Schools of tiny animals and fish might be a huge and previously unnoticed contributor to the churning of ocean water, on a par with the wind and tides. Researchers have observed a dramatic increase in water turbulence in a coastal inlet as a dense layer of shrimplike krill ascended for their evening meal, hinting at similar mixing off coasts around the globe. Rough estimates of the water-stirring potential of underwater organisms suggest they could be responsible for one third of all ocean mixing.
The mixing of ocean water plays a number of roles, from dredging nutrients up from the deep and regulating the exchange of gases between water and the atmosphere to maintaining the so-called global conveyor belt that links ocean currents. Wind and tides were long considered the main sources of mixing, even though microscopic plants at the ocean's surface seemed to be getting more nutrients from below than these physical mechanisms could explain. For decades researchers largely ignored the idea that migrating zooplankton (small marine animals) might be a significant source of underwater turbulence, says biological oceanographer John Dower of the University of Victoria in British Columbia.
So Dower and co-worker Eric Kunze decided to check for themselves. From a boat sitting on Saanich Inlet in British Columbia, they repeatedly dropped a probe to the bottom. On each descent it recorded centimeter-scale variations in water shear and temperature. At dusk, their radar picked up a dense school of krill, each about 1.5 centimeters long, ascending from deeper waters. The probe¿s turbulence readings suddenly jumped 1,000- to 10,000-fold compared with its daytime measurements, enough to significantly increase the inlet's average daily mixing, Dower, Kunze and their colleagues report in the September 22 Science. "This is a pretty short-lived pulse," Dower says. "You start to wonder--we know these krill are abundant everywhere--are we looking at a general phenomenon that simply hasn't been detected elsewhere?"
Ocean physics allows for this possibility. About 1 percent of the energy generated by microscopic ocean plants is likely converted to mechanical energy in the form of swimming zooplankton and fish, according to an upcoming paper in the Journal of Marine Research by William Dewar of Florida State University and colleagues. Globally, that could translate to one terawatt of energy--a third of the amount required to sustain the ocean's observed rate of mixing. The wind and tides each contribute another third. "We believe the biosphere is comparable in importance to other physical processes," Dewar says.