| Zooplankton Fight the Flow |
|
5 May 2005, for ScienceNOW Tiny marine creatures, known as zooplankton, regularly form vast aggregations hundreds of kilometers long. You might think these zooplankton conferences result solely from whims of ocean currents, but a new study in the 6 May issue of Science shows they swim hard to stay together. That's important because these dense patches are crucial feeding spots for larger predators, including whales. An early guess at the reason why zooplankton congregate, which mathematical models have shored up, was that zooplankton seek to stay at a constant depth, perhaps to up their chances of finding tiny algae and other food. If zooplankton—everything from minute crustaceans to sea urchin larvae—passively ride the ocean currents, they could be swept to less favorable depths. But if they swim against the currents to stay at constant depth, the models predicted, this would explain the aggregations. No one had been able to directly observe this mechanism, however. Now a study tracking hundreds of thousands of individual zooplankton shows they can swim prodigiously against the current to maintain their depth. The researchers, led by biological oceanographer Amatzia Genin of the Hebrew University in Israel, used a high-frequency, high-resolution sonar system to follow zooplankton as small as 1 millimeter across. By also tracking the water flow, the researchers found the bulk of these creatures react quickly to changing currents, up or down, fast or slow. "This is the first evidence that zooplankton are responding behaviorally to current," Genin says.The work provides "strong evidence" that the combination of vertical currents and vertical swimming are a key mechanism leading to zooplankton patches, says Cabell Davis, a zooplankton ecologist at Woods Hole Oceanographic Institution in Massachusetts. He speculates that congregating could also help zooplankton, which are otherwise sparsely spread, find mates. |