by Playfuls Staff |
22nd September 2006
Measurements in a coastal inlet revealed turbulence that was three to four orders of magnitude larger during the dusk ascent of a dense acoustic-scattering layer of krill than during the day, elevating daily-averaged mixing in the inlet by a factor of 100.[more]
Because vertically migrating layers of swimming organisms are found in much of the ocean, biologically generated turbulence may affect the transport of inorganic nutrients to the often nutrient-depleted surface layer from underlying nutrient-rich stratified waters to affect biological productivity and the exchange of atmospheric gases such as CO2 with the stratified ocean interior, which has no direct communication with the atmosphere.
Krill are shrimp-like marine invertebrate animals. These small crustaceans are important organisms of the zooplankton, particularly as food for baleen whales, Mantas, whale sharks, Crabeater seals and other seals, and a few seabird species that feed almost exclusively on them. Their scientific name is Euphausiids, after their taxonomic order Euphausiacea. The name Krill comes from the Norwegian word krill meaning "young fry of fish".
Krill occur in all oceans of the world. They are considered keystone species near the bottom of the food chain because they feed on phytoplankton and to a lesser extent zooplankton, converting these into a form suitable for many larger animals for whom krill makes up the largest part of their diet. In the Southern Ocean, one species, the Antarctic Krill, Euphausia superba, makes up a biomass of hundreds of millions of tonnes, similar to the entire human consumption of animal protein. Over half of this biomass is eaten by whales, seals, penguins, squid and fish each year, and replaced by growth and reproduction. Most of the species display large daily vertical migrations making a significant amount of biomass available as food for predators near the surface at night and in deeper waters during the day.
Krill could have a big impact on ocean life, contributing to mixing gases in the ocean, such as carbon dioxide, and thus playing an important role in the global warming process. A great portion of the total amount of carbon dioxide emitted annually is being “captured” underwater and this is why the consequences of the greenhouse effect are being diminished.
Until now, scientists have known the fact that oceans’ surface, continuously swept by winds, is the cradle of a lot of life, but they were unable to explain why, since the amount of nutrients that rise from deep under were not enough to sustain the teeming life above.
The researchers, from the University of Victoria in Canada, investigated swarms of krill in Saanich Inlet, a fjord on Vancouver Island. In British Columbia, Canada, billions of the tiny swimmers churn the seas as they migrate each night between their safe daytime havens deep underwater to food-rich surface waters.
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.
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.”
"I was initially skeptical that biologically linked turbulence could be significant. I was surprised at just how large it could be," researcher Eric Kunze, an ocean physicist, told LiveScience.
"The question now is how frequently and how significant any biologically linked turbulence really is," Kunze said.
