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

Abstract

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 the support of fisheries. We analysed 26 years of pigment data (14,824 publicly available samples between 32°S and the Antarctic coast) from over 50 oceanographic voyages (1996 – 2022), using the phytoclass software, representing the largest analysis of Southern Ocean phytoplankton pigments to date. The analysis showed that the Antarctic Polar Front (APF) separates phytoplankton communities between diatom and haptophyte domination, confirming a biological analogue to the recognised Biogeochemical Divide in regional carbon dioxide flux and global export production. Furthermore¸ community composition was remarkably similar south of the APF despite significant spatial variation in biomass. These insights into the geospatial distribution of phytoplankton community composition will improve modelling and projection of future change in ecosystems, carbon and nutrient dynamics in the Southern Ocean.