Southern Ocean regime shift: divergent trends in surface phytoplankton community composition using a diagnostic pigment framework

Phytoplankton communities in the Southern Ocean (SO) play a central role in regulating global climate and ecosystems through the biological carbon pump (BCP), which is influenced by ocean warming and sea-ice variability. Here, we combine an expanded circumpolar surface pigment dataset with satellite ocean-colour observations to develop a SO-diagnostic pigment framework that resolves phytoplankton community composition for austral spring and summer between 1998 and 2024. We compare multi-year mean composition for 1998-2014 and 2015-2024 periods separated by a ~ 7% step decrease in September sea-ice extent. Between these two periods, the mean diatom contribution to total chlorophyll increased by up to ~ 10% during spring in sea-ice-influenced regions but decreased by up to ~ 3% over the Antarctic shelf during summer. These findings suggest a seasonally and spatially variable reorganisation of phytoplankton community composition that may alter bloom dynamics, carbon-export efficiency, and the functioning of the SO BCP under continued climate-driven sea-ice variability. The study reveals that Southern Ocean phytoplankton communities reorganised after the 2014 Antarctic sea-ice decline, with contrasting spring and summer responses that could reshape marine food webs and carbon storage.