Physics could unite plankton

The oceanic version of wind shear can disorient marine microorganisms and trigger formation of thin, densely populated layers

Phytoplankton sometimes come together in the ocean because they can’t tell which way is up, new research suggests.

TAKING A TUMBLE Phytoplankton often accumulate in thin layers below the ocean surface. The boundary between two currents could be where. This illustration shows upward swimming plankton becoming disoriented and stalling once they hit a sharp change in current velocity. IMAGE CREDIT: Credit: G. Gorick, W. Durham and R. Stocker

Oceanographers have long known that certain species of phytoplankton often form kilometer-wide layers only a few centimeters thick. Researchers have sought an explanation because these layers are often the source of toxic algal blooms known as red tides.

Now researchers have one idea for how the layers form: Lab experiments hint that conditions inside the thin layer of water separating a surface current from a deeper one flowing in a different direction can disorient the phytoplankton, disrupting their swim to the surface and causing them to accumulate in profusion at a single depth.

These microscopic and typically single-celled algae at the base of the sea’s food chain often migrate to the surface in the daytime to take advantage of sunlight. Then they drop back to the safety of the depths at night. Because the individuals in many of the species are lopsided either in body shape or in weight distribution, they can discern up from down, says Roman Stocker, a microbial ecologist at MIT. But lab tests indicate that an organism’s simple sense of direction can be easily confused in certain circumstances, Stocker and his colleagues report in the February 20 Science.

The researchers investigated marine conditions that mimic wind shear in the atmosphere, a situation that arises when the speed or direction of the wind at one altitude is dramatically different from that of an adjacent layer. As organisms pass through the narrow interface between such layers in water, the sharp change in velocities causes individual plankton to tumble, Stocker says.

In the ocean, such shear forces could cause a small, upward-swimming organism to become disoriented — it literally wouldn’t know which way is up, he notes. When a loose group of plankton ascends into an ocean layer where strong shear conditions exist, they swim in random directions and become trapped. The layers didn’t form in experiments with dead plankton, further bolstering the idea that the formation has to do with swimming.

Plankton concentrations in these layers are often hundreds of times higher than those in waters above and below, says Stocker. The plankton layers are biological hot spots, “akin to a watering hole on the savannah,” he notes. Fish larvae and other small creatures are drawn to these concentrations of prey.

The new theory for thin layer formation “is a really clever idea,” says Peter J.S. Franks, a biological oceanographer at the Scripps Institution of Oceanography in La Jolla, Calif. Strong shear forces are a reasonable mechanism for how plankton might become disoriented, he notes, “but it’s a big extrapolation from the lab to the ocean.” Sharp changes in velocity across distances smaller than a microbe would likely cause turbulence that would rip such layers apart almost as quickly as they could form, he notes.

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