The small crustacean Euphausia superba (Antarctic krill) is the food source for many animals in the Southern Ocean - and one of the most abundant species in the world. New findings from researchers in Oldenburg now show that competition for food is the main cause of regular fluctuations in krill populations in the Southern Ocean.
Huge shoals of the almost six-centimetre-long krill frolic in the waters around Antarctica. Whales, seals, penguins - they all prey on the small crustacean, which plays a major role in the Antarctic ecosystem. However, Antarctic krill populations fluctuate regularly over a period of five to six years. The biomass changes more than tenfold in the process. Experts previously assumed that climatic factors, such as the highly seasonal polar climate, triggered the so-called krill cycle. However, there is no real evidence for this.
An international team led by physicist Prof Dr Bernd Blasius and marine biologist Prof Dr Bettina Meyer has taken a closer look at the population fluctuations of krill - and has come to some surprising conclusions: Feedback within the population, such as competition for food, is the main cause of the cycle. The researchers concluded this from analyses of long-term data and from calculations of a mathematical model with which they investigated the changes in the populations. The marine scientists have published their findings in the renowned journal "Nature Ecology and Evolution".
In the Antarctic autumn in particular, the small crustaceans within the shoal increasingly compete for food. This is because larvae and adults need to build up sufficient fat reserves for the approaching winter during this time. At the same time, the krill's food, the microscopic algae of the phytoplankton, decreases sharply due to the shortening days. Larger krill populations have to starve, hibernate and reproduce for longer periods of time. All of this causes the population size to fluctuate.
"We can show that it is primarily competition within the krill population that causes the fluctuations in autumn," says Dr Alexey Ryabov, who also conducts research at the Institute of Chemistry and Biology of the Marine Environment. Until now, scientists had assumed that winter was more critical for the survival of krill larvae. This is because when large parts of the Southern Ocean are covered in ice, there is little food for the small crustaceans. "However, our results shed new light on this assumption," says Meyer.
The scientists were able to understand the processes using a specially developed bioenergetic model, which, among other things, depicts the growth of the krill from egg to adult and links it to the availability of food. "The results of these simulations corresponded well with the cycles actually observed over a period of 18 years," says Blasius.
According to the researchers, their simulations also help to better understand the food web in the Southern Ocean as a whole. This is because the model results show that a decrease in large predators, such as whales, is likely to increase the fluctuations in krill stocks. "This could destabilise the food web in the region and further reduce the number of large predators," says Meyer. Conversely, increased hunting pressure on krill could even stabilise their populations. "Any factor that changes this delicate balance can have a drastic impact on the entire Antarctic ecosystem," concludes Blasius.