Distinct phytoplankton groups as key drivers modulating summer CO2 uptake in the northern Antarctic Peninsula

Data collection in the northern Antarctic Peninsula (NAP) remains scarce and is generally limited to low-resolution surveys, which makes linkages between phytoplankton composition and biogeochemical processes extremely valuable. In this study, extensive phytoplankton (pigments and microscopy) and physico-chemical data were collected at the sea surface during January 2019 along the NAP. The influence of phytoplankton community composition on sea-air CO 2 partial pressure difference (Δ p CO 2 ) and CO 2 flux (FCO 2 ) was assessed across 6 key subregions: Gerlache and Bransfield Straits, coastal and oceanic Weddell Sea, Drake Passage, and Bellingshausen Sea shelf. The Gerlache Strait and coastal Weddell regions acted as CO 2 sinks (Δ p CO 2 = -14 µatm and FCO 2 = -0.37 mmol m -2 d -1 ; -54 µatm and -3.02 mmol m -2 d -1 , respectively), while the oceanic Weddell area was in equilibrium (Δ p CO 2 ~1 µatm and FCO 2 = 0.72 mmol m -2 d -1 ). Conversely, Bransfield Strait, Drake Passage, and Bellingshausen regions acted as CO 2 sources, i.e. Δ p CO 2 (FCO 2 ) = 7 µatm (1.16 mmol m -2 d -1 ), 39 µatm (4.72 mmol m -2 d -1 ), and 8 µatm (1.13 mmol m -2 d -1 ), respectively. Across all subregions, CO 2 drawdown was driven by non-thermal processes, predominantly biological, with phytoplankton playing a significant role. Subregions dominated by diatoms showed stronger CO 2 drawdown than those dominated by cryptophytes. Among diatoms, regions dominated by centric rather than pennate diatoms showed higher CO 2 uptake, suggesting differences in CO 2 uptake between diatom types, although separating their effects is challenging. These results highlight the importance of considering phytoplankton community composition when evaluating sea-air CO 2 exchanges in the Southern Ocean.