The divers frequented the underwater setup to sample the animals, reorient the transducer and measure the vertical currents using various methods, including releasing and tracking fluorescent dye and with state-of-the-art acoustic current meters.
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.
The scientists say the ecological implications of the zooplankton's depth-keeping behavior carry far-reaching consequences for predatory fishes, whales and humans.
Predators can dependably locate zooplankton aggregations by tracking well-defined cues, for example, sharp temperature gradients that occur across downwelling fronts where cold and warm waters meet. Vertical currents are common in many oceanic regions, including mid-ocean fronts, shelf breaks, submarine canyons and submerged banks.
"That small zooplankton are capable of remaining at a constant depth with a precision of centimeters, sometimes in the face of strong vertical currents, is incredible," said Genin. "It implies that these organisms have extremely sensitive depth sensors, the nature of which is yet unknown. That 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."