South African Journal of Science

Abstract

Is the southern Benguela a significant regional sink of CO 2 ?This study was undertaken to characterise the seasonal cycle of air-sea fluxes of carbon dioxide (CO 2 ) in the southern Benguela upwelling system off the South African west coast.Samples were collected from six monthly cross-shelf cruises in the St. Helena Bay region during 2010.CO 2 fluxes were calculated from pCO 2 derived from total alkalinity and dissolved inorganic carbon and scatterometer-based winds.Notwithstanding that it is one of the most biologically productive eastern boundary upwelling systems in the global ocean, the southern Benguela was found to be a very small net annual CO 2 sink of -1.4 ± 0.6 mol C/m 2 per year (1.7 Mt C/year).Regional primary productivity was offset by nearly equal rates of sediment and sub-thermocline remineralisation flux of CO 2 , which is recirculated to surface waters by upwelling.The juxtaposition of the strong, narrow near-shore out-gassing region and the larger, weaker offshore sink resulted in the shelf area being a weak CO 2 sink in all seasons but autumn (-5.8, 1.4 and -3.4 mmol C/m 2 per day for summer, autumn and winter, respectively). MethodsWater samples were collected for six months (January, April, May, June, August and September) during 2010 along the St. Helena Bay Monitoring Line (SHBML).A conductivity, temperature and depth rosette with General Oceanics-Niskin bottles was lowered to various depths at each of the 12 stations (Figure 1).Samples were analysed for dissolved inorganic carbon (DIC) and total alkalinity (TA) using a Versatile Instrument for the Determination of Titration Alkalinity and Total Carbon (VINDTA 3C) by Marianda (Kiel, Germany).14 The precision achieved for DIC and TA was ±3.7 µmol/kg and 2.5 µmol/kg, respectively.Partial pressure of CO 2 (pCO 2 ) was calculated from DIC and TA using dissociation constants by Dickson and Millero 15 .A propagation of error analysis of pCO 2 resulted in