Researchers from the University of California San Diego's Scripps Institution of Oceanography developed underwater robots to provide scientists with a new tool for studying currents in the ocean and the ocean's most abundant organism: plankton.

Miniature Autonomous Underwater Explorers

For the study published in the journal Nature Communications, Jules Jaffe, Scripps oceanographer, designed and made miniature autonomous underwater explorers to examine small-scale processes occurring in the ocean.

As ocean probes, M-AUEs are fitted with sensors for measuring conditions in the water and equipment to help underwater robots in "swimming" up and down the ocean and adjust buoyancy so they can stay at a uniform depth. Depending on research needs and goals, the underwater robots can be released in swarms with anywhere between hundreds and thousands of units to capture in 3D how ocean currents interact with marine life, and vice versa.

Along with Peter Franks and other colleagues, Jaffe sent out a 16-unit swarm of underwater robots the size of grapefruits, each one designed to mimic plankton swimming behavior underwater. The researchers' goal was to test out theories regarding plankton formation under the ocean's surface, which commonly occurs during instances of red tides.

Studying Plankton

Franks had long suspected that this plankton formation could help in feeding and protecting predators, as well as aid in their reproduction. He came up with a mathematical theory 20 years ago predicting that plankton swimming would form dense clusters when the organisms are moved by internal waves, or the giant but slow-moving waves occurring beneath the ocean's surface.

However, he was unable to test his theory back then before because that would call for tracking each plankton's movement, the technology for which was not available at the time. But with the robotic plankton from the current study, Franks finally has a tool at his disposal capable of imitating plankton movement.

"The big engineering breakthroughs were to make the M-AUEs small, inexpensive, and able to be tracked continuously underwater," said Jaffe. It was because the underwater robots were economical to produce that the researchers were able to deploy a swarm of them.

GPS doesn't work underwater so the researchers turned to using acoustic signals for tracking the robots while they were submerged for five hours. Within that period, location data was collected in 12-second intervals.

Overall results were closely identical to predictions Franks made and the M-AUEs revealed that plankton form tight patches in warm waters with interval wave troughs but disperse over wave crests. According to him, this is the first instance that a mechanism like that was tested underwater.

With this, the researchers were able to get confirmation that free-floating plankton are capable of using the ocean's physical dynamics, like internal waves, to increase population concentration and fulfill their fundamental needs for survival.

Future Research

The researchers said their swarm-sensing approach can take ocean exploration to a new level, such as attaching cameras to M-AUEs to map corals. In the future, they want to build more of the underwater robots for further study involving larval movement in protected marine areas, monitoring blooms of red tide, and tracking oil spills.

Aside from Jaffe and Franks, researchers for the study included Adrien Boch, Ryan Kastner, Curt Schurgers, Diba Mirza, and Paul Roberts.