Linking Surface Phytoplankton Dynamics to Small‐Particle Fluxes in the Mesopelagic Zone: Insights From High Latitude Bioregions Using BGC‐Argo Floats

Abstract Understanding factors controlling the biological carbon pump (BCP) at the regional scale is of major interest for better characterizing carbon sequestration into the deep ocean and, therefore, the ocean's role in climate regulation. This study focuses on high‐latitude marine regions, which are responsible for the majority of marine CO 2 absorption. Using data from Biogeochemical‐Argo floats, a bioregionalization method was performed on 335 annual time series of chlorophyll a concentration and particulate backscattering coefficient, variables from which particulate organic carbon (POC) could be estimated. This analysis highlighted six regimes characterized by distinct seasonality in productivity, export, and transfer of small POC (<100 μm). Both hemispheres exhibited regimes with strong summer blooms and others with deep chlorophyll maxima. Across these regimes, variations in phytoplankton phenology and particle assemblages drove three distinct systems of BCP strength and efficiency for small particles. Despite these differences, processes such as gravitational sinking, the mixed layer pump, or particle fragmentation facilitated the export of small particles down to ∼1,000 m across all regions. This resulted in an average annual contribution of ∼10% of small particles to total organic carbon fluxes at depth, highlighting the role of small particles in long‐term carbon sequestration. These findings emphasize the need for future investigations into processes driving small‐particle carbon export and transfer in the mesopelagic zone at annual and seasonal scales.