Changing Biogeochemistry in the South China Sea

Abstract

The tropical/subtropical South China Sea (SCS) is the largest marginal sea in the world. Like other warm bodies of water, its sea surface temperature (SST) is rising, albeit more slowly (0.012 °C/yr between 1998 and 2016) than that of cold-water regions at high latitudes. The chlorophyll concentration increased at 0.0012 μg/L/yr during that period, and the Secchi disk depth (SDD) increased by 0.035 m/yr. The changes of SST, chlorophyll concentration and SDD, the factors governing changes in ocean biogeochemistry, in the SCS exhibit high temporal-spatial variability, and these parameters varied in opposite directions during the periods 1998–2008 and 2008–2016. The first period witnessed declining SST and SDD and increasing chlorophyll concentration, referring to enhancing primary productivity. The second period witnessed increasing SST and SDD but falling chlorophyll concentration, referring to declining primary productivity. These changes and increasing anthropogenic activities on land may be related to changing biogeochemistry such as decreasing dissolved oxygen concentration in coastal regions. In the SCS basin, however, particulate organic carbon and nitrogen seem to be on the rise.