— Krill small shrimp-like invertebrates that feed on plankton are capable of turning the calm nocturnal sea into a churning, frothing Jacuzzi, researchers report.
The phenomenon of biologically generated turbulence may have a previously unappreciated effect on climate change, they add.
Although tiny, krill is present in such vast numbers in the worlds oceans that its biomass is at least equal to all the animal protein consumed by humans each year.
Now Eric Kunze and John Dower, at the School of Earth and Ocean Sciences at the University of Victoria in British Columbia, Canada, and colleagues have measured the turbulence generated at dusk by krill as they rise to the surface of the ocean to feed.
This is the first example of measurements of biological turbulence made under field conditions, says Dower.
During the day krill hide about 100 metres below the surface. As night falls they ascend in vast numbers. In the Saanich Inlet, a coastal bay just northwest of Victoria, for example, krill occur in concentrations of up to 10,000 per cubic metre, Kunze says. The total number of Antarctic krill alone is estimated to weigh between 50 and 150 million tonnes.
Turbulence was measured by dropping a tethered sensor, called a microstructure profiler, through the water. The instrument measures shear forces, temperature and conductivity on a microscale, allowing the scientists to determine the amount of turbulence. Shear sensors are like old phonograph needles that are very sensitive to bending, Kunze explains.
The results were clear cut, but unexpected, because the idea that biology could influence marine turbulence was disputed, says Dower. So we were really surprised when the layer of krill came up and we immediately saw that the turbulence levels were higher.
And they were much higher: the krill elevated the turbulence by 3 to 4 orders of magnitude, and increased mixing between water layers by a factor of 100.
One of the reasons we care about mixing is that surface layers mix with atmospheric gases, says Dower. This could play a role in the drawing down of carbon dioxide from the atmosphere into the ocean.
It is not yet known if biological turbulence caused by krill will turn out to be a widespread phenomenon, but if so, then it will have to be taken into account in models of CO2 accumulation and climate change.
And if it is a general phenomenon then it stands to reason that if we remove species then it will have an effect on mixing, says Dower. Wed like other people to check for biological turbulence in the areas of the world.
Geraint Tarling, of the British Antarctic Survey in Cambridge, UK, says that models have already suggested that biological turbulence may be a significant mixing force in the oceans, but confirms that this is one of the first direct demonstrations of its magnitude.
Tarling says that the turbulence generated by krill in the Southern Ocean may bring more micronutrients to the surface and enhance the growth of phytoplankton, the main food source of krill. In effect, krill reap the rewards of their own efforts."
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