Phytoplankton communities in the Southern Ocean (SO) regulate global climate through the biological carbon pump, yet their contribution has weakened under ongoing warming and sea-ice loss. Here, we combined circumpolar pigment data with satellite ocean-colour observations (1998-2024) to resolve phytoplankton functional types and size structure.
We identified an abrupt sea-ice tipping point in September 2014 (~7% decline), marking a climate-driven regime shift toward earlier and more extensive spring retreat. Following this transition, the austral spring exhibited basin-wide declines in surface chlorophyll across the SO, with the strongest declines in the Pacific sector. Concurrently, microphytoplankton (diatoms) declined across shelf and sea ice-influenced regions but became confined to a narrower band near the maximum sea-ice extent, where localised increases persisted. In contrast, nano- and picophytoplankton expanded across stratified shelf and subantarctic regions, indicating a shift toward smaller phytoplankton during the spring bloom period.
By summer, these changes intensified, with chlorophyll strongly reduced along the Antarctic margin in both the Atlantic and the Pacific, while microphytoplankton remained suppressed across most regions. Nanophytoplankton became more dominant in sea ice-affected zones, and picophytoplankton expanded in the stratified Pacific open ocean, with only limited microphytoplankton recovery in the Indian sector.
This reorganisation reflects a shift toward smaller, recycling-dominated communities, reducing carbon export efficiency and weakening the SO biological carbon pump under continued warming.