Phytoplankton strengthen CO₂ uptake in the South Atlantic Ocean

The influence of phytoplankton groups on carbon dynamics was investigated during six oceanographic spring cruises (three cruises were carried out in 2009, and another three were conducted in 2011) between the western (Brazil) and eastern (Africa) South Atlantic margins. A seventh cruise crossed the South Atlantic during the early winter of 2015. Sea surface temperature and salinity, oceanic and atmospheric partial pressure of CO₂ (pCO₂), chlorophyll a and other phytoplankton pigment data were gathered. Net CO₂ fluxes were calculated for each cruise, characterizing the ability of each region to take up atmospheric CO₂. We quantified phytoplankton chemotaxonomic groups using the HPLC/CHEMTAX approach. Thus, this study aimed to improve our understanding of the distribution of phytoplankton groups and their connection with the carbon biogeochemical cycle in the South Atlantic Ocean. Our results showed significant variations in both the zonal and meridional patterns of phytoplankton groups and the associated CO₂ uptake magnitudes. Diatoms and haptophytes dominated the coastal regions of Brazil and Africa, respectively, whereas the open ocean was dominated by haptophytes and the picoplanktonic cyanobacteria Prochlorococcus and Synechococcus. The CO₂ uptake capacity increased eastward from −7.1 mmol CO₂ m⁻² d^–1 on the Brazilian coast to −27.6 mmol CO₂ m⁻² d^–1 on the African coast. There was a significant negative relationship (p < 0.05) between the phytoplankton biomass and the difference in sea-air pCO₂ (ΔpCO₂), with increasing CO₂ uptake corresponding to increases in the biomasses of diatoms and haptophytes. Therefore, according to our analysis, haptophytes and diatoms were the main phytoplankton groups related to a high uptake of CO₂ along the South Atlantic Ocean regions covered in this study. Thus, we encourage further investigations on their traits and vulnerabilities to future environmental change scenarios.