Seasonally migrating zooplankton strongly enhance Southern Ocean carbon sequestration

High‐latitude zooplankton can sequester millions of tons of carbon due to their seasonal migration from the surface ocean to depth, and their respiration and mortality during overwintering. This seasonal vertical migration pump (SVMP) efficiently removes carbon but not limiting nutrients such as iron from the surface layers. However, this process is not included in Earth System Models and whole Southern Ocean estimates are still lacking. Here, we compile large datasets of Southern Ocean zooplankton biomass and physiology to estimate that the SVMP transports 65 Mt carbon annually to sequestration‐achieving depths of > 500 m. Mesozooplankton are the main agents (80%), followed by krill (14%), and salps (6%), with respiration and mortality at depth contributing a similar share. This SVMP adds greatly to existing modeled or measured estimates of Southern Ocean carbon sequestration, equating to 38–56% of particulate organic carbon flux at 500 m and 78–103% of the flux at 1000 m. Given their large biomass but projected change under polar warming, understanding how zooplankton transport carbon and nutrients will underpin improved model projections of ocean carbon storage in a warmer world.