Krill "deposits" transport atmospheric carbon to the ocean depths
Krill may be small, but scientists at the British Antarctic Survey (BAS) say that these tiny crustaceans play an important role in the transportation of carbon within the oceans. Not only are they a key element of the ocean's food pyramid, but they also move through the depths in such a way that they could have a major impact on the world's climate. According to a recent study, the way in which krill move between different depths can accelerate how atmospheric carbon moves into the deep ocean.
An individual krill isn't much to look at. Under a magnifying glass, the adults grow up to 2 cm (0.8 in) long and resemble something like semi-transparent shrimp, though they also share anatomical characteristics with the lobster and freshwater crayfish. But it's when they get together that it's time to sit up and take notice.
Collectively, krill make up the single largest concentration of biomass in the oceans. At any one time, there are 400 million tonnes of them swimming about in every ocean of the world. Their place in the ecosystem is as the second tier in the food pyramid. The krill feed on the phytoplankton and zooplankton and as they digest the tiny plants and marine animals they convert them into food that larger animals can eat.
In fact, krill are eaten by many fish, seals, and penguins and they make up the main diet of baleen whales, including the blue whale – the largest animal ever seen on Earth. In addition, over 100,000 tonnes are harvested by fishermen in the Southern Ocean alone for use in animal feed, pharmaceuticals, dietary supplements, and certain local delicacies.
To give some idea of how abundant krill are, they became a major topic of study by the major naval powers during the Cold War because the tremendous noise made by shoals of these minute animals is so great that they can form a biological barrier against sonar under which submarines can hide.
In a recent study, BAS scientists Prof. Geraint Tarling and Dr Sally Thorpe used advanced acoustic technology to follow the movements of about 2,000 swarms of Antarctic Krill (Euphausia superba). What they found was that the krill do not remain on one level in the wild, but, like many forms of plankton, they migrate from the deeper levels to shallower ones to feed before returning to the depths in what is called satiation sinking.
They do this in a continuous cycle with well-fed krill being replaced by hungry ones. According to the researchers, this cycling means that as the satiated krill relieve themselves in the deep, their tiny, carbon-rich droppings sink down into the deeper ocean. Tarling says that this could result in 23 million tonnes of carbon, or the equivalent to the annual UK residential greenhouse gas emissions, going out of circulation each year thanks to the krill.
"This behavior has previously been observed in laboratory studies on individual krill onboard our research vessel the RRS James Clark Ross," says Thorpe. "What's really exciting in this study is that we now have evidence that suggests that krill perform satiation sinking in swarms in the open ocean and not just in the lab. This knowledge of the behavior of krill will help us to improve models of the transfer of carbon to the deep sea."
The research was published in the Proceedings of the Royal Society B.