Tiny Sea Creatures Play Major Role in Fighting Climate Change, Study Finds
An obscure, minuscule sea creature—often sold as fish food—has been quietly helping protect the planet from climate change by undertaking an extraordinary annual migration, according to new research.
These “unsung heroes”, known as zooplankton, gorge themselves on food during the spring, storing fat before descending hundreds of metres into the depths of the Southern Ocean, where they slowly burn off their reserves.
In doing so, they effectively lock away carbon in the deep ocean—equivalent to the yearly emissions of around 55 million petrol-powered cars—preventing it from entering the atmosphere and contributing to global warming, researchers say.
This carbon sequestration is far greater than previously estimated. Yet, just as scientists begin to understand the crucial role these creatures play, zooplankton are increasingly under threat.
For years, scientists have studied the migration patterns of these animals in Antarctic waters and their impact on the global climate system.
Lead author Dr Guang Yang of the Chinese Academy of Sciences described the findings as “remarkable”, saying they prompt a reassessment of how much carbon the Southern Ocean stores.
“The animals are an unsung hero because they have such a cool way of life,” says co-author Dr Jennifer Freer from British Antarctic Survey.
Despite their enormous ecological importance, zooplankton receive little attention compared to more iconic Antarctic species like penguins and whales. To many, they are known only as aquarium feed.
Among the most studied are copepods—a type of zooplankton related to crabs and lobsters, measuring just 1–10 mm in length. They spend most of their lives dormant at depths between 500 metres and 2 kilometres below the ocean surface.
Under a microscope, copepods reveal stores of fat that look like long sausages inside their bodies, with fat droplets also visible in their heads, explains Professor Daniel Mayor, who photographed them in Antarctica.
Without these tiny creatures, our planet’s atmosphere would be significantly warmer.
The oceans have absorbed about 90% of the excess heat generated by burning fossil fuels. Of that, the Southern Ocean accounts for approximately 40%—a large portion of which is thanks to zooplankton.
Millions of pounds are being invested globally to better understand how these creatures store carbon.
Scientists had long known that zooplankton help sequester carbon through a daily cycle, whereby their carbon-rich waste sinks to the deep ocean. However, their seasonal migrations in the Southern Ocean had not been fully quantified—until now.
The recent study focused on copepods, krill, and salps. These creatures feed on phytoplankton at the ocean’s surface—microscopic plants that absorb carbon dioxide through photosynthesis. This carbon is converted into fat, which the zooplankton store and then transport deep into the ocean.
“Their fat is like a battery pack. When they spend the winter deep in the ocean, they just sit and slowly burn off this fat or carbon,” explains Prof Daniel Mayor at University of Exeter, who was not part of the study.
“This releases carbon dioxide. Because of the way the oceans work, if you put carbon really deep down, it takes decades or even centuries for that CO2 to come out and contribute to atmospheric warming,” he says.
This process, called the seasonal vertical migration pump, moves an estimated 65 million tonnes of carbon per year to depths of at least 500 metres.
Of that total, copepods were found to be the largest contributors, followed by krill and salps. That’s roughly equivalent to the emissions from 55 million diesel cars annually, according to a greenhouse gas emissions calculator by the US EPA.
The study used data dating back to the 1920s to calculate how much carbon has been stored by this process.
But research is still ongoing. Earlier this year, Dr Amy Freer and Prof Mayor spent two months aboard the RRS Sir David Attenborough research vessel near South Georgia and the South Orkney Islands.
Using large nets, they collected zooplankton samples to study their life cycles and carbon storage mechanisms more closely.
“We worked in complete darkness under red light so we didn’t disturb them,” says Dr Freer.
“Others worked in rooms kept at 3-4C. You wear a lot of protection to stay there for hours at a time looking down the microscope,” she adds.
However, their future may be in jeopardy. Warming waters and increasing commercial krill fishing pose significant risks to zooplankton populations.
“Climate change, disturbance to ocean layers and extreme weather are all threats,” explains co-author Prof Angus Atkinson from Plymouth Marine Laboratory.
A reduction in zooplankton could, in turn, limit the ocean’s ability to store carbon, researchers warn.
In 2020, nearly half a million tonnes of krill were harvested—legally—according to the UN. Although permitted under international law, the practice has come under fire from environmental groups and was recently highlighted in David Attenborough’s Ocean documentary series.
Scientists say these new findings should be incorporated into global climate models, to more accurately forecast future warming.
“If this biological pump didn’t exist, atmospheric CO2 levels would be roughly twice those as they are at the moment. So the oceans are doing a pretty good job of mopping up CO2 and getting rid of it,” explains Prof Atkinson.
The study is published in the journal Limnology and Oceanography.
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