Sea-air CO2 exchanges, pCO2 drivers and phytoplankton communities in the southwestern South Atlantic Ocean during spring

Hydrographic properties and carbon dioxide partial pressure (pCO₂) were measured through underway survey of surface waters during spring 2014, mainly along the Surface Haline Front in the continental shelf-break domain in the southwestern South Atlantic Ocean margin. Additionally, discrete seawater surface samples were collected along the ship track to identify the phytoplankton community and measure seawater chemical properties. This study aims to identify the drivers of the marine CO₂‑carbonate chemistry and the role played by the phytoplankton composition on changes in the surface marine carbonate properties and the sea-air CO₂ exchanges in two biogeochemical provinces (i.e., South Brazil Bight – SBB, and Southern Brazilian Shelf – SBS) governed by the dynamics of the Brazil Current system in the South Atlantic Ocean. The water masses identified on the surface of the region were Tropical Water (mostly present at offshore regions), Subtropical Shelf Water (mostly present over the continental shelf and slope), and Plata Plume Water (present in the south coastal domain of the SBS). On average, the study area behaved as a weak net CO₂ outgassing zone of 1.2 ± 2.3 mmol m⁻² d⁻¹ during the spring, despite some subregions behaving as CO₂ ingassing zones. The CO₂ uptake verified in the SBB was related with mesoscale activity bringing cold waters in the region while CO₂ uptake in the continental shelf domain of SBS was associated with the presence of cooler and fresher Plata Plume Water. Changes in total alkalinity and dissolved inorganic carbon at surface were mainly governed by CaCO₃ production in SBB and seawater dilution in SBS, although other processes may also have influenced on their spatial variability. The dominant phytoplankton groups were haptophytes (31 %), Trichodesmium (21 %), and picocyanobateria (28 %), corresponding to Synechococcus (17 %) and Prochlorococcus (11 %). The dominance of the diatom group was associated with a decrease in sea surface pCO₂ (mainly at coastal zones at southern areas), although the sea-air CO₂ exchanges were regulated by cooling process due the presence of Plata Plume Water in that region. Changes in surface pH were related to high concentration of Trichodesmium slicks at offshore zones with the highest microalgae concentration, leading to pH drops of up to 0.4. Trichodesmium slicks likely allowed the development of haptophytes in offshore oligotrophic waters due to its role on N₂ fixation. An increase of ∼20 % in the dominance of haptophytes contribution was verified in that situation, which was likely in a post-bloom development stage, since an increased dissolved inorganic carbon content was observed, associated with a prevalence of net respiration processes.