Antarctic phytoplankton communities restructure under shifting sea-ice regimes

Phytoplankton are critical to the Antarctic marine food web and associated biological carbon pump, yet long-term shifts in their community composition are poorly understood. Here, using a machine learning framework and combining pigment samples and environmental samples from austral summertime 1997–2023, we show declines in diatoms and increases in haptophytes and cryptophytes across much of Antarctica’s continental shelf. These trends—which are linked to sea ice increases—reversed after 2016, with a rebound in diatoms and a large increase in cryptophytes, coinciding with the loss of sea ice. Significant changes (P < 0.05) across the 25-year dataset include diatom chlorophyll a (chl-a) declines of 0.32 mg chl-a m⁻³ (~33% of the climatology) and increases for haptophytes and cryptophytes of 0.08 and 0.23 mg chl-a m⁻³, respectively. The long-term shifts in phytoplankton assemblages could reduce the dominance of the krill-centric food web and diminish the biologically mediated export of carbon to depth, with implications for the global-ocean carbon sink.