Spatiotemporal Variability in the Vertical Distribution of Phytoplankton Biomass in the Ice‐Free Southern Ocean: A Bioregionalization Approach

Abstract The Southern Ocean plays a major role in the global oceanic sink of carbon dioxide (CO 2 ). However, substantial uncertainties remain regarding how phytoplankton production influences this carbon sink. Observations of the spatiotemporal variability in vertical phytoplankton biomass distribution are therefore essential to advance our understanding of the Southern Ocean CO 2 sink. This study analyzes a 25‐year (1998–2022) database of spatially gridded, weekly averaged, vertical profiles of chlorophyll‐ a concentration and particulate backscattering coefficients to characterize seasonal to inter‐annual dynamics in phytoplankton biomass. A two‐step Principal Component Analysis workflow was used to extract the dominant modes of vertical variability in the profiles and to characterize their seasonal dynamics. The resulting seasonal descriptors were then used as input to a Gaussian Mixture Model to delineate phytoplankton‐based bioregions across the Southern Ocean. The bioregionalization was strongly shaped by iron limitation and the position of the Subantarctic Front. Each year, approximately 43% of the Southern Ocean exhibited expected annual cycles of vertical phytoplankton biomass, while the remaining regions were characterized by interannual variability or undefined phenological patterns. At high latitudes in summer, the regional detection of recurring subsurface phytoplankton biomass maxima might be constrained by stratification dynamics. In subtropical regions, interannual variability in spring phytoplankton biomass may be indirectly influenced by the Southern Annular Mode through its modulation of wintertime vertical mixing. This bioregionalization provides a spatial framework for further investigating biologically driven carbon fluxes in the Southern Ocean.