Swarms of tiny animals as small as brine shrimp—known to kids the world over as Sea-Monkeys—could have an outsize effect on ocean currents when they swim together in giant herds, according to new research.
Plankton, tiny marine creatures often thought of as mere drifters, actually aren't always so passive. Many move up and down in the ocean in dense layers throughout the day. These collective movements might produce currents large enough to mix seawater, says study co-author John Dabiri, an engineer studying biological physics at Caltech in Pasadena. (See "Sea Animals Change Climate Via Flutters and Flaps?")
If so, this mixing may need to be accounted for in simulations of Earth's future climate, Dabiri says.
Wind-and tide-driven currents move nutrients, heat, and salt around the ocean, and help to regulate the planet's temperature, Dabiri says. In recent years, scientists have started to seriously consider whether collective animal movements—like plankton swimming up and down en masse—could also be contributing to currents.
Dabiri and a colleague found in 2009 that jellyfish can actually move water over distances greater than their body length just by swimming. The layer of water closest to the jellyfish clings to the animal, with the "stickiness" of the water diminishing as it gets farther away from the jellyfish. A halo of water around the jellyfish gets dragged along by the animal as it moves.
"This was the first hint that animals could transport water over distances much longer than their body size," Dabiri says.
Dabiri and colleague Monica Wilhelmus, also at Caltech, are extending that work to other important kinds of vertically migrating animals such as krill and copepods.
Krill are common in the ocean but difficult to keep in the laboratory, so the researchers used brine shrimp. Although they aren't part of the vertically migrating layers in the ocean, the brine shrimp swimming motion is similar to that of krill, they are easier to raise, and they are highly attracted to light. This made it possible to use lasers to trigger brine shrimp migration across tanks of water in the laboratory.
By seeding the water with silver-coated, hollow glass spheres, the researchers could see the fluid flow produced by their sea-monkey herd as it followed the lasers up and down in the tank.
The jet of water the animals produced behind them as they swam was moving faster than the surrounding water, says Dabiri. The edges of that "fast lane" snagged on the edges of the surrounding slower water, creating swirls and eddies known as Kelvin-Helmholtz instabilities.
Those swirls were much larger than the animals—which are half an inch (15 millimeters) long as adults—the researchers report Tuesday in the journal Physics of Fluids. The finding suggests that much larger groups of plankton could mix seawater by creating currents as they swim, Dabiri says.
It's a Big Ocean
Dabiri says that even he was surprised at how clearly the effect showed up in the laboratory experiments.
"My friends who are physical oceanographers have a healthy skepticism of [this] idea," he says. "But you have to remember that there are billions of [plankton] in the ocean, and the whole is greater than its parts."
Christian Noss, an environmental physicist at the University of Koblenz-Landau in Germany, says that he's not convinced the effect would scale up from the laboratory to the ocean. The study was well designed, he says, but unlike water in a small tank, water in the ocean is often stratified, with denser layers lying underneath lighter ones.
Noss's work with another tiny crustacean, known as Daphnia, showed that stratified conditions dampened the mixing produced by these animals.
Dabiri says he plans to test the stratification question and hopes to perform the same experiments at a larger scale in the ocean.
"To me, an interesting aspect of this work is to see animals that seem to be at the mercy of the water play a role in shaping their own environment," Dabiri says. "It's something that we hadn't appreciated, but these experiments are showing [that this] might be a common occurrence in the ocean."
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