We may be able to increase the amount of carbon locked away in the ocean depths by making zooplankton produce faecal pellets that sink faster – and new research suggests we could do it by supplementing the tiny animals’ diets with clay. “They eat about anything and everything,” says at Dartmouth College in New Hampshire.
Photosynthetic plankton in the ocean take up tens of billions of tonnes of CO2 from the atmosphere every year. But most of that carbon returns to the air when the plankton and the organisms that eat them die and decompose.
The exception is the carbon contained within the “marine snow” of carcasses and excrement that sinks into the deep ocean as a key component of what is known as the biological carbon pump. The carbon “pumped” into deep waters may not cycle back to the atmosphere for centuries or longer.
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In efforts to slow climate change, researchers have – often controversially – explored ways to increase the amount of CO2 taken up by the biological carbon pump by fertilising plankton blooms. But they have not explored ways to increase the amount of that carbon that makes it to deeper waters. “Just producing a bloom isn’t enough,” says at the Bigelow Laboratory for Ocean Sciences in Maine.
Desai and her colleagues, including Fields, tested how feeding clay to two different species of small crustaceans that feed on photosynthetic plankton – Acartia tonsa and Calanus finmarchicus – would affect the composition of their faecal pellets, and thus the rate at which they sank. Faster sinking pellets may be more likely to reach the deep ocean without breaking apart or being eaten and recycled by other organisms, says Desai.
In tanks, some of the crustaceans were fed only planktonic algae. Others were fed on algae and varying amounts of clay. The researchers then used a high-speed camera to carefully measure the sinking rates of the resulting faecal pellets.
They found adding the clay didn’t affect the amount of algae eaten by either species of crustacean or visibly influence their health. “They’re eating normally. They’re pooping normally,” says Desai. However, as she explained at the American Geophysical Union’s Ocean Sciences Meeting on 22 February, .
When fed clay, faecal pellets from A. tonsa sank twice as fast as when they were fed on algae alone. Faecal pellets from C. finmarchicus sank more than three times as fast in some cases. “They are producing these faecal pellets that drop like a brick,” says at Dartmouth College, the project’s lead investigator. He first had the idea to use clay after studying the close link between carbon and clay in rocks that made up the seafloor hundreds of millions of years ago.
at Vassar College in New York, who attended the presentation, says the study is exciting because it presents an unexplored way to enhance a physical, rather than biogeochemical, aspect of the biological carbon pump. “It definitely inspires some hope that there are still ideas out there to solve the climate crisis,” he says, although he points out the research is in a very early stage.
Sharma says they next plan to test the clay approach in a larger aquarium in Germany in March. If the results hold up there, they hope to test the clay in the ocean off the coast of California. “We can hack the biological pump,” he says.
