Each Winter, These Tiny Ocean Travelers Bury Millions of Tons of Carbon

Every year, billions of microscopic ocean drifters—copepods, krill, and other zooplankton—perform a breathtaking migration in the Southern Ocean, diving hundreds of meters into the deep.

As they descend to hibernate for the winter, they carry carbon from the surface with them and, through respiration and mortality, lock it away beneath 500 meters. This newly quantified “seasonal migrant pump” moves around 65 million tonnes of carbon annually, a hidden natural process that plays a massive role in regulating Earth’s climate.

Zooplankton’s Hidden Role in Carbon Storage

A major international study has uncovered that some of the ocean’s smallest inhabitants, zooplankton such as copepods, krill, and salps, play a much bigger role in storing carbon in the Southern Ocean than previously understood.

Published in Limnology and Oceanography, the research provides the first detailed measurement of how these tiny creatures help trap carbon through their seasonal vertical migrations. Scientists have long known that the Southern Ocean is one of the planet’s most important regions for locking away carbon, but until now, it was widely believed that most of this process depended on the sinking of organic detritus created by larger zooplankton like krill.

The “Seasonal Migrant Pump” Explained

The study highlights a less familiar but highly significant process known as the “seasonal migrant pump.” Each year, countless zooplankton travel from surface waters to depths greater than 500 meters to survive the winter months. During this deep hibernation period, they release carbon through respiration and mortality, effectively transferring it into the deep ocean where it can remain stored for centuries or longer.

Traditionally, scientists thought carbon reached these depths primarily when zooplankton consumed phytoplankton near the surface and their waste, known as particulate organic carbon (POC), passively sank. The new findings reveal that the seasonal migrant pump adds another powerful pathway. By descending to deep waters each autumn, zooplankton directly inject an estimated 65 million tonnes of carbon into the ocean’s depths every year, making them far more influential in global carbon storage than previously recognized.

Building a Century of Zooplankton Data

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, said: “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), added: “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.

Reference: “Seasonally migrating zooplankton strongly enhance Southern Ocean carbon sequestration” by Guang Yang, Angus Atkinson, Evgeny A. Pakhomov, Katrin Schmidt, Weilei Wang, Jennifer J. Freer and Geraint A. Tarling, 23 June 2025, Limnology and Oceanography.
DOI: 10.1002/lno.70120

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.

Never miss a breakthrough: Join the SciTechDaily newsletter.