Arctic Seabeds A Growing Carbon Sink

They may look delicate, but bryozoans like this are removing carbon from the oceans as sea ice melts. Credit: Lebendkulturen.de

The seafloor around Antarctica is becoming biologically richer as a result of sea ice reductions, and in the process mopping up some of the carbon dioxide that is causing the loss of that same sea ice. How important this will be to placing a check on global warming remains uncertain, but it's a rare piece of good news in a generally bleak picture.

Any system as complex as the global climate is bound to have some unexpected responses to disturbance. So far, most of the ones we have discovered are bad, either in their implications for human or animal life, or by releasing more greenhouse gasses, accelerating the rate of change. Nevertheless, there are exceptions, including last month's announcement that acidic soils in the Arctic absorb methane at higher temperatures.

A new example of a beneficial response comes in the form of a study, published in Current Biology, of the continental shelf around Antarctica. The Arctic Ocean, has suffered a devastating plunge in sea ice extent, with this year the fourth lowest on record. The situation in the Antarctic has been mixed however, with sea ice disappearing from some areas as it is expanding in others.

Measuring the ice extent is easy in an age of satellites, but the life forms on the ocean floor are a different matter. However, David Barnes of the British Antarctic Survey has combed these depths and found an explosion of bryozoan growth in those areas where the sea-ice has retreated above the continental shelf.

Bryozoans are aquatic invertebrates, many of which live on the Arctic sea floor. Barnes found their rate of growth has almost doubled over 20 years in areas of the West Antarctic. Small as they are, the bryozoans Barnes investigated consume a lot of carbon, removing more than 200,000 tonnes per year.

It is already known that arctic forests and high latitude phytoplankton blooms have been expanding in response to global warming and drawing down more carbon, but Barnes said in a statement that the bivalves, brachiopods and bryozoans of the Arctic and Antarctic “could be more important than both."

Extrapolating to the species he did not study, Barnes estimates is that if other seafloor dwelling lifeforms are benefiting as much as the bryozoans they will collectively remove 2.9 million tonnes of carbon a year, which he notes equals 50,000 hectares of tropical rainforest. With much greater sea-ice loss in the Arctic, the figure could be even larger there. Moreover, Barnes thinks that where forests can release their carbon when they burn or rot, much of this carbon may be buried on the sea floor, removed for geological periods of time.

"The forests you can see are important with respect to the carbon cycle and climate change, but two-thirds of our planet is ocean, and below it the life you can't see is also very important in climate responses," Barnes said.

Helpful as these collective marine creatures are, it should be remembered cement production and the burning of fossil fuels release 10 billion tonnes of carbon a year.

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