Zooplankton diel vertical migration drives a major, variable and underestimated carbon sink in the Southern Ocean

Abstract The biological carbon pump, a key process for oceanic CO 2 sequestration, is primarily attributed to the gravitational sinking of particles. However, vertically migrating zooplankton actively transport carbon to depth through respiration and excretion during diel vertical migration (DVM). Here, we quantify the magnitude and variability of DVM‐mediated active carbon flux (AF_DVM) by mesozooplankton in the Southern Ocean and assess its contribution relative to gravitational export. We estimated AF_DVM using paired day–night, vertically resolved net sampling across ecologically contrasting regions of the Southern Ocean during austral summers between 2018 and 2023. Migrating biomass was combined with temperature‐dependent metabolic rates to calculate respiratory and excretory carbon fluxes. Regional estimates were compared with modeled particulate organic carbon fluxes and extrapolated to the circumpolar scale using a long‐term zooplankton biomass synthesis (1932–2020). Diel vertical migration–mediated active fluxes exhibited strong regional variability, with relatively consistent export in copepod‐dominated communities of the Amundsen Sea and substantially higher variability in the Cosmonaut Sea. Across the 27 stations, AF_DVM ranged from 1.1% to 136.6% of modeled gravitational particulate organic carbon fluxes, with a geometric mean of 27.2% and a median of 31.8%. Circumpolar extrapolation indicates that summer AF_DVM reaches approximately 38.9 Mt C (95% uncertainty interval: 27–60 Mt C), representing ~ 13% of total summer carbon export at 200 m depth. Our results demonstrate that DVM constitutes a substantial and previously under‐quantified component of the Southern Ocean carbon cycle. Incorporating zooplankton‐mediated active transport alongside sinking fluxes is therefore essential for a more complete assessment of carbon export in polar oceans.