New study quantifies carbon captured by algae living in Antarctic sea ice

Antarctic sea ice plays a crucial role in the planet’s climate and ecosystem, yet our understanding of its contribution to the carbon cycle has been limited. 

A new study led by Laura Dalman (University of Tasmania) and co-authored by iC3 researcher Sébastien Moreau provides the most comprehensive estimate in over 30 years of how much carbon is captured by algae living in the sea ice. The results, based on few hundreds of ice core samples, show that these tiny organisms contribute significantly to the Southern Ocean’s carbon balance.

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Analysing data from 362 ice cores

Until now, estimates of how much organic carbon is produced in Antarctic sea ice have been based on a handful of measurements. 

The new study provides a far more robust estimate. Researchers compiled data from 362 ice cores collected over 30 years to provide a more accurate and representative picture of carbon production by sea ice algae. 

They estimate that these algae contribute 26.8–32.9 million tonnes of carbon per year, accounting for at least 15%–18% of total primary production in the Antarctic sea-ice zone. The new estimate replaces an outdated figure from the early 1990s, which was based on far fewer observations and may have overestimated the contribution of sea ice algae.

The updated data aligns more closely with recent computer models, improving confidence in predictions of how the Southern Ocean’s ecosystem will respond to climate change.

"I think that this study will be a stepping stone in our understanding of the role of sea ice in the Southern Ocean carbon cycle," said Sébastien. 

Ice algae from an ice core (Credit: Sébastien Moreau)

Carbon is distributed throughout the ice

To estimate carbon production, the researchers compiled and analyzed organic carbon data from ice cores collected across different locations and seasons. They focused on net community production, which accounts for the balance of carbon produced by the algae and consumed by heterotrophic organisms.

The large dataset allowed them to track seasonal changes, showing that carbon production peaks in spring and early summer before declining as the ice melts. 

The data also revealed that carbon is distributed throughout the ice, not just in surface layers. This suggests that algae thrive across different depths of the ice, making them a more significant part of the ecosystem than previously thought.

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Implications for marine food webs and climate models

The study provides a new benchmark for understanding Antarctic sea ice productivity. It will help refine climate models and guide future studies on how marine ecosystems respond to environmental change. 

 Sea ice algae are a crucial part of the Southern Ocean food web. They provide an early-season food source for krill, which in turn support whales, seals, and seabirds. Understanding their role helps scientists predict how changes in sea ice will impact marine life. 

The study also has implications for climate science. By capturing carbon through photosynthesis, sea ice algae contribute to the ocean’s ability to absorb carbon dioxide from the atmosphere. As climate change alters the extent and duration of sea ice cover, this process could change, affecting the global carbon cycle.

 Sébastien emphasises the significance of the study’s findings:

 "Such an estimate had not been produced in the literature in over three decades and was only based on a handful of sea ice cores. This new estimate is based on several hundred sea ice cores from all over the Southern Ocean and is thus much more powerful and accurate."

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The study “Observation‐Based Estimate of Net Community Production in Antarctic Sea Ice” is available open access in Geophysical Research Letters.

Sébastien Moreau is a researcher with the iC3 Polar Research Hub and the Norwegian Polar Institute specializing in phytoplankton and the biogeochemical cycle of carbon in polar oceans. If you want to learn more about his research, have a look here.