Carbonate chemistry in the Northern South China Sea Shelf-sea in June 2010

The distributions of dissolved inorganic carbon (CT), total alkalinity (AT) and pH at 25 °C (pH25) were determined in the Northern South China Sea Shelf-sea (NoSoCS) in early June, 2010 during a low flow period. The distributions of the partial pressure of CO2 (pCO2), in situ pH, and temperature normalized pCO2 (NpCO2) were derived from the measured values. The distribution of AT is linearly related to salinity indicating that its distribution is controlled primarily by mixing between the surface water and the subsurface North Pacific Tropical Water. Aside from physical mixing, the distribution of CT is also influenced by its loss through biological uptake and CO2 evasion to the atmosphere. The net community production (NCP) rate in the NoSoCS is estimated to be 10±20 (with a range of 4–13) mmol C m−2 d−1. Within the NoSoCS, the NCP is elevated in the coastal upwelling area, where it is estimated to reach 30±17 mmol C m−2 d−1. In addition to temperature, upwelling and biological uptake also affect the distribution of the surface pCO2 and in situ pH.The waters in the NoSoCS are super-saturated with respect to aragonite at all depths as the saturation horizon is at 600–800 m in the open northern South China Sea. Nevertheless, the aragonite saturation state, ΩAr, in the surface water, which is mostly between 3.3 and 3.5, is already within the range that has been suggested as barely adequate to marginal for the growth of the tropical shallow-water corals. The ΩAr is linearly related to in situ pH. At the reported rate of pH decrease in the oceans as a result of ocean acidification, ΩAr might reach a value that is considered “extremely marginal” within several decades and the existence of this type of coral reef ecosystem in the NoSoCS may then be threatened.