Krill mix up the ocean

Titanium dioxide particles mixed in sea water and illuminated by a laser, help scientists trace the movement of water around krill.
Small tracer particles of titanium dioxide were mixed into the krill tanks. A laser was then used to illuminate the particles and observe the movement of water around the krill.
Photo: Kimberly Caton
Turbulence in the ocean, caused by the wind, tides and currents, plays an important role in regulating the global ocean circulation and the movement of gases and nutrients between the surface and deeper waters. Now new research suggests that organisms as small as krill can contribute to this turbulence.

According to Science magazine, scientists have hypothesised that schools of krill, which migrate up through the water column at night to feed, generate large turbulence patches, potentially increasing the nutrient exchange across the stratified (layered – with warmer water at the surface) ocean. This means that krill could be fertilising the surface waters, boosting the production of phytoplankton.

It's not hard to imagine that large organisms, such as whales or big fish, could produce such 'biologically generated turbulence' or 'biomixing' at a local scale. Indeed, scientists speculate that the removal of whales and stocks of big fish over the past 200 years could have removed enough biomixing to affect climate (through, for example, a change in the exchange of carbon dioxide between the atmosphere and the ocean). But the idea that krill could generate enough turbulence to efficiently mix ocean waters is controversial.

Research by the Australian Antarctic Division's krill biologist, Dr So Kawaguchi, and his collaborators at the Georgia Institute of Technology in the United States, is lending weight to the hypothesis that biomixing by krill does occur.

The team filmed krill in the Australian Antarctic Division's aquarium. High resolution analyses of the flow fields generated by the krill showed that when a small group of krill (less than five individuals) were moving in a coordinated way – similar to schooling – the energy generated to displace the water was of the same magnitude as that of turbulence observed at sea. Thus, in stratified conditions, a large enough school of krill could transport less dense water deeper into the water column, leading to increased mixing.

'This work shows that the group effect is significant and suggests that the hypothesis for biomixing by krill is valid,' Dr Kawaguchi says.

Wendy Pyper

Corporate Communications, AAD

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