DOI: 10.5281/zenodo.11395489
title: Biogeochemical data from diel cycles in a turbid-water pond and a clear-water pond in Brussels

description: The dataset comprises one file containing geo-referenced information with corresponding timestamps. The names of the two ponds are written in French according to the official name defined by Brussels Environment (BE) (i.e. Leybeek and Silex).

Field sampling

Sampling was done every hour from a pontoon by collecting directly surface waters with 60ml polypropylene syringes for gases (CO2, CH4, N2O). Contents of the syringes were transferred with a silicone tube in 60 ml borosilicate serum bottles (Weathon) for CH4 and N2O, poisoned with 200 µl of a saturated solution of HgCl2, and sealed with a butyl stopper and crimped with aluminium cap, without a headspace for further analysis at home laboratory. CO2 measurements were carried out directly on the field with a Li-Cor Li-840 CO2/H2O gas analyser using the headspace technique by equilibrating four syringes with 30 mL of sample water and 30 mL of atmospheric air by vigorous shaking during 5 min (Borges et al., 2019). The Li-Cor Li-840 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%. Water temperature, specific conductivity, and %O2 were also measured every hour in-situ with VWR MU 6100H probe. 2L polyethylene containers were filled with water three to four times a day and processed at home laboratory for nutrients (soluble reactive phosphorus (SRP), ammonium (NH4+), nitrate (NO3-), and nitrite (NO2-)), chlorophyll-a (Chl-a) and total suspended matter (TSM). 

Meteorological data

Meteorological data including hourly air temperature, rainfall, wind speed and atmospheric pressure were retrieved online from https://wow.meteo.be/en from the closest meteorological station of the two ponds (Institute of St-Lambert in Brussels, at 50.8408 °N, 4.4234 °E) located from 2.5km of Pêcheries pond and 5km from Silex pond.

CH4 and N2O measurements by gas chromatography and δ13C-CH4 by cavity ring-down spectrometry

Measurements of N2O and CH4 concentrations dissolved in water and in the gas were made with the headspace technique (20ml of ultra-pure N2, Air Liquid Belgium, Weiss, 1981) and a gas chromatograph (GC) (SRI 8610C) with a flame ionisation detector for CH4 and an 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.

The δ13C-CH4 was measured in the headspace gas (20 ml of synthetic air, Air Liquid Belgium) equilibrated with the water sample (total volume 60 ml). The gas samples were diluted to achieve a final CH4 partial pressure below 10 ppm, aligning with the instrument's recommended operational concentration range. This prepared gas was then injected into a cavity ring-down spectrometer (G2201-I, Isotopic Analyzer, Picarro) equipped with a Small Sample Introduction Module 2 (SSIM, Picarro). The data were corrected using calibration curves of δ13C-CH4 as a function of concentration, based on two gas standards from Airgas Specialty Gases with certified δ13C-CH4 values of -23.9±0.3 ‰ and -69.0±0.3 ‰.

Chlorophyll-a, total suspended matter, and dissolved inorganic nutrients

Water was filtered through Whatman GF/F glass microfiber filters (porosity 0.7 µm) with a diameter of 47 mm for TSM and Chl-a determination. Chl-a was extracted from filters that were kept frozen before analysis (-20°C) with 90% acetone and concentrations was determined by fluorimetry (Kontron model SFM 25) (Yentsch and Menzel, 1963). Filters used for determination of TSM were pre-weighed before filtration and weighed after filtration of a known volume of water (after oven drying at 50°C). Filtered water was used for the determination of dissolved nutrients. NH4+ was measured by the nitroprusside-hypochlorite-phenol staining method (Grasshoff and Johannsen, 1972), NO2- and NO3- were measured before and after reduction of NO3- to NO2- by a cadmium-copper column, using the Griess acid reagent staining method (Grasshoff and Kremling, 2009), SRP was measured by the ammonium molybdate, ascorbic acid and potassium antimony tartrate staining method (Koroleff, 1983).

References

Borges AV, F Darchambeau, T Lambert, C Morana, G H Allen, E Tambwe, A Toengaho Sembaito, T Mambo, J Nlandu Wabakhangazi, J-P Descy, CR Teodoru, S Bouillon (2019) Variations in dissolved greenhouse gases (CO2, CH4, N2O) in the Congo River network overwhelmingly driven by fluvial-wetland connectivity, Biogeosciences, 16, 3801-3834. https://doi.org/10.5194/bg-16-3801-2019 

Grasshoff, K., and Johannsen, H (1972). A new sensitive and direct method for the automatic determination of ammonia in sea water. ICES J. Mar. Sci. 34 (3), 516–521. https://doi.org/10.1093/icesjms/34.3.516.

Grasshoff, K., Kremling, K., and Ehrhardt, M. (2009). Methods of Seawater Analysis: Determination of Nitrite. John Wiley & Sons.

Koroleff, J. (1983). Determination of total phosphorus by alkaline persulphate oxidation. Methods of Seawater Analysis. Verlag Chemie, Wienheim, pp. 136–138.

Weiss, R. F. (1981). Determinations of carbon dioxide and methane by dual catalyst flame ionization chromatography and nitrous oxide by electron capture chromatography. Journal of Chromatographic Science, 19(12), 611-616. doi.org/10.1093/chromsci/19.12.611 

Yentsch, C. S., & Menzel, D. W. (1963). A method for the determination of phytoplankton chlorophyll and phaeophytin by fluorescence. In Deep Sea Research and Oceanographic Abstracts (Vol. 10, No. 3, pp. 221-231). Elsevier. https://doi.org/10.1016/0011-7471(63)90358-9