Competition for food in the sea ice zone in autumn drives large variations in the Antarctic krill population, according to new research by German and Australian scientists.

Published in Nature Ecology & Evolution in June, the study challenges the view that climate and environmental factors cause a huge expansion and then contraction of the krill population every five to six years.

Instead, large numbers of adult and larval krill compete for phytoplankton (microscopic marine plants) during autumn, which declines in abundance as winter approaches. If there are too many krill for the food available, many starve and die before winter.

“Scientists have assumed that factors such as the El Nino-Southern Oscillation index and changes in sea surface temperature or sea ice duration are the main reason for a more than 10-fold change in krill biomass every five to six years,” Australian Antarctic Division krill biologist, Dr So Kawaguchi, said.

“While these factors do play a role, this study has theoretically shown competition for food in autumn could be the main driver of the ‘krill cycle’.”

The study, led by Professor Bernd Blasius and Professor Bettina Meyer of the University of Oldenburg’s Institute for Chemistry and Biology of the Marine Environment, and the Alfred Wegener Institute for Polar and Marine Research, used modelling of krill reproduction, growth and energy requirements, to link the ebb and flow of krill biomass with phytoplankton abundance.

“We found that an abundant krill population accelerates the decline of the phytoplankton concentration in autumn and depletes it early, leading to a long starvation period over late autumn and winter,” Prof Blasius said.

“In contrast, a low krill population has a smaller impact on phytoplankton density, enabling both adult and larval krill to lay down sufficient fat reserves to survive the winter.

“A new cohort of larvae only survives when the existing adult population has fallen below a critical level, thereby reducing the number of adults and larvae grazing on phytoplankton.”

The research suggests that increasing the population pressure on krill, through consumption by whales, for example, could stabilise krill populations. On the flip side, a decline in krill predators such as seals, penguins and albatross due to climate-related changes, could amplify the fluctuations in krill biomass and potentially destabilise the Antarctic food web.

“This research will play a role in highlighting to conservation managers the importance of maintaining the delicate balance of the krill-centric ecosystem,” Dr Kawaguchi said.