The structure of the phytoplankton community is strongly influenced by environmental variables linked with variations in sea–air CO₂ net fluxes (FCO₂). However, compared to physical parameters, the relationship between phytoplankton and CO₂ dynamics has been largely unexplored. The complex interplay between CO₂ 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 FCO₂ 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 CO₂ partial pressure were evaluated, and phytoplankton groups were linked to CO₂ exchanges. The results highlight the importance of biology on the modulation of FCO₂ 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 CO₂ 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 CO₂-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 CO₂-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 CO₂-uptake rates. This link between the shifts in phytoplankton community structure and CO₂-uptake rates is a potential target to shed light on long-term CO₂-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 CO₂ uptake, especially considering the warming ocean conditions associated with a shift toward smaller phytoplankton cells.

_{浮游植物群落的结构受到与海气 CO 2}净通量 (FCO ₂ )变化相关的环境变量的强烈影响。_{然而，与物理参数相比，浮游植物与CO 2}动力学之间的关系在很大程度上尚未得到探索。在南大西洋西南部（20°S-50°S）研究了沿海海洋吸收CO ₂与不同浮游植物群的优势之间的复杂相互作用，主要是在春季。我们通过对 FCO _{2的研究区域进行天气表征来应对这一挑战}和浮游植物色素成分。因此，我们通过通过高效液相色谱 (HPLC) 获得的色素组成信息来辨别不同组中的浮游植物生物量，并使用 CHEMTAX 方法进一步确定浮游植物组。评估了生物学和温度对海面CO ₂分压的影响，并将浮游植物群与CO ₂交换联系起来。结果突出了生物学对研究区域中FCO _{2调节的重要性。因此，我们根据浮游植物和 CO 2}的分布模式将南大西洋西南部大陆架划分为两个不同的生物地球化学区域，由一个过渡带（~ 35°S）划分行为。在 35°S 以北，较高的海表温度和盐度，加上较低的浮游植物生物量，与通常非常小的蓝细菌和 CO ₂释气行为有关。在过渡区（35°S–40°S），盐度和温度的变化促进了主要浮游植物群的转变，从而将海洋表面行为从 CO ₂释气区转变为吸气区。再往南，在 40°S 和 50°S 之间，由硅藻产生的较高的浮游植物生物量与较低的海面温度和盐度值相关，与较高的 CO ₂吸收率呈正相关。浮游植物群落结构的变化与 CO _{2之间的这种联系}吸收率是揭示南大西洋西南部长期 CO _{2通量调节的潜在目标。}因此，这里的主要发现可能与预测未来气候驱动的海洋 CO ₂吸收变化的潜在后果相关，特别是考虑到与向更小的浮游植物细胞转变相关的变暖海洋条件。