Variability in pH, fCO2, oxygen and flux of CO2 in the surface water along a transect in the Atlantic sector of the Southern Ocean

Underway sampling and measurements of pH, fCO2, oxygen and Chlorophyll a (Chl a) were performed in the surface waters from Cape Town (South Africa) to Queen Maud Land (Antarctica) in the Atlantic sector of the Southern Ocean during the austral summer 1997/1998. From direct measurements of these parameters and from calculated fCO2 the oceanic carbon dioxide system was studied and related to hydrological and biological parameters. fCO2 was in general undersaturated relative to the atmosphere and showed a large variability with values ranging from 313 to 377 μatm with a mean value of 346±13 μatm. The undersaturation was more pronounced in areas associated with fronts where high Chl a and high pH in situ values were observed. Using shipboard wind speed data, estimates of the CO2 flux were made along the transect and during three mesoscale surveys on the northward return transect in the area of the Spring Ice Edge (SIE), the Winter Ice Edge (WIE) and in the Antarctic Polar Front (APF). The undersaturation observed during the transect caused the ocean to act as a sink for CO2 with a mean sea–air flux for the entire transect of −3±5 mmol m−2 d−1 with a large variability between −20 mmol m−2 d−1 (oceanic uptake) to 1.3 mmol m−2 d−1 (oceanic source). The lowest fCO2 values (largest oceanic uptake of CO2) were found at the southern boundary of the APF at 53°S, which coincided with a supersaturation in oxygen and high pH values. Oxygen concentrations were measured from 50°S to 63°S and varied between 324 and 359 μmol kg−1 with a mean value of 347±9 μmol kg−1. In general only small deviations from equilibrium oxygen saturation were observed (mean value=99±2%). However, in the SIE oxygen was clearly undersaturated, probably an effect of upwelling of oxygen poor deep water which had not yet been compensated for by biological production. Three weeks later, the ice edge had retreated in the SIE region and the Chl a concentration had increased three-fold, suggesting the start of a phytoplankton bloom. This was also seen in the oxygen concentration which had increased and showed supersaturation. This coincided with an increased oceanic uptake of CO2 in the SIE during the mesoscale survey.