Using lasers to guide herds of swimming sea monkeys, researchers show how the migration of billions of tiny krill and other swimming plankton might affect global ocean circulation. The work was published in Physics of Fluids this week.

Forces created by winds and tides drive the vertical mixing of oceans, which stirs up nutrients and transports heat downwards. Sea monkeys (or brine shrimp, Artemia salina) are only about half a centimeter long, and even with all of their 10 or so small leaf-like fins flapping about, they hardly make any waves. But could billions of similarly tiny critters together move oceans? The idea had been proposed but never tested. 

So, to see if swimming plankton can generate enough swirling flow to influence the large-scale circulation of water, Monica Wilhelmus and John Dabiri of Caltech used a combination of blue and green lasers to coax sea monkeys to migrate upward inside a tank of water. In the wild, brine shrimp swim toward the surface of saltwater lakes at night to feast on photosynthetic algae without fear of predators. By daytime, they sink back down into the dark depths. 

The green laser at the top of the tank provides a bright target for the shrimp to swim toward, and the blue laser rising along the side of the tank illuminates a path to guide them. The tank of water is also filled with tiny, silver-coated hollow glass spheres that are 13 microns across. The duo used high-speed cameras to track the motion of the spheres, and a red laser (invisible to the critters) was used to measure how the shrimp's swimming causes the surrounding water to swirl.

The collective swimming motion of sea monkeys, they found, creates strong swirls that are noticeably larger than the size of a single organism. When two or more swim in close proximity to each other, the eddies they produce interact to create more powerful swirling fluid forces that could alter water circulation on a wider scale. 

And their swimming stroke is very similar to other organisms. So if the effect of all the zooplankton in the ocean were added up, that would inject as much as a trillion watts of power into the oceans to drive global circulation, Dabiri explains in a Caltech release. Winds and tides together contribute two trillion watts.

“This research suggests a remarkable and previously unobserved two-way coupling between the biology and the physics of the ocean,” Dabiri says in an Institute of Physics statement. “If similar phenomena occur in the real ocean, it will mean that the biomass in the ocean can redistribute heat, salinity and nutrients.” After all, small organisms make up the bulk of oceanic biomass. 

If the results are replicated in field studies out in the ocean, Dabiri tells Washington Post, "it would suggest that we've been missing a variable in modeling with ocean currents."

Images: M. M. Wilhelmus and J. O. Dabiri/Caltech