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Tiny ocean migrants play a massive role in Southern Ocean carbon storage, study finds

A new study has revealed that small but mighty zooplankton—including copepods, krill, and salps—are key players in the Southern Ocean's ability to absorb and store carbon.

Led by an international team of researchers, the study quantifies for the first time how these tiny creatures collectively enhance carbon sequestration through their seasonal, vertical migrations.

The Southern Ocean is a key region for carbon storage. Traditional thinking is that the carbon storage in the Southern Ocean is dominated by gravitational sinking of detritus produced by large zooplankton grazers, such as krill.

This new research concerns another more recently described process called the "seasonal migrant pump." This process sees zooplankton migrate each year from surface waters to depths below 500m, storing carbon via their respiration and mortality during this deep overwintering phase.

The team first built a big database of zooplankton collected in thousands of net hauls from around the Southern Ocean, dating from the 1920s to the present day. From these they quantified the extent of the zooplankton's annual descent to overwinter at great depths, where they respire CO₂—directly and efficiently injecting carbon into the deep ocean.

Their key findings, in the journal Limnology and Oceanography, reveal that 65 million tons of carbon are stored annually: The seasonal, vertical migration of zooplankton transports roughly 65 million tons of carbon to depths below 500 meters. Copepods dominate the seasonal migrant pump, with mesozooplankton (mainly small crustaceans called copepods) accounting for 80% of this carbon flux, while krill and salps contribute 14% and 6%, respectively.

The Southern Ocean is a critical carbon sink, but current Earth system models overlook this zooplankton-driven process. As warming shifts species distributions (e.g., declining krill, increasing copepods, changing food sources), the carbon storage dynamics may change dramatically.

Why does the 'seasonal migrant pump' matter?

The Southern Ocean absorbs approximately 40% of all human-made COâ‚‚ taken up by oceans, yet the role of zooplankton has been underestimated.

Unlike sinking detritus, which removes both carbon and essential nutrients like iron, migrating zooplankton efficiently inject carbon into the deep ocean while recycling nutrients near the surface. This seasonal migrant pump could become even more important as marine ecosystems respond to climate change.

This research stresses the urgent need for updates to climate models to include zooplankton-driven carbon fluxes. It also highlights the necessity to manage and protect Southern Ocean ecosystems, where industrial fishing and warming threaten krill populations—a key species that supports both carbon export and Antarctica's unique biodiversity.