Krill more important than previously assumed | Polarjournal
Krill probably is the largest accumulation of animal biomass in the Southern Ocean. Experts suspect that up to 400 million tons dwell in the freezing waters of Antarctica as shoals or swarms. The crustaceans, which are almost 6 cm long and weigh 2 grams, eat algae and thus incorporate the carbon fixed by the algae into their carapaces (shells), which they moult every 2 weeks. Photo: Ocean Treasures Library

Krill, which is only 6 centimetres in size, is probably the most important species in the Southern Ocean. Because basically all other larger animal species such as penguins, seals and whales feed on the small crustaceans. However, the importance of krill is high not only as a food source, but also in the fixation of CO2 from the atmosphere. The crustaceans eat algae and the faeces fall down as pellets onto the seafloor, where they keep the carbon bound. Now, researchers from the British Antarctic Survey have found that another part of the crustaceans is an important carbon sink: their carapaces.

“Krill are really unusual crustaceans in moulting so frequently,” he said.

Prof. Geraint Sterling, British Antarctic Survey

The crustaceans have to re-form their exoskeleton every two weeks starting in spring, when they start feeding on algae again, because the old shell becomes too small for them. The old exoskeleton then is shed off and sinks to the seafloor, where it remains. Thus, the carbon, which is a component of the building material, is bound and is no longer in the atmosphere. The researchers around Dr. Clara Manno of the BAS have found that such large amounts of carbon are bound and stored. Up to 300 kilograms of carbon would be transported by the entire amount of krill (about 380 million tons) per day from the atmosphere to the seafloor, the scientists write. “Krill are really unusual crustaceans in moulting so frequently,” explains the study’s co-author, Professor Geraint Sterling of BAS.

Krill lay eggs that first sink to the seabed. After hatching, the larvae rise, grow to the reproductive stage and then die after about 6 years. They form a new carapax (moulting) almost every two weeks. Exoskeletons, faecal pellets and the dead crustaceans form the largest proportion of organic carbon from summer to winter on the way to the seafloor. Picture: Michael Wenger

However, the research team not only looked at the shells, but also calculated how much of the various krill portions (exoskeletons, fecal pellets and carcasses) make up of the total amount of organically bound carbon that sinks to the seafloor. And here something surprising was found: Each year, the three parts account for about 87 percent of the total amount of so-called POC (particulate organic carbon) per square meter of ocean. “This is exciting news because it almost doubles the previous estimate of how much atmospheric carbon is transported into deep ocean layers by krill. Our study reveals that large krill swarms could remove a significant amount of carbon from the atmosphere”, says Dr. Clara Manno. In summer it is mainly the exoskeletons, because at that time the animals grow quickly due to the high food intake. In autumn, it is the fecal pellets, as the crustaceans slow down their growth and prepare for winter. In winter and spring it is mainly the dead animals that either die due to age (krill can grwo up to six years) or die due to too few reserves.

Krill is the most important food source for many Antarctic inhabitants. Especially Adélie and chinstrap penguins are highly dependent on the krill quantities in their immediate breeding areas. Since krill is dependent on sea ice and that has retreated in the area of the Antarctic Peninsula, the colonies there have also decreased. Experts assume the amount of krill at around 380 million tonnes per year. Picture: Michael Wenger

In addition to South Georgia, other areas of the Scotia Sea and the Antarctic Peninsula are also important krill areas. The Ross Sea and some sub-Antarctic islands are also important ecological regions. It is therefore not surprising that these regions also have the highest densities of penguins, seals and whales. But the researchers also warn in their work. These areas are acutely threatened by warming and ice loss, which in turn affects not only the livelihoods of Antarctic animals, but also the Southern Ocean’s ability to fix carbon from the atmosphere. Other studies have shown that, thanks to the Southern Ocean, the increase in levels of carbon dioxide in the atmosphere had been dampened. Now we know that the small crustacean has played a more important role than previously thought.

Dr Michael Wenger, PolarJournal

Link to the study: Manno, C., Fielding, S., Stowasser, G. et al. Continuous moulting by Antarctic krill drives major pulses of carbon export in the north Scotia Sea, Southern Ocean. Nat Commun 11, 6051 (2020). https://doi.org/10.1038/s41467-020-19956-7

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