CO2 fluxes in the South African coastal region

Abstract

The air-sea exchange of CO2, its distribution and trends in the South African continental shelf over 8 years (2005–2012) has been studied using data from 28 journeys of a volunteer observing ship (VOS). Sea water properties, strongly controlled by the oceanographic dynamic, showed a complex distribution pattern between Cape Town and Durban. Three different upwelling cells were identified, which dominated the regional variability, together with the presence of eddies, filaments and the core of the Agulhas Current. From west to east the variation in CO2 fugacity (fCO2) followed the temperature increase, resulting in oversaturated (> 600 μatm) or undersaturated (~ 215 μatm) waters. After removing seasonality, seawater fCO2 decreased while sea surface temperature (SST) and salinity did not show a clear trend. From 18°25′E to 20°00′E (the west region), surface water was undersaturated and presented an averaged flux of − 4.27 ± 0.07 mol m− 2 year− 1. The south region (20°00′E-25°30′E) reached a value of − 4.39 ± 0.08 mol m− 2 year− 1, while between 25°30′E and 28°30′E (the south-east region) the averaged flux was − 5.03 ± 0.11 mol m− 2 year− 1. Between 28°30′E and 31°15′E (the east region) the flux was the lowest (− 1.65 ± 0.04 mol m− 2 year− 1). The full studied area acted as a sink with a mean value of − 3.83 ± 0.04 mol m− 2 year− 1, decreasing at an annual rate of 0.13 ± 0.16 mol m− 2 year− 1 from 2005 to 2012 related to the highest rate of increase in fCO2,sw over that in fCO2,atm. The estimated carbon uptake by the coastal region with a surface area of 99 × 109 m2 was − 4.6 ± 0.1 TgC year− 1.