Seasonal Variability of Phytoplankton Vertical Distribution in a Contrasted South Pacific Ocean From BioGeoChemical‐Argo Profiling Floats

Abstract The seasonal variability of phytoplankton vertical distribution is investigated in the South Pacific where observations are scarce and scattered. We used 13 BioGeoChemical‐Argo floats deployed across diverse oceanic environments. The seasonal latitudinal displacement of the Tasman front induces transitions from mesotrophic to oligotrophic conditions. This shift results in Chlorophyll‐a concentration vertical distribution changing from bloom types to Subsurface Chlorophyll Maxima (SCM) types, with intermediate hybrid types between these extremes. Such hybrid profiles frequently occur in the equatorial Pacific, highlighting a large‐scale pattern rather than local island mass effect. In oligotrophic regions, seasonal variations of light availability and stratification dynamics below the mixed layer likely relate SCM to an increase in carbon biomass or photoacclimation. A biomass increase is frequently observed, contrary to previous studies, suggesting that subsurface phytoplankton biomass may have been largely underestimated. This calls for further observations of the water column in these remote undersampled open ocean areas.