Antarctic krill are well-known for their role at the base of the Southern Ocean food web, but also play an important role in removing CO2 from the atmosphere and causing it to be buried in the sea floor.
A new study, funded by The Pew Charitable Trusts, and including scientists from PML, the Australian Antarctic Division, the British Antarctic Survey and research institutes and universities from the UK, Germany, and the US, highlights the influence of krill on atmospheric carbon levels and urges consideration of the impact of commercial krill fishing on ocean chemistry and the global climate.
Led by Dr Emma Cavan, a researcher formerly of the Institute of Marine and Antarctic Studies at the University of Tazmania and now at Imperial College London, the study reviewed current scientific knowledge of the role of krill in processes that remove up to 12 billion tonnes of carbon from Earth’s atmosphere each year.
Through eating phytoplankton and excreting carbon- and nutrient-rich pellets which sink to the seafloor, Antarctic krill form an integral part of the carbon cycle and are a key contributor of iron and other nutrients that fertilise the ocean. The research found that krill faecal pellets constitute the majority of sinking carbon particles identified by scientists in both shallow and deep waters in the Southern Ocean.
Swarming in vast numbers, the krill's combined contribution to the movement of ocean carbon and other nutrients can be huge despite their size - Antarctic krill grow to approximately six centimetres long and weigh around one gram. The Southern Ocean is one of the largest global carbon sinks, so krill have an important influence on atmospheric carbon levels and the global climate.
As krill are the Southern Ocean's largest fishery, it is important to address the lack of knowledge around the extent of krill's ability to affect the carbon cycle. Dr Cavan said management of the krill fishery currently centres on sustainability and krill’s role in supporting megafauna such as whales, with little attention given to assessing the significance of krill to the carbon cycle and ocean chemistry.
She said: “Today the fishery takes less than 0.5% of the available krill and only adults are targeted. But there is no consensus on the effect that harvesting Antarctic krill could have on atmospheric carbon and ocean chemistry nor, for that matter, how growing whale populations might also affect krill numbers.
"We don’t know, for example, whether a decline in krill might actually lead to an increase in the biomass of phytoplankton, which are also integral in transporting carbon to the seafloor. Conversely, a decline in krill would decrease the beneficial fertilisation effect that their faecal matter has on phytoplankton biomass, at the same time also jeopardising the important part krill play in circulating iron and other nutrients."
The study indicates a need for further research to address these and other questions about the significance of krill, as well as for more accurate estimates of their biomass and distribution. This would inform understanding of both biogeochemical processes in the Southern Ocean and the management of the krill fishing industry.
The authors also recommended that measures be put in place to ensure that as fishing technology advances, the fishery does not encroach on larval krill habitat near sea-ice, and steps should be taken to prevent potential larval by-catch when fishing for adults, avoiding spawning hotspots.
Dr Angus Atkinson at Plymouth Marine Laboratory who co-authored the study said: “Scientists have known for some time that Antarctic krill have important roles in exporting carbon from the surface layers of the Southern Ocean, but we are still learning about their roles in the cycling of key nutrients such as iron and nitrogen. This paper synthesises this rapidly changing knowledge and recommends some potential areas for fruitful research in the future.”