Israeli, U.S., German Researchers Use Acoustic 3-D Imaging System To Unveil Remarkable Behavior Of Ocean Plankton

An international team of scientists from Israel, the United States and Germany, led by Prof. Amatzia Genin of the Hebrew University of Jerusalem and the Interuniversity Institute for Marine Sciences in Eilat, has provided, for the first time, evidence of the remarkable dynamics responsible for the formation of large aggregations of microscopic animals in the ocean.

From the surface, the ocean appears to be vast and uniform. But beneath the surface, countless number of tiny, nearly transparent animals, called zooplankton, are swept into clusters and patches by ocean currents. The very survival of many zooplankton predators—from invertebrates to whales—and the success of fishermen catches can depend on their success at finding those patches. The new findings indicate that zooplankton are passively drifting with the current, as their name implies (“planktos” = “drifting” in Greek), but only in the horizontal direction, not in the vertical. Indeed, in the vertical, these creatures show a great ability to go “against the flow.”

Although scientists and fishermen have known for a long time that zooplankton spend their life suspended in a constantly flowing environment, an understanding of their responses to ocean currents has remained elusive, mainly due to technological limitations in tracking the motion of the minuscule animals.

Now, the recent development of a three-dimensional, acoustic imaging system by Jules Jaffe of the Scripps Institution of Oceanography at the University of California, San Diego, has opened the door for a team of researchers to track several hundred thousand individual zooplankton at two coastal sites in the Red Sea. In addition to Prof. Genin, the team included his graduate student Ruth Reef; Dr. Jules Jaffe and Prof. Peter Franks from the Scripps Institution of Oceanography; and Dr. Claudio Richter from the Center for Tropical Marine Ecology in Bremen, Germany.

Their findings, reported in the May 6 issue of the prestigious journal Science, show that these small animals effectively keep their depth by “treadmilling” against upwelling and downwelling currents at speeds of up to several tens of body-lengths per second. Downward-flowing water in the ocean is always accompanied by horizontal flows, forming a convergence, or “downwelling” zone. When zooplankton swim upward against such a downward current, they form patches as more and more individuals are brought in with the horizontal currents and concentrated in the downwelling zone.

“Clumped distribution, termed ‘patchiness,’ is one of the most ubiquitous characteristics of oceanic zooplankton,” said Genin, lead author of the Science paper. “Aggregations (of the tiny animals) are found on all scales, from millimeters to areas covering hundreds of kilometers. Understanding the mechanisms that produce zooplankton patchiness is a central objective in biological oceanography.”

The new imaging system, Fish TV, uses multibeam sonar to uniquely measure animal movement. The system allowed the researchers to analyze the swimming behavior of more than 375,000 individual zooplankton swimming against vertical currents. Swimming in this manner allows the plankton to keep their depth, a behavior which was postulated long ago but had never been measured in the ocean until now. The scientists say it is remarkable that the small zooplankton are capable of remaining at a constant depth with such high precision in the face of such strong vertical currents. The ecological implications of this behavior carry far-reaching consequences for predatory fishes, whales and humans.

The results of the multinational research project were captured during three experiments lasting several weeks at two sites in the Red Sea, near the coral reef of Eilat in Israel and at Ras Burka off the coast of Egypt’s Sinai Peninsula. At the sites, scuba divers attached Fish TV’s sonar head (“transducer”) on a large underwater tripod, raised some 20 feet above the sea floor. The transducer was cabled to a control and data-acquisition unit consisting of a computer and other electronic hardware. Fish TV’s transmitters sent out 1.6 megahertz “pings” that bounced off the zooplankton and returned data to the instrument’s receivers. It’s a system not unlike those used in ultrasound procedures for biomedical applications.

“That small zooplankton are capable of remaining at a constant depth with a precision of centimeters, sometimes in the face of strong vertical currents, implies that these organisms have extremely sensitive depth sensors, the nature of which is yet unknown,” said Genin. “That this depth-keeping behavior has evolved in so many different species implies that this energetically demanding behavior provides significant, yet poorly understood benefits. Revealing those benefits and the nature of depth sensing will be a major and exciting challenge for future research in zooplankton ecology and evolution.”

The research was funded by the German Ministry for Education and Research through the “Red Sea Program” and the U.S.-Israel Binational Science Foundation. Jaffe was supported by the National Science Foundation, the Office of Naval Research and California Sea Grant.