Tiny crustaceans called copepods are the most abundant animals in the oceans, and if a new study is correct, they are more important than previously realized at helping those waters remove carbon dioxide from the atmosphere, where it drives global warming. Certain species in the North Atlantic, for example, appear to haul massive amounts of carbon to the deep ocean in their fat, which they burn off as they hibernate far below.
The findings provide “a new take on the way that oceans move carbon,” says Daniel Mayor, a zooplankton biologist with the National Oceanography Centre, Southampton in the United Kingdom. “I think this is really exciting.” If the estimates are confirmed, they could help biogeochemists better understand the natural cycle of carbon, but they are unlikely to have any direct bearing on human-caused climate change.
One of the major ways that oceans store carbon is through a process called the biological pump. The pump is primed by microscopic creatures called phytoplankton, which use energy from the sun and carbon dioxide to grow through photosynthesis. When phytoplankton die, they may sink to the deep ocean, below 1000 meters, where the carbon in their bodies can remain for centuries or millennia. Many other phytoplankton make the same journey indirectly; they are eaten near the surface by copepods, krill, and other zooplankton, whose feces sink toward the seabed. All told, the biological pump is thought to store roughly 1 gigaton of carbon per year in the deep ocean—a huge amount, but still only about 10% of the carbon emitted by burning fossil fuels each year. The pump seems to be particularly efficient in the North Atlantic, which accounts for perhaps as much as 25% of the carbon stored this way in deep water.
Thanks to their unusual life cycle, copepods are likely sending a significant amount of carbon to the deep ocean in still another way, says Sigrún Jónasdóttir, a zooplankton biologist at the Technical University of Denmark, Charlottenlund. The most abundant copepods in the North Atlantic, Calanus finmarchicus and two related species, spend only a few months each year near the surface, while phytoplankton bloom. They pack on fat, building their stores up to 50% of their body weight. Then they swim down 1000 meters to spend the rest of the year hibernating in the cold, dark waters, which slows their metabolism and helps them save energy until the next phytoplankton bloom. (The bulked-up copepods don’t bob up to the surface because most of their body fat consists of wax esters, which compresses and becomes denser at depth, making the copepods neutrally buoyant.) Before the phytoplankton bloom again, the copepods migrate back to the surface to reproduce.
Jónasdóttir knew that Calanus continue to breathe while hibernating and burn off most of their body fat. This means they leave behind carbon dioxide in the deep ocean—a phenomenon Jónasdóttir has dubbed “the lipid pump.” Curious about the total amount of carbon sunk this way, she and several colleagues combined surveys of Calanus abundance and distribution from oceanographic cruises in the North Atlantic, and measurements of copepod biology, such as their fat content and respiration rates during hibernation. In the region of the North Atlantic that they studied, copepods may be leaving below about 1 million to 3 million tons of carbon a year, they estimate in a preprint posted online this month. That’s about the same amount as contained in all the dead phytoplankton and feces that sink each year in the same place. “It is a relatively conservative estimate,” Jónasdóttir writes in an email. “We did not want to hype a story that is pretty amazing!”
The numbers for Calanus are still quite rough, mostly because of uncertainty about respiration rates and the length of hibernation. And it’s not clear how much carbon might be taken down by other kinds of copepods, and in other oceans, so the overall size of the lipid pump is hazy. “Getting the numbers is difficult at best,” says Jefferson Turner, a zooplankton biologist at the University of Massachusetts, Dartmouth, who nonetheless calls the results “interesting and potentially important.”
Still, don’t count on these tiny marine creatures to curtail global warming. Jónasdóttir does not expect that copepods will begin taking more carbon from the atmosphere to the deep ocean. “I cannot see a potential mitigation of the CO2 buildup.” And what will climate change mean for the copepods? Although it's hard to predict, Mayor says that warmer seas should generally raise the copepods’ respiration rate. In turn, that could potentially reduce the rate at which copepods produce lipid and haul its carbon to the depths, Mayor says: "This would reduce the strength of the ‘lipid pump’—bad.”