The tiny swirls created by brine shrimp and other minuscule aquatic creatures could mix the seas’ upper layers as well as winds and waves do, a new study suggests. Such “biomixing” could play an important role in redistributing heat, salt, and nutrients in the upper layers of the ocean. However, some researchers question how effectively biomixing blends the waters of the wave-thrashed sunlit surface with those from the cool, calm depths.
Winds, waves, and tides are crucial for mixing the surface waters of lakes and seas, transporting heat downward and simultaneously bringing nutrient-rich waters up to the surface where light-harvesting phytoplankton need them to thrive. But small marine creatures help such processes as they migrate to the ocean surface each night to forage and then return to the relative safety of unlit depths during daylight hours, some researchers think. One of the most familiar of these travelers, known to kids worldwide as the sea monkey, is the brine shrimp Artemia salina, says John Dabiri, a fluid dynamicist at the California Institute of Technology (Caltech) in Pasadena. Although the small swirls created by the fast-churning legs of a single sea monkey are not strong enough to significantly stir the seas, the eddies kicked up by billions of them might do the trick, Dabiri and others have proposed. To test the notion, he and Monica Wilhelmus, also of Caltech, measured the tiny currents triggered by artificially induced migrations of brine shrimp in the lab.
Dabiri and Wilhelmus used blue and green lasers to induce thousands of 5-millimeter-long brine shrimp to “migrate” to and from the bottom of a 1.2-meter-deep tank. The creatures are strongly attracted to those colors, Dabiri says. The researchers shone the blue laser into the tank and moved it slowly up and down to control the crustaceans’ vertical movements. The tank’s solid walls could strongly affect the flow patterns generated by the shrimp as they swam, so the researchers kept the shrimp away from the edges of the tank by shining the green laser beam directly down into the center. To help visualize the swirls and eddies generated by the shrimp, the researchers added copious amounts of silver-coated microspheres to the water and illuminated them with a red laser, a color that doesn’t seem to affect the shrimps’ behavior.
The team’s high-speed videos of the teeming, laser-lit migrations captured images of swirls much larger than the creatures themselves, which resulted from the interactions of smaller flows created by individuals. The larger the swirls, the more effective the mixing might be, Dabiri says. “So even for slow migrations, there could be strong effects,” he notes.
Previous studies suggest that light-harvesting phytoplankton, the base of the ocean’s food chain, collect about 60 terawatts of solar energy, Dabiri says. Even if marine organisms that consume phytoplankton convert only 1% of that power into mixing the oceans, that’s collectively comparable to the mixing power of winds and tides, Dabiri and Wilhelmus report online today in Physics of Fluids.
“This is a really innovative experimental setup that provides a nice illustration of flow velocities,” says Christian Noss, a fluid dynamicist at the University of Koblenz-Landau in Germany. Jeannette Yen, a biological oceanographer at the Georgia Institute of Technology in Atlanta, agrees. “I like the idea of using [the shrimps’] behavior to lure them to the camera,” she says.
But scientists disagree on how effective billions of churning sea monkey legs might be in blending ocean layers that are hundreds of meters deep. "I wouldn’t want to say just yet that [biomixing] is important at a global scale” solely based on a lab experiment, says Stephen Monismith, a fluid mechanicist at Stanford University in Palo Alto, California. André Visser, a physical oceanographer at the Technical University of Denmark in Charlottenlund, agrees. “Most of the energy [from the shrimp] probably goes into heating the water” rather than mixing it, he says.
In fact, the upper and lower layers of the seas have measurable differences in density, a stratification that, according to theory, would reduce the efficiency of any biomixing. And in May, experiments similar to Dabiri's suggested that stratification stifles mixing. In that research, Noss and colleague Andreas Lorke, also of Koblenz-Landau, studied the effects of large crowds of aquatic creatures called Daphnia (commonly known as water fleas) as they migrated up and down in a tank of mildly stratified water. As expected, the stratification squelched the biomixing generated by the swimming Daphnia, Noss says. Those results aren’t surprising, Visser says. “It’s difficult to lift heavy water up and to push light water down.”
Dabiri and his colleagues’ next set of lab experiments will look at the effects of sea monkey migrations in stratified waters, he says. Those experiments should reveal whether sea monkeys are better mixers than water fleas.