The South China Sea (SCS) frequently encounters mesoscale eddies due to the influence of the Kuroshio intrusion and monsoonal patterns. However, the impact of these eddies on methane (CH4) biogeochemical cycling in the SCS remains poorly understood. Dissolved methane and dimethylsulfoniopropionate (DMSP) samples were collected from the eddy pair (cyclonic-anticyclonic eddy pair) at stations ME1−ME7, located near the western side of Hainan Island, on September 27, 2020. Additionally, methane concentration and oxidation rates were assessed within anticyclonic eddy stations between September 13 and 23, 2022. The distribution of SLA were obtained from the Copernicus Marine Environment Monitoring Service at 0.25° resolution. Seawater samples collected by 12 L Niskin bottles equipped with SBE 911 plus a conductivity-temperature-depth (CTD) probe. The CH4 sample was collected in 120 mL brown bottles and then 50 μL saturated HgCl2 was added to inhibit biological activity. Samples were sealed with aluminum caps containing Teflon septa and temporarily stored in the dark at 4°C.Dissolved methane was measured using a cryogenic purge-and-trap setup connected to an gas chromatograph (GC-8890, Agilent, USA) with a flame ionization detector (FID) according to the detailed methods given in (Zhang et al., 2004, doi:10.1029/2004JC002268). The 3H radiotracer approach was used to measure the aerobic MOx rate (Bussmann et al. (2015, doi:10.5194/bg-14-4985-2017); Mao et al. (2022, doi:10.1038/s41467-022-35082-y); Liu et al. (2024, doi:10.1029/2023JC020280). Seawater samples (triplicate samples and one killed control) were filled into 20 ml Hungute tubes with butyl rubber stoppers without headspace. 50 μl ³H-CH⁴ gas was injected using airtight needles into each sample by displacing the same volume of seawater. Controls were killed with a final concentration of 3.7% formaldehyde prior to tracer additions. All samples were incubated in the dark at in situ sampled temperature for 48 hours. The concentration of CH⁴ added to the samples was less than ~2 nM. Once the incubation was finished, ~100 μl subsample was collected in 2 ml glass vials filled with scintillation liquid, which was used to measure total radioactivity (DPM-³H-H²O and DPM-³H-CH⁴) using a scintillation counter (Tri-Carb 3110TR, USA). The remaining sample was transferred to a 50 ml falcon tube containing 2 ml 37% formaldehyde and purged with N² for 45 min to remove the unused ³H-CH⁴. Then 2 ml samples added 5 ml scintillation liquid was quantified for the measurement of DPM-³H²O.
