Methane and Dimethylsulfoniopropionate (DMSP) concentration in the South China Sea

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). For total dimethylsulfoniopropionate (DMSPt), 5 ml seawater were preserved with the addition of 5 μl of 50% sulfuric acid to oxidize dimethylsulfide (DMS) into nonvolatile compounds and inhibit biological activity. Seawater samples intended for dissolved DMSP (DMSPd) determination were pre-filtered by gravity using a 0.45 μm filter. Particulate DMSP (DMSPp) was derived by subtracting DMSPd from DMSPt. All DMSP samples were stored in darkness at 4℃ until analysis. Briefly, a 2 ml sample was mixed with 200 μl of 10 mol/potassium hydroxide (KOH) and incubated at room temperature for 6 h to ensure complete conversion of DMSP into DMS. Seawater DMS samples were analyzed immediately after collection using a modified purge-and-trap method on a gas chromatograph (GC-2014, Shimadzu, Japan) equipped with a flame photometric detector (FPD) (Yang et al., 2012, doi:10.1016/j.marchem.2012.05.003).