Twenty-six years of phytoplankton pigments reveal a circumpolar Class Divide around the Southern Ocean

Regional taxonomic variation of phytoplankton communities in the Southern Ocean remains largely uncharacterised despite the distinct trophic and biogeochemical roles of different taxa in anthropogenic carbon uptake, biogeochemical processes, and as the primary source of energy for marine ecosystems. Here we analysed 26 years of pigment data (14,824 samples between 32°S and the Antarctic coast) from over 50 voyages (1996 – 2022), using the phytoclass software. The analysis confirms that the Antarctic Polar Front (APF) is a circumpolar phytoplankton class boundary, separating haptophyte dominated communities to the north from diatom domination of chlorophyll a in the south, and thereby a biological analogue corresponding to the Biogeochemical Divide. Furthermore, community composition was remarkably similar in different zones south of the APF despite substantial spatial variation in biomass. This circumpolar characterisation of the geospatial distribution of phytoplankton community composition will contribute to improved modelling and projection of future change in ecosystems and carbon in the Southern Ocean.