There Are So Many Tiny Animals in the Sea That They May Affect Currents

Tiny zooplankton (white) create swirling currents in seawater, visualized in this lab experiment by particles illuminated by laser light (yellow dots). M. Wilhelmus and J.O. Dabiri/Caltech

The wind and tides are major drivers of the ocean’s global circulation, moving its waters all over the planet and mixing up its temperature, salinity, and nutrients. But according to new research, there might be another crucial force in ocean circulation that scientists haven’t accounted for: the billions upon billions of small marine animals that live in its depths.

Throngs of tiny organisms called zooplankton inhabit the ocean—everything from microscopic protozoans to krill to jellyfish. Many of these animals live deep underwater during the day to avoid predators, and migrate en masse, sometimes hundreds of meters, to the surface to feed at night. Caltech fluid dynamicist John Dabiri thinks zooplankton’s daily collective movements may have a profound influence on ocean dynamics by mixing up its waters, and his new study, published in Physics of Fluids, backs up this theory.

To mimic zooplankton migration in the ocean, Dabiri and his research partner, Monica Wilhelmus, devised an automated laser robot that shoots moving blue light through a water tank filled with thousands of brine shrimp. The shrimp (the same creatures sold to curious kids as Sea Monkeys) followed the laser light as it swept from the bottom of the tank to the top, and as they swam, they kicked back water behind them.

Individually, a sea monkey’s kick doesn’t move much water, but as Dabiri discovered, their collective migration creates large eddies. In the ocean, this could potentially mix up the nutrients and salinity of warmer surface saltwater with cold brine from deeper depths. Dabiri thinks that when untold numbers of zooplankton migrate up and down the ocean’s water column every day, they may have an effect on circulation as substantial as the wind and tides by adding about a trillion watts of energy to the ocean system.

Many physical oceanographers are skeptical of this theory (called ‘biomixing’), particularly since zooplankton migration is much harder to measure in the real world than the wind and tides. “It’s hard to go from a lab experiment in a tank and extrapolate to the ocean,” said physical oceanographer André Visser of the Technical University of Denmark. “I’m not convinced that this is a credible mechanism in ocean mixing.”

But Dabiri thinks his lab experiments prove the physics of the phenomenon. “The ocean is much bigger than the tank in our lab, but the tank had only a few thousand of these organisms versus billions and billions of them in the ocean,” he said.

If zooplankton do, in fact, move ocean waters as Dabiri predicts, this might help scientists model climate change more precisely. The ocean is Earth’s largest carbon sink, soaking up more than a quarter of CO2 that human activity emits, and zooplankton may play a key role in that process. “We may need to rethink our models of the ocean,” he said. “Perhaps there are significant factors we’re missing right now.”