene vials. NO3- and NO2- were determined with the sulfanilamide colorimetric with the vanadium reduction method (APHA, 1998), and NH4+ with the dichloroisocyanurate-salicylate-nitroprussiate colorimetric method (SCA, 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.
References
APHA, 1998. Standard methods for the examination of water and wastewater, American Public Health Association.
Borges, A. V., Darchambeau, F., Teodoru, C. R., Marwick, T. R., Tamooh, F., Geeraert, N., Omengo, F. O., Guérin, F., Lambert, T., Morana, C., Okuku, E., and Bouillon, S.: Globally significant greenhouse gas emissions from African inland waters, Nature Geosci., 8, 637-642, doi:10.1038/NGEO2486, 2015.
Descy, J.-P., Hardy, M.-A., Sténuite, S., Pirlot, S., Leporcq, B., Kimirei, I., Sekadende, B., Mwaitega, S. R., and Sinyenza, D., 2005. Phytoplankton pigments and community composition in Lake Tanganyika. Freshw. Biol., 50, 668-684.
Frankignoulle, M., Borges, A., Biondo R., 2001. A new design of equilibrator to monitor carbon dioxide in highly dynamic and turbid environments. Water Res., 35, 1344-1347.
Standing committee of Analysts: Ammonia in waters. Methods for the examination of waters and associated materials. 16 pp., 1981.
Weiss, R.F., 1981. Determinations of carbon dioxide and methane by dual catalyst flame ionization chromatography and nitrous oxide by electron capture chromatography. J. Chromatogr. Sci., 19, 611-616.
