Sampling was conducted during two cruises aboard the R/V Dongfanghong 3 under the NSFC Ship Time Sharing Project: the first from 21 August to 4 September 2021, and the second from 21 August to 27 September 2023. Seawater was collected using 12-L Niskin bottles mounted on a Seabird SBE 911plus CTD rosette (USA). For dissolved methane (CH4) analysis, water was collected in 120 mL brown glass bottles. Each bottle was overflowed with at least two volumes of sample to eliminate headspace, preserved with saturated HgCl2 to halt microbial activity, sealed with butyl rubber stoppers and aluminum crimp caps, and stored at 4 °C in the dark until analysis. Samples for bacterial production (BP) and methane oxidation (MOx) rate measurements were also stored at 4 °C in the dark prior to analysis.Accurate estimation of the air-sea flux of methane (CH4) is hindered by dynamic oceanic processes, particularly mesoscale eddies. The South China Sea (SCS), a region of intense mesoscale activity, provides an ideal setting to investigate the impact of these eddies on CH4 release. Data were collected during two research cruises in the northern SCS. The first cruise aboard the R/V Dongfanghong 3 conducted station sampling from 21 August to 4 September 2021. The second cruise, from 20 to 23 September 2023, sampled a transect across a cyclonic eddy during its decay phase (stations MS01–MS11). Seawater samples were collected using a CTD-rosette system. Dissolved CH4 concentrations were determined via purge-and-trap gas chromatography with flame ionization detection. Methane oxidation (MOx) rates were measured using 3H-CH4 tracer incubations, and bacterial production (BP) was quantified via 3H-leucine incorporation.

This dataset has been carefully reviewed and curated in accordance with PANGAEA's exceptionally high quality standards. However, due to discontinued communication from the authors' side, it did not receive the usual formal approval by the authors and therefore lacks final scientific validation.Analytical methods details:1. Dissolved CH4 concentration was determined using a purge-and-trap system coupled to a gas chromatograph equipped with a flame ionization detector (GC-FID, Agilent 7890B, USA).2. MOx rates and rate constants (k) were measured via 3H-CH4 radiotracer incubation. MOx rates were calculated as MOx = k × [CH4], where the first-order rate constant k was derived from the conversion of 3H-CH4 to 3H-H2O (Bussmann et al., 2021; Li et al., 2025). Briefly, ~20 mL of seawater was collected in headspace-free Hungate tubes and injected with ~50 μL of 3H-CH4 (>10^5 DPM; American Radiolabeled Chemical, Inc.) while displacing an equal volume of water. Killed controls received 3.7% formaldehyde. Samples were incubated in the dark at in situ temperature for 48 h. Total radioactivity was measured in a 100-μL subsample mixed with scintillation cocktail. The remaining sample was transferred to 50 mL tubes containing 2 mL of 37% formaldehyde, sparged with N2 for 45 min to remove residual 3H-CH4, and then 2-mL aliquots were mixed with 4 mL of cocktail for 3H-H2O quantification on a Tri-Carb 3110TR liquid scintillation analyzer (USA).3. BP was quantified via 3H-leucine incorporation following Kirchman (2001). Seawater samples were dispensed into 2 mL sterile centrifuge tubes. For each sample, 25 μL of 3H-leucine (0.012 nmol/L; specific activity >10^6 DPM; American Radiolabeled Chemical, Inc.) was added to three replicate tubes and one killed control. Killed controls received 100 μL of 100% trichloroacetic acid (TCA) prior to incubation to halt bacterial activity. All tubes were incubated in the dark at in situ temperature for 6 h. After incubation, 100 μL of 100% TCA was added to each replicate tube to terminate incorporation. Samples were then centrifuged at 10300 rpm for 15 min, followed by sequential washing with 5% TCA and 80% ethanol to remove unincorporated tracer. The resulting pellet was dissolved in 2 mL of scintillation cocktail, and radioactivity was measured using a liquid scintillation counter.