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

Weddell Sea, Southern Ocean. 66North | Unsplash

A groundbreaking study published in Limnology and Oceanography reveals 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.

Key Findings:

• 65 Million Tonnes of Carbon Stored Annually: The seasonal, vertical migration of zooplankton transports roughly 65 million tonnes of carbon to depths below 500 meters.
• Copepods Dominate the ‘Seasonal Migrant Pump’: Mesozooplankton (mainly small crustaceans called copepods) account for 80% of this carbon flux, while krill and salps contribute 14% and 6%, respectively.
• Climate Implications: 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 CO₂ 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.

Dr Guang Yang, first author and Marine Ecologist from Institute of Oceanology, Chinese Academy of Sciences, said

“Our work shows that zooplankton are unsung heroes of carbon sequestration. Their seasonal migrations create a massive, previously unquantified carbon flux—one that models must now incorporate”.

Prof. Angus Atkinson MBE, co-author and Senior Marine Ecologist at Plymouth Marine Laboratory, added:

“This study is the first to estimate the total magnitude of this carbon storage mechanism. It shows the value of large data compilations to unlock new insights and to get an overview of the relative importance of carbon storage mechanisms”.

Dr Katrin Schmidt, co-author and Marine Ecologist at the University of Plymouth, continued:

“The study shows the ‘seasonal migrant pump’ as an important pathway of natural carbon sequestration in polar regions. Protecting these migrants and their habitats will help to mitigate climate change”.

Dr Jen Freer, co-author and Ecological Modeller at the British Antarctic Survey (BAS), concluded:

“Krill are famous for their role in the Antarctic food web, but we find that copepods significantly dominate carbon storage overwinter. This has big implications as the ocean warms and their habitats may shift”.

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.

This international study was a collaboration among scientists from China, UK, and Canada, and leverages a century’s worth of data on zooplankton biomass, distribution, respiration and mortality across the Southern Ocean.

The UK component of the study was carried as part of the  PICCOLO and BIOPOLE science programmes, which are both looking at how ecosystem processes in the polar regions sequester atmospheric carbon into the deep ocean.