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Sedentary Seabed Creatures Create a Carbon Sink

New research finds that, as sea ice disappears, filter feeders are flourishing on the sea floor, and in the process are locking up carbon in their tissues as they grow.
An icefish swims over bryozoans on the Weddell Sea. (Photo: Thomas Lundalv)

An icefish swims over bryozoans on the Weddell Sea. (Photo: Thomas Lundalv)

Sea ice in the West Antarctic—one of the world's fastest warming regions—is disappearing at a record rate. But that retreating ice appears to actually be a boon for the tiny filter feeders anchored to the seafloor of the freezing cold waters. David Barnes, an ecologist with the British Antarctic Survey, has been studying the seabed animals known as Bryozoans for more than 20 years, and he reports today in Current Biology that colonies of coral-like creatures are ballooning, and may be turning the seabed there into a carbon sink.

"What started as a study of growth in a fairly obscure group of animals at the bottom of the sea at the end of the world had lead to the discovery of a major feedback on climate change," Barnes says. "Who would have thought?"

When Bryozoans die, they become buried by sediment, and the carbon in their tissues becomes indefinitely locked up in the seabed.

To measure the growth of Bryozoans over time, Barnes and his colleagues dropped cameras down to the seafloor to capture images of the blooming colonies, and compared them to ice loss data from above water over the same 20-year period. The researchers collected Bryozoan specimens from continental shelves at South Georgia and the South Orkney Islands, as well as the Bellingshausen, Amundsen, and Weddell seas.

"Bryozoans are very suited for this sort of work," Barnes says. The immobile creatures are easy to capture, abundant, and as they grow they form annual rings—"like tree rings, but even better," he says, "because they are on the outside, so they can even be seen on photographs." Barnes has found that these creatures are one stop on a one-way flow of carbon from the atmosphere to the seabed: Carbon dioxide in the air dissolves in the oceans, where it is picked up by microscopic algae for photosynthesis. Bryozoans gobble up the algae, using the carbon to form their skeletons and other tissues. When the animals die, they become buried by sediment, and the carbon in their tissues becomes indefinitely locked up—or sequestered—in the seabed. (Carbon does most of its environmental damage when it's in the atmosphere as the greenhouse gas carbon dioxide, so a carbon sink on the seafloor is a good thing for the environment.)

Over the last two decades the growth of Bryozoans has nearly doubled, the study finds, with the colonies sucking down an increased 75,000 tons of carbon. But the finding is not a happy ending for the sad saga that is climate change. The effect needs further research to be placed into the larger context of how global warming is altering the carbon cycle. Eventually, though, a better understanding of the feedbacks involved will lead to better climate models.

"These animals are sequestering about the same amount of carbon to the seabed as approximately 50,000 hectares of tropical rainforest," Barnes says. And that number is likely to climb even higher. "Global sea ice area is vast," he notes, "and the likelihood is that it will continue to retreat and whilst it does these animals may keep increasing the amount of carbon they take up."

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