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The ocean plays a key role in carbon sequestration, particularly the Southern Ocean, which absorbs 40% of all human-made CO2 taken up by oceans. A groundbreaking study by an international team of researchers, published in Limnology and Oceanography, has revealed that zooplankton, including copepods, krill, and salps, are key players in the Southern Ocean’s ability to absorb and store carbon.

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 study shows that a winter process known as the ‘seasonal migrant pump’ also leads to substantial deep carbon storage.

The zooplankton migrate downwards in autumn to overwinter below 500m, where their respiration and death directly inject around 65 million tonnes of carbon annually into the deep ocean. Unlike sinking detritus, this process recycles nutrients.

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.

 “Our work shows that zooplankton are unsung heroes of carbon sequestration,” said Dr Guang Yang, first author and Marine Ecologist from the Institute of Oceanology, Chinese Academy of Sciences.

He further highlighted that their seasonal migrations create a massive, previously unquantified carbon flux that Earth system models must now incorporate.  

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.

According to Prof. Angus Atkinson MBE, co-author and Senior Marine Ecologist at Plymouth Marine Laboratory, 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 protecting zooplanktons and their habitats will help to mitigate climate change, as “the study shows the ‘seasonal migrant pump’ as an important pathway of natural carbon sequestration in polar regions.”

The study reveals that in the fight against climate change, more attention should be given to zooplankton. The research further emphasizes the need to update Earth climate models to include zooplankton-driven carbon fluxes and 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.