Distribution of the surface partial pressure of CO2 in the southern Yellow Sea and its controls

Surface partial pressure of CO2 (pCO2), dissolved inorganic carbon (DIC), temperature, salinity and chlorophyll a (Chl a) at grid stations were measured in the southern Yellow Sea (SYS; 32–37°N to 120–125°E) during four cruises conducted in March 2005 (winter), April 2006 (spring), May 2005 (late spring), and July 2001 (summer). Factors influencing pCO2 spatial and seasonal variations are explored.Surface seawater pCO2 during winter was oversaturated with respect to the atmosphere in the entire study area (380–606 μatm), primarily due to the complete mixing of the water column in winter which brought CO2-enriched bottom water to the surface. However, during spring, surface pCO2 in the central SYS was undersaturated relative to the atmosphere with a low range between 274 and 408 μatm. The net CO2 sink in the central SYS was mainly due to the consumption of CO2 by the strong phytoplankton activity and to the weak water stratification, whereas surface pCO2 in the nearshore area was oversaturated for the atmosphere owing to vertical mixing and terrestrial inputs. During summer, surface pCO2 varied between 125 and 599 μatm over the entire sampling area. In the Changjiang (Yangtze River) Diluted Water (CDW) area, surface pCO2 was undersaturated because of the nutrient inputs via the Changjiang, triggering strong phytoplankton activity, whereas surface pCO2 was oversaturated in other areas. We conclude that the nearshore area behaves as a source of atmospheric CO2 during the entire investigated periods owing to vertical mixing and terrestrial inputs as well as upwelling, whereas the central region generally shifts from a source of CO2 in March to a sink in the remaining time of the investigation.