Tiny shrimp may be mixing ocean water as much as the wind and waves

Ask any oceanographer how rich surface waters reach the ocean’s depths, and they will probably tell you two things: wind and waves. Now, a new study suggests they should add a third factor: shrimp. Collectively, say scientists, these tiny invertebrates push around so much water that their actions—and those of other swarming sea-dwelling creatures—should be included in models of ocean circulation, which help predict the role the seas play in climate change.

“Whether or not swarming adds up to genuine mixing has been the big question in this business for the past decade or so,” says Nicholas Butterfield, a paleobiologist at the University of Cambridge in the United Kingdom who was not involved with the work. “This study makes a pretty good claim for nailing it.”

Intrigued by the idea that marine creatures might affect ocean circulation, engineer John Dabiri, who specializes in fluid mechanics at Stanford University in Palo Alto, California, and colleagues came up with a way to study shrimp swarming in the lab. They filled small tanks with brine shrimp and water of different densities. Because the water refracts light according to its density, they could see the water as it swirled off the shrimp. Dabiri was surprised at how big these eddies were, relative to the size of the shrimp.

So his team built two large tanks, one 1.2 meters tall and outfitted with blue light-emitting diodes and one 2 meters tall illuminated with a blue laser. The scientists added water of two different salinities and up to 135,000 brine shrimp to each. After the shrimp settled to the bottom, the researchers turned on the lights or the laser, causing the shrimp to swim to the surface. In some experiments, they added microscopic particles so they could better detect water flow.

The shrimp caused the two layers of water to mix 1000 times faster than they would if left to their own devices, Dabiri and his colleagues report today in Nature. The individual eddies created by each shrimp don’t do much, but as they swirl together, they add up to a significant downward flow. The difference is akin to what happens when you stir cream into coffee rather than letting the cream just sink in. “This is how the animals have a collective effect,” Dabiri says.

In the ocean, much larger krill gather by the millions in swarms up to hundreds of meters long for daily upward migrations as long as a kilometer. So Dabiri suspects their impact on ocean circulation is profound, perhaps even more than that of the wind. This migration can transport heat, nutrients, microbes, and carbon downward. Butterfield, too, thinks this effect is quite powerful, and may have even helped early life evolve. Today, he says, the animals’ downward push even affects how much carbon is sequestered in the deep oceans. “It’s now clear that animal ecology needs to be factored into models for how the modern oceans work.”