Biogeochemical data from two clear-water and two turbid-water urban ponds in Brussels (Belgium) from June 2021 to December 2023

The dataset comprises three files, each containing geo-referenced information with corresponding timestamps. The names of the four ponds are written in French according to the official name defined by Brussels Environment (BE) (i.e. Leybeek, Pêcheries, Tenreuken and Silex).

·        DATA_Dissolved contains water temperature (°C), Chlorophyll-a concentration (µg L^-1), oxygen saturation (%O₂, in %), total suspended matter concentration (TSM, in mg L^-1), inorganic nutrients (nitrate : NO₃^-, nitrite : NO₂^-, ammonium : NH₄⁺, soluble reactive phosphorus : SRP, in µmol L^-1), partial pressure of CO₂ (pCO₂, in ppm) , dissolved CH₄ concentration (CH₄, in nmol L^-1), N₂O saturation level (%N₂O, in %) and ¹³C/¹²C ratio of CH₄ (δ¹³C-CH₄, in ‰) collected from June 2021 to December 2023 in four ponds in Brussels.

·        DATA_Ebullitive contains wind speed (m s^-1), water temperature (°C), atmospheric pressure (atm), bubbles flux measured with inverted funnels (mL m^-2 d^-1), CH₄ content in bubbles (%CH₄, in %) and ¹³C/¹²C ratio of CH₄ (δ¹³C-CH₄, in ‰) of CH₄ in the bubbles measured with three bubble traps in spring, summer, and fall in 2022 and 2023, totaling 8 days in the Leybeek, Pêcheries, and Tenreuken ponds and 24 days in the Silex pond.

·        DATA_perturbed_sediments contains ¹³C/¹²C ratio of CH₄ (δ¹³C-CH₄, in ‰) of CH₄ in the bubbles present in the sediment directly sampled with bubble traps by physically perturbing the sediment with a wooden rod the fourth September 2023 in the four ponds.

Field sampling and meteorological data

Sampling was done from a pontoon, with 60mL polypropylene syringes for gases (CO₂, CH₄, N₂O) and a 2L polyethylene water container for processing at the home laboratory for other variables. Water temperature and %O₂ were measured in-situ with VWR MU 6100H probe. pCO₂ was measured with a Li-Cor Li-840 infrared gas analyser (IRGA) based on the headspace technique with 4 polypropylene syringes (Borges et al., 2019). 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 transferred from the syringes with a silicone tube in 60 mL borosilicate serum bottles (Weathon), poisoned with 200 µl of a saturated solution of HgCl₂ and sealed with a butyl stopper and crimped with aluminium cap, without a headspace.

Three bubble traps were deployed at 50 cm apart for measuring ebullitive CH₄ flux. The bubble traps consistent in inverted polypropylene funnels (diameter 23.5cm) mounted with 60mL polypropylene syringes and attached with steel rods to a polystyrene float. The volume of gas collected in the funnels was measured every 24 hours with 60mL syringes. The collected gas was stored in pre-evacuated 12 mL vials (Exetainers, Labco, UK) for the analysis of CH₄ concentration and δ¹³C-CH₄.

Wind speed and atmospheric pressure, were retrieved from https://wow.meteo.be/en for the meteorological station of the Royal Meteorological Institute of St-Lambert (50.8408°N, 4.4234°E) in Brussels, located between 2.5 and 5 kilometers from the surveyed ponds.

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 samples from bubbles 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 (with a methanizer for CO₂) 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.

The δ¹³C-CH₄ was measured in gas of the headspace (20mL of synthetic air, Air Liquid Belgium) equilibrated with the water sample (total volume 60mL) for water samples and directly on gas stored in Exetainers for gas samples from the bubble traps. The gas samples were diluted to obtain a final partial pressure of CH₄ in the cavity below 10 ppm to fall within the recommended operational concentration range of the instrument, prior to injection into a cavity ring-down spectrometer (G2201-I, Isotopic Analyzer, Picarro) with a Small Sample Introduction Module 2 (SSIM, Picarro). Data were corrected with 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 total suspended matter (TSM) and Chlorophyll-a (Chl-a). Filters for TSM were dried in the oven at 50C° and filters for Chl-a were kept frozen (-20°C). The weight of each filter was determined before and after filtration of a known volume of water using an Explorer™ Pro EP214C analytical microbalance (accuracy: ±0.1mg) for determination of TSM. Filtered water was stored in 50 mL plastic bottles and frozen (-20°C) for analysis of dissolved nutrients. Chl-a was measured on extracts with 90% acetone by fluorimetry (Kontron model SFM 25) (Yentsch and Menzel, 1963) with a limit of detection of 0.01 µg L^-1. Ammonium (NH₄⁺) was determined by the nitroprusside-hypochlorite-phenol staining method (Grasshoff and Johannsen, 1972), with a limit of detection of 0.05 µmol L^-1. Nitrite (NO₂^-) and nitrate (NO₃^-) were determined before and after reduction of NO₃^- to NO₂^- by a cadmium-copper column, using the Griess acid reagent staining method (Grasshoff and Kremling, 2009), with a detection limit of 0.01 and 0.1 µmol L^-1, respectively. Soluble reactive phosphorus (SRP) was determined by the ammonium molybdate, ascorbic acid and potassium antimony tartrate staining method (Koroleff, 1983), with a limit of detection of 0.1 µmol L^-1. Concentration of dissolved inorganic nitrogen (DIN) was calculated as the sum NH₄⁺, NO₂^- and NO₃^- concentrations.

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

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

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