Each night, all over the ocean, swarms of animals wriggle and kick their way from deep below the waves to feed at the surface. Each creature is tiny — less than a centimeter long, and sometimes much smaller — and there are trillions of them.
New research suggests this nightly migration might be helping mix the ocean on a grand scale, sending columns of water down as the animals swim up. It's a radical idea, and one that is just starting to take hold among scientists who study the oceans and who have long assumed that wind and waves, not animals, are the drivers of ocean-mixing.
"People assumed that they were just too small to make a difference," says John Dabiri, an engineering professor at Stanford University. It's difficult to imagine how swimming animals less than a centimeter long could stir up water over meters or even kilometers, which is the distance water would need to travel to have any meaningful effect on ocean mixing.
Unless they're swimming in huge groups.
"You have this massive migration vertically every day of literally trillions of organisms," Dabiri explains. "The question we wanted to ask is whether, as those animals migrate vertically, could they be inducing large-scale flows."
They tested their hypothesis in the lab, using small creatures called brine shrimp (also known as sea monkeys) in a tall tank. The shrimp are drawn to light, so the team used lasers to make the animals swim en mass up and down, over and over, and studied the eddies of water generated by the group.
"Imagine almost a ball of these animals swimming toward the light," Dabiri says. "As they start swimming upward, each of them kicks a little bit of fluid backward." The next shrimp kicks the same parcel of water a little more. And the next shrimp and the next shrimp and "pretty soon you have this vertical stampede upward of these shrimp, and [the water is] getting rushed downward by this successive series of kicks."
The result is that, even though each animal is very small, the group generates a powerful jet of water. The findings were published Wednesday in the journal Nature, along with videos that use dye and tiny glass beads to show how the water moves as the animals swim.
(The visualization method was developed a few years ago with a grant from the National Science Foundation, and was held up as an example of wasteful government spending by Sen. Tom Coburn, who described the research as "synchronized swimming for sea monkeys.")
A group of brine shrimp in a tank swim upward toward a light. On the right, the animals are visible as specks illuminated by a blue laser. On the left, the water is dyed yellow to show how the fluid moves down as the shrimp swim up.
Analyzing the videos, the team found the jets of water created by the shrimp move at about a centimeter per second. "It's not going to be as strong as a rip current on the beach. But certainly much stronger than what we call natural upwelling events" caused by wind at the surface, Dabiri explains.
The results suggest that animals may be affecting how nutrients are transported in the ocean, and how oxygen and bacteria are distributed below the surface.
Understanding what drives ocean mixing is also important because oceans are an enormous and dynamic sponge for carbon dioxide, sucking it from the air into the water, and then trapping the carbon as the surface water mixes with lower layers of the sea.
Waves and wind churn and mix ocean layers on a huge scale. When scientists create mathematical models of ocean dynamics – such as the models governments rely on to predict atmospheric CO2 levels — they factor in the ocean mixing effects of both waves and wind. But those models currently don't consider the mixing effects of swimming animals.
"It will still be important to confirm these findings in the real ocean, and frankly I would encourage my colleagues in oceanography, before they go changing their models of the ocean, we really do need to go to the ocean and do these measurements," Dabiri says.
But there's no guarantee that ocean-based research will confirm the laboratory findings. A study of krill off the coast of Canada did find evidence that the animals can significantly increase the turbulence of the water in the upper layers of the ocean. But a subsequent study by the same team at the University of Victoria in British Columbia suggested that powerful turbulence generated by large groups of small animals is rare.
"I think it's fair to say there is growing acceptance of the idea that biologically generated turbulent mixing can sometimes be significant," says John Dower, one of the authors of the krill studies, and an oceanographer at the University of Victoria. "The key word here is 'sometimes.' "
Dower acknowledges it's possible that dense, turbulence-generating groups of small swimming organisms may be more common than the current research suggests. But he says it's unlikely that there will be a lot more research in this area in the near future. "Most biological oceanographers don't measure turbulence, and most physical oceanographers aren't interested in zooplankton," Dower says.