Size‐Fractionated Photosynthetic Parameters and Their Controlling Factors in the Cosmonaut Sea, Southern Ocean

Abstract The size structure of phytoplankton is a key determinant of oceanic energy transfer and biogeochemical cycling. Accurate estimation of primary productivity relies on photosynthetic parameters specific to different phytoplankton size classes. However, the broad size distribution of Phaeocystis antarctica poses challenges for pigment‐based diagnostic approaches in resolving size‐fractionated photosynthetic characteristics in the Southern Ocean. To overcome this limitation, we applied direct size‐fractionated filtration to assess photosynthesis‐irradiance (P–E) curves across the Cosmonaut Sea during the austral summer of 2022. Our results revealed that smaller phytoplankton exhibited greater photosynthetic efficiency especially under low‐light and nutrient‐depleted conditions. Primary productivity showed substantial spatial variation with the highest rates observed near the continental shelf. Incorporating phytoplankton size structure alongside environmental parameters markedly improved the estimation accuracy of photosynthetic physiological traits particularly for Nano + Pico phytoplankton. To this end, we developed and validated a size‐fractionated model for photosynthetic parameters using in situ measurements. The model demonstrated high predictive performance with coefficients of determination ( r 2 ) of 0.96 for P B m (RMSD = 0.200 mg C [mg Chl a ] −1 hr −1 ) and 0.97 for α (RMSD = 0.001 mg C [mg Chl a ] −1 hr −1 [μmol quanta m −2 s −1 ] −1 ). The model results underscore the importance of incorporating size‐specific physiological traits into photosynthetic parameter models to enhance the predictive accuracy of primary production in polar marine ecosystems.