Methane distributions and sea-to-air fluxes in the Pearl River Estuary and the northern South China sea

We measured dissolved methane (CH₄) at various depths in the Pearl River Estuary and the shelf, slope, and basin of the northern South China Sea (SCS) during cruises from May to July 2014, October 2014, June 2015, and March 2017. High CH₄ concentrations (9.4–430.3 nM) were observed in the Pearl River Estuary. There was a negative correlation between the CH₄ concentration and the salinity in the surface waters of the Pearl River Estuary, indicating rapid CH₄ loss via air-sea exchange and aerobic CH₄ oxidation. The CH₄ concentrations in the surface waters ranged from 1.7 to 5.4 nM in the shelf and slope region and from 2.2 to 3.0 nM in the basin of the northern SCS. The injection of a Pearl River plume rich in CH₄ and driven by eddies had a profound impact on the CH₄ distribution in the shelf and slope region in June 2015. The vertical profiles of CH₄ showed spatial variations that can be attributed to complex interactions among physical, chemical, and biological processes. The subsurface CH₄ maxima occurred ubiquitously in the euphotic zones, probably due to in situ CH₄ production in anoxic microniches and aerobic CH₄ formation from potential substrate precursors such as dimethylsulfoniopropionate (DMSP). The gross CH₄ production rate in the mixed layer was estimated at 0.11 nM d^-1 in the summer and -0.19 nM d^-1 in the autumn. The surface waters of the Pearl River Estuary were oversaturated in CH₄ with respect to atmospheric equilibrium having a saturation level of 5066±5908% (mean±SD) in early autumn and 6166±3611% in early summer. The CH₄ fluxes (estimated using the W14 relationship) were estimated at 314.3±464.9 μmol m^-2 d^-1 in early autumn and 184.2±187.5 μmol m^-2 d^-1 in early summer. In the shelf and slope areas, the CH₄ saturation was 178±37% in early autumn, 181±60% in early summer, and 129±14% in winter. The sea-to-air CH₄ fluxes were 8.0±4.3 μmol m^-2 d^-1 in autumn, 4.1±5.2 μmol m^-2 d^-1 in summer, and 1.1±0.8 μmol m^-2 d^-1 in winter. The CH₄ saturation ratio was less variable in the deep basin, ranging from 108% to 184% with an average of 140±16%. The CH₄ flux in the deep basin area was 1.9±1.2 μmol m^-2 d^-1. Overall, we estimate the annual CH₄ emission from the SCS as 9.9 × 10⁹ mol yr^-1. Hence the SCS is a net source of atmospheric CH₄.