The role of phytoplankton in the deep chlorophyll maximum CO2 dynamics along a zonal section (34.5°S) in South Atlantic Ocean.

Marine phytoplankton acts directly on the carbon biogeochemical cycle as key mediators of the biological carbon pump, and its efficiency is dependent on the composition of phytoplankton communities. Therefore, in the present study, we aimed to investigate the composition of phytoplankton communities in the deep chlorophyll maximum and the role of the different phytoplankton groups in CO2 dynamics and biogeochemical processes along the South Atlantic Ocean. Environmental and biological data were collected in the deep chlorophyll maximum in 59 stations along a zonal section (34.5°S) in the South Atlantic Ocean. Sampling stations were grouped into Brazil, South Atlantic Gyre and Africa provinces. We identified the chemotaxonomic phytoplankton groups using the HPLC-CHEMTAX approach and evaluated the thermal and non-thermal effects on CO2 partial pressure (pCO2) variability in the region. The relationship between biogeochemical variables and phytoplankton groups was investigated using a canonical correspondence analysis. Our results showed strong gradients in the distribution of biogeochemical parameters among the different provinces, with higher pCO2 and thermal effects in the coastal regions due to the influence of ocean circulation processes, and lower pCO2 and higher non-thermal effects in the open ocean due to the upwelling of deeper waters rich in dissolved inorganic carbon and nutrients associated to mesoscale eddies. Haptophytes and Prochlorococcus dominated the deep chlorophyll maximum in the study area, with the highest phytoplankton biomass being recorded in the Africa province. Haptophytes were associated with mixed nutrient-rich waters, and showed higher influence in pCO2 variation, whereas Prochlorococcus were associated with stratified oligotrophic waters. The results found in this study elucidate the variability in the role of the different phytoplankton groups on CO2 dynamics across the different biogeochemical provinces of the South Atlantic Ocean. Thus, further investigations on the spatial and temporal patterns of phytoplankton distribution are encouraged to better decouple the thermal and non-thermal effects driving pCO2 variations in subsurface waters in the region.