Carbonate dynamics in a tropical coastal system in the South China Sea featuring upwelling, river plumes and submarine groundwater discharge

This study examined carbonate dynamics in the northwestern South China Sea(NWSCS),an area jointly influenced by upwelling,river plumes and submarine groundwater discharge.Data were obtained from two cruises conducted in summer 2009 and 2012.In 2009,a high salinity-low temperature water mass occurred nearshore off northeastern Hainan Island,indicative of upwelling,commonly referred to as HNEU.A river plume fueled primarily by local rivers and characterized by low salinity and high temperature was observed in the NWSCS off the mainland roughly along the 30 m isobath.In 2012,coastal upwelling off northeastern Hainan Island was not detectable at the surface,but was observed at a different location off eastern Hainan Island(HEU).River plume waters in 2012 were patchily distributed,with a low salinity zone further westerly than that in 2009 and another on the mid-shelf of the NWSCS sourced from the Pearl River which reached out~250 km from the mouth of the Pearl River Estuary.In 2009,elevated dissolved inorganic carbon(DIC)and total alkalinity(TA)occurred in the coastal plume,where submarine groundwater discharge contributed DIC and TA additions of 38.9±20.5 and 42.5±22.3μmol kg^(−1),respectively,with a DIC/TA ratio of~0.92,which made a minor contribution to the variation of seawater partial pressure of CO_(2)(pCO_(2)),pH and the aragonite saturation state index(Ω_(arag)).Additionally,high surface phytoplankton production consumed DIC of 10.0±10.4μmol kg^(−1) but did not significantly affect TA,which dominated pCO_(2) drawdown in the coastal plume water and increased the pH andΩ_(arag) at surface.Submarine groundwater discharge was also observed in the region influenced by upwelling,but to a lesser degree than that impacted by coastal plume.Lower pH andΩ_(arag) and higher pCO_(2) values than in offshore waters were observed downstream of the upwelling system,attributable largely to organic matter remineralization with a DIC addition of 23.8±8.4μmol kg^(−1).In 2012,submarine groundwater discharge was not detected but high phytoplankton production dominated carbonate dynamics in the coastal plume water with a net DIC consumption of 104.2μmol kg^(−1),which markedly drew down sea surface pCO_(2) and increased pH andΩ_(arag).In the Pearl River Plume,the solubility-driven CO_(2) sink exceeded biological CO_(2) uptake,resulting in an additional decrease of pH andΩ_(arag) and increase of seawater pCO_(2).Taken together,this study demonstrated complex spatial and year-to-year variability,and the controls of the carbonate system under the joint modulations of upwelling,river plumes and submarine groundwater discharge.A first order estimate that considered the rise of atmospheric CO_(2) and seawater temperature further suggested a high risk of ocean acidification in this coastal area by the end of this century,which could be amplified under the stresses of river plumes,submarine groundwater discharge and organic matter remineralization.