The southwestern South Atlantic continental shelf biogeochemical divide

The structure of the phytoplankton community is strongly influenced by environmental variables linked with variations in sea–air CO 2 net fluxes (FCO 2 ). However, compared to physical parameters, the relationship between phytoplankton and CO 2 dynamics has been largely unexplored. The complex interplay between CO 2 uptake by the coastal ocean and the dominance of different phytoplankton groups was investigated in the southwestern South Atlantic Ocean (20°S–50°S), mostly during spring. We addressed this challenge by synoptically characterizing the study region for both FCO 2 and phytoplankton pigment composition. Thus, we discern the phytoplankton biomass in different groups by pigment composition information obtained through high-performance liquid chromatography (HPLC), with further determination of phytoplankton groups using the CHEMTAX approach. The effects of biology and temperature on sea surface CO 2 partial pressure were evaluated, and phytoplankton groups were linked to CO 2 exchanges. The results highlight the importance of biology on the modulation of FCO 2 in the study region. Hence, we delimited the southwestern South Atlantic continental shelf into two distinct biogeochemical regions divided by a transitional zone (~ 35°S) according to the distribution patterns of both phytoplankton and CO 2 behavior. North of 35°S, higher sea surface temperature and salinity, combined with lower phytoplankton biomass, were associated with a domination of generally very small cyanobacteria and CO 2 -outgassing behavior. In the transitional zone (35°S–40°S), changes in both salinity and temperature promoted a shift in dominant phytoplankton groups and, consequently, changed the ocean surface behavior from a CO 2 -outgassing zone to an ingassing zone. Farther south, between 40°S and 50°S, the higher phytoplankton biomass produced by diatoms, associated with lower values of both sea surface temperature and salinity, was positively related to stronger CO 2 -uptake rates. This link between the shifts in phytoplankton community structure and CO 2 -uptake rates is a potential target to shed light on long-term CO 2 -flux modulation in the southwestern South Atlantic Ocean. Thus, the main findings here can be relevant for predicting the potential consequences of future climate-driven changes in ocean CO 2 uptake, especially considering the warming ocean conditions associated with a shift toward smaller phytoplankton cells.