biostudies-literatureUnknown9(48)Flynn RFAcross the Southern Ocean, large (≥20 μm) diatoms are generally assumed to be the primary vector for carbon export, although this assumption derives mainly from summertime observations. Here, we investigated carbon production and export potential during the Atlantic Southern Ocean's spring bloom from size-fractionated measurements of net primary production (NPP), nitrogen (nitrate, ammonium, urea) and iron (labile inorganic iron, organically complexed iron) uptake, and a high-resolution characterization of phytoplankton community composition. The nanoplankton-sized (2.7 to 20 μm) diatom, <i>Chaetoceros</i> spp., dominated the biomass, NPP, and nitrate uptake across the basin (40°S to 56°S), which we attribute to their low iron requirement, rapid response to increased light, and ability to escape grazing when aggregated into chains. We estimate that the spring <i>Chaetoceros</i> bloom accounted for >25% of annual export production across the Atlantic Southern Ocean, a finding consistent with recent observations from other regions highlighting the central role of the phytoplankton "middle class" in carbon export.Science advanceseadi3059https://www.ebi.ac.uk/biostudies/studies/S-EPMC10691778biostudies-literatureNanoplankton: The dominant vector for carbon export across the Atlantic Southern Ocean in spring.PMC10691778Roychoudhury ANHaraguchi LMutseka Lunga PBurger JMStirnimann LMcQuaid JFawcett SEFlynn RFSamanta SfalseNanoplankton: The dominant vector for carbon export across the Atlantic Southern Ocean in spring.Across the Southern Ocean, large (≥20 μm) diatoms are generally assumed to be the primary vector for carbon export, although this assumption derives mainly from summertime observations. Here, we investigated carbon production and export potential during the Atlantic Southern Ocean's spring bloom from size-fractionated measurements of net primary production (NPP), nitrogen (nitrate, ammonium, urea) and iron (labile inorganic iron, organically complexed iron) uptake, and a high-resolution characterization of phytoplankton community composition. The nanoplankton-sized (2.7 to 20 μm) diatom, <i>Chaetoceros</i> spp., dominated the biomass, NPP, and nitrate uptake across the basin (40°S to 56°S), which we attribute to their low iron requirement, rapid response to increased light, and ability to escape grazing when aggregated into chains. We estimate that the spring <i>Chaetoceros</i> bloom accounted for >25% of annual export production across the Atlantic Southern Ocean, a finding consistent with recent observations from other regions highlighting the central role of the phytoplankton "middle class" in carbon export.2023-01-01T00:00:00Z2023 Dec2026-05-28T23:53:31.369Z2025-04-04T14:18:46.601ZS-EPMC106917783803936310.1126/sciadv.adi3059