The Diatom Disconnect: Retention near surface waters limits carbon transfer to the deep ocean

The Southern Ocean, a region highly vulnerable to climate change, plays a critical role in regulating global nutrient cycles and atmospheric CO₂ via the biological carbon pump. Diatoms, a group of photosynthetically active plankton with dense opal skeletons, are central to this process, as their exoskeletons are thought to enhance the transfer of particulate organic carbon to depth, making them main vectors of carbon storage. However, conflicting observations obscure the mechanistic link between diatoms, opal, and particulate organic carbon fluxes, especially in the twilight zone where the greatest flux losses occur.

Here we present direct springtime flux measurements from different sectors of the subpolar Southern Ocean, demonstrating that across large areas of the subpolar twilight zone, carbon is efficiently transferred to depth: however, not by diatoms. Instead, opal is retained near the surface ocean, indicating that processes such as diatom buoyancy regulation and grazer repackaging can negate the ballast effects of diatoms’ skeletons. Using image data, we further reveal species-specific differences in diatom flux dynamics, highlighting the complexity of their role in the carbon cycle.

Our findings challenge the assumption that diatom-rich surface waters are necessarily associated with effective carbon export and transport in the Southern Ocean. They suggest that shifts in phytoplankton community composition driven by climate change may have a smaller impact on biological carbon storage than current models predict.