Have you ever wondered why plankton sometimes converge together at the ocean surface, creating a so-called red tide? So have the scientists at Scripps Institution of Oceanography at the University of California San Diego.
Plankton, the tiny organisms at the bottom of the ocean food chain that so much of marine life depends on, drift with the ocean currents, but sometimes come together in dense patches under the surface that can later rise to the surface as red tides. The Scripps researchers realized that by studying plankton and their movements to solve this mystery, they could also find out a lot about internal ocean movements in general.
To do this they built a swarm of grapefruit-sized robots called miniature autonomous underwater explorers, or M-AUEs, that mimic the way plankton drift through the ocean. The robots are equipped with sensors for measuring temperature and other conditions and adjust their buoyancy to maintain a steady depth. When deployed in large numbers, the robots can capture a 3D picture of the interaction between ocean movements and the affect they have on marine life.
To study how large patches of plankton occur, the scientists dropped 16 of the bots in the water, forming a 300-meter diameter swarm that stayed 10-meters deep off the coast of Torrey Pines, Calif. Since GPS doesn't work underwater, the robots were tracked by acoustic signals. Every 12 seconds, their locations were recorded as they bobbed beneath the surface and moved with the currents.
The researchers found that internal water temperature shifted as giant internal waves moved through. The swarm of bots formed a tightly-packed patch in the warm internal wave troughs and then dispersed over the wave crests. This shows that plankton could use the internal wave movements to congregate in large concentrations for reproduction, which is likely what's happening in those red tides.
“This swarm-sensing approach opens up a whole new realm of ocean exploration,” said Scripps research oceanographer Jules Jaffe, who designed and built the M-AUEs.
The next step will be adding cameras and hydrophones for capturing images and sounds underwater as well as movement. Larger swarms of these robots could help to monitor the health of coral reefs, the movement of larvae in marine protected areas, harmful red tide blooms, and even track oil spills.