Does the carbon cycle in the Southern Ocean work differently than we thought?
Diatoms, a key player in the Southern Ocean food web, may contribute less to the biological carbon pump than previously thought.
In a new study led by the UK’s National Oceanography Centre (NOC), an international research team has made a discovery that challenges our fundamental understanding of the biological carbon pump in the Southern Ocean.
For years, it was believed that diatoms, which often dominate the community of plant plankton in the Southern Ocean, played a crucial role in transporting carbon dioxide into the deep sea, where it is stored for a very long time and removed from the atmosphere.
Because of their relatively dense shells made of silicon dioxide, diatoms were thought to naturally sink, making them a key mechanism for transporting carbon to the deep ocean.
However, the research team, which also included scientists from France, Finland, Spain, and Norway, has discovered that diatom shells tend to remain near the ocean surface, while carbon sinks to the deep ocean through other processes.
“The surprising discovery that diatoms’ silica skeletons stay near the surface while carbon makes it down to the deep ocean forces us to rethink the ecological processes in what we call the biological carbon pump,” says Dr. Sari Giering, a researcher at NOC, in an institute’s press release.
The biological carbon pump
This model describes various biological processes from the uptake of carbon dioxide by microalgae to the transport of CO2 into the deep sea via sinking organic material.
Carbon dioxide from the atmosphere is absorbed by the ocean and bound as biomass by tiny, single-celled algae called phytoplankton during photosynthesis in the sunlit surface waters (down to a depth of about 100 meters). These microscopic plants, which form the foundation of the food web, remove billions of tons of carbon from the atmosphere annually.
Phytoplankton serves as a food source for animal plankton, or zooplankton, including krill, which plays a crucial role in the Southern Ocean’s food web. Numerous species of fish, seabirds, penguins, seals, and whales rely directly or indirectly on krill and other plankton organisms for their survival.
As carbon moves through the food web, a portion is again converted into biomass, another part is respired and released back into the seawater as CO2, while the rest sinks to the seabed in the form of excrements. These sinking particles are commonly referred to as “marine snow”.
Dead algae and other marine organisms also sink to depth, where they are either decomposed by bacteria or eaten by deep-sea organisms on their way to the sea floor.
The portion of the carbon that finds its way to the sea floor is stored in the sediment and remains there for millions of years, ultimately reducing the concentration of CO2 in the atmosphere.
The progression of climate change largely depends on how much biomass sinks below the ocean’s surface layer, which is mixed by wind and waves. The carbon contained in this biomass remains stored in the intermediate and deep waters for decades to centuries.
Models and studies show that the biological carbon pump transports around 10 billion tons of carbon into the deep sea every year. Without these processes, significantly more CO₂ would remain in the atmosphere, with concentrations reaching nearly double the current level of about 420 ppm (parts per million).
The biological carbon pump is therefore a crucial component of the global carbon cycle and plays a key role in regulating the climate. Without it, the greenhouse effect would intensify, resulting in more severe impacts of climate change.
The researchers collected the data for the study during two expeditions in the Southern Ocean, focusing on the barely explored twilight zone between 100 and 1,000 meters deep, where they observed fewer diatoms than expected.
Carbon storage little dependent on diatom productivity
According to the authors, the findings, published in Nature Geoscience in November 2024, suggest that climate-driven changes in the composition of the phytoplankton community have less impact on the efficiency of the biological carbon pump in the Southern Ocean than previously assumed.
“The Southern Ocean is vulnerable to ocean warming, which may alter the availability of nutrients and reduce diatom numbers in future,” said Jack Williams, post-graduate researcher at the University of Southampton and lead author of the study, in the press release. “But our results suggest these changes may not impact the strength of Southern Ocean carbon storage as much as previously thought.”
The processes actually responsible for carbon transport into the depths remain unclear and will need to be explored in future studies.
“Understanding these processes and how they govern carbon uptake in this hugely important part of the ocean is crucial for accurately predicting how the oceans may store carbon in the future,” Williams said.
Julia Hager, Polar Journal AG
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