Populations of Antarctic krill (Euphausia superba) in the Southern Ocean’s southwest Atlantic sector have moved south since the early 20th century, researchers have found.
The latitude marking the species’ mean north–south distribution was 57°S in the early 20th century but is 61°S today. That change has been accompanied by a drop in krill density, and an even steeper decline in the population of juvenile krill.
Angus Atkinson of Plymouth Marine University, UK, Simeon Hill of the British Antarctic Survey and colleagues from Canada, Germany and the UK mapped the krill’s movements from 1926 onwards using catch data from thousands of samples in KRILLBASE.
The researchers attribute these effects to a change in the weather brought about by a band of circumpolar winds shifting periodically towards the south. Since the 1920s the Southern Annular Mode (SAM) – an index of atmospheric circulation – has seen more positive anomalies, bringing warm, cloudy and windy conditions to the Southern Ocean and later formation of sea ice. How exactly this affects krill egg production and larvae survival is still uncertain.
“We do not know exactly what the mechanisms are at this stage, but we found that this link between SAM and krill density held throughout the austral summer, autumn and winter,” says Atkinson.
The researcher speculates that overcast skies and greater ocean mixing limit the availability of phytoplankton for larvae to feed on during the summer. Then, late ice formation in autumn leads to fewer sea-ice algae to eat during winter.
The poles are forecast to warm disproportionately as climate changes. Antarctic krill are especially sensitive to temperature but because lines of longitude converge at these latitudes, moving south into colder water means occupying ever-smaller habitat.
Krill populations have already become concentrated on the coast of the Antarctic Peninsula. Further southward movement will eventually be blocked by the greater Antarctic continent. What will happen if warming oceans and diminishing sea ice restrict krill to the continental shelf?
“This is not known,” says Atkinson. “The larvae historically tend to be found over deeper water (more than a kilometre), suggesting that krill really do need access or proximity to deep water to spawn successfully.”
“Our results suggest that southern areas have been relatively resilient to these changes and that juvenile krill can still be plentiful in some years,” Hill adds. “These differences offer some hope that krill and its predators will survive into the future. Nonetheless, this is a clear sign that ecosystems can change rapidly in a changing climate.”
Antarctic krill is found throughout the Southern Ocean but is especially common in the southwestern part of the Atlantic. Not only is the creature a crucial component of marine food webs – its huge total biomass directly or indirectly supports all of the region’s large animal species – but its digestive processes fertilize the oceans by mobilizing iron bound up in glacial sediment.
Angus Atkinson and colleagues reported their findings in Nature Climate Change.