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

Abstract

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.