The secret life of plankton

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Posted by on June 04, 2005 at 20:17:50:

From above, the ocean appears vast and uniform but beneath the surface, an infinite number of tiny, nearly transparent animals called zooplankton are swept into clusters and patches by ocean currents. Serving as the basis of the ocean's food chain, they sustain predators from invertebrates to whales. Fishermen also depend on them for their catches. Evidence of the remarkable dynamics involved in the formation of large aggregations of microscopic animals has been provided for the first time by a team from Israel, the US and Germany.

The researchers, headed by Prof. Amatzia Genin of the Hebrew University and the Interuniversity Institute for Marine Sciences in Eilat, discovered that zooplankton drift with the current, as their name implies (planktos is Greek for "drifting"), but only in a horizontal direction. In the vertical, these creatures show a great ability to "go against the flow." Although scientists and fishermen have long known that zooplankton spend their life suspended in a constantly flowing environment, an understanding of their responses to current has remained elusive, mainly due to technological limitations in tracking the minuscule creatures.

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 made it possible to track several hundred thousand zooplankton at two coastal sites in the Red Sea. The team also included Genin's 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 Science, show that these small animals effectively keep their depth by "treadmilling" against "upwelling" and "downwelling" currents at speeds of up to tens of body lengths per second. In the ocean, downward-flowing water is always accompanied by horizontal movement, forming a convergence or "downwelling" zone. When zooplankton swim against such a downward current, they form patches as more and more individuals are concentrated in the downwelling zone.

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

The results of the multinational 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 these sites, scuba divers attached Fish TV's transducer to a large underwater tripod nearly seven meters above the sea floor. This in turn was connected to a control and data-acquisition unit, with transmitters sending out 1.6 megahertz "pings" that bounced off the zooplankton and returned to the instrument's receivers a system similar to medical ultrasound scans.

"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 still unknown," said Genin. "That this depth-holding behavior has evolved in so many species implies that such 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 research in zooplankton ecology and evolution."

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