Methane concentration and methane oxidation rate along the western Guangdong coast

This dataset includes hydrographic parameters (temperature, salinity) and methane (CH4) concentrations across the full water column, as well as aerobic methane oxidation (MOx) rates in surface seawater, collected along the western Guangdong coast. Seawater samples were obtained using 12 L Niskin bottles mounted on a SBE 911 plus CTD system. For CH4 analysis, samples were collected in 120 mL brown glass bottles following standard dissolved gas procedures, preserved with 50 μL saturated HgCl2, and later analyzed using a headspace equilibrium method coupled with gas chromatography (GC-FID, Agilent 8090). Briefly, a nitrogen headspace was created in the vial, and after shaking and equilibration, the headspace gas was injected into a GC equipped with an HP-PLOT/Q capillary column (30 m × 0.32 mm × 20 μm, oven at 90°C). For MOx rate measurements, surface seawater was gently transferred into 1 L polycarbonate bottles without bubble formation, stored at 4°C in the dark, and processed using the 3H radiotracer method (Bussmann et al., 2015). Approximately 20 mL of sample was incubated with 3H–CH4 (>105 DPM) for 48 h in the dark at in situ temperature, with formaldehyde-killed controls. The produced 3H–H2O was separated by N2 purging and quantified using a liquid scintillation counter (Tri-Carb 3110TR). This dataset provides key insights into methane distribution and microbial oxidation dynamics in a coastal ecosystem. Air‑sea CH4 fluxes were calculated from the concentration difference between surface seawater and the atmosphere using the gas transfer velocity of Wanninkhof (2014). Atmospheric CH4 concentrations were obtained from the NOAA dataset (monthly averages at Dongsha Island, Taiwan), and wind speed data were retrieved from the Global Ocean Monthly Mean Sea Surface Wind and Stress product (0.25° resolution, EU Copernicus Marine Service).Reference:Bussmann, I., Matousu, A., Osudar, R., & Mau, S. (2015). Assessment of the radio 3H-CH4 tracer technique to measure aerobic methane oxidation in the water column. Limnology and Oceanography: Methods, 13(6), 312-327. http://doi.org/https://doi.org/10.1002/lom3.10027Wanninkhof, R. (2014). Relationship between wind speed and gas exchange over the ocean revisited. Limnology and Oceanography: Methods, 12(6), 351-362. http://doi.org/https://doi.org/10.4319/lom.2014.12.351