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Researchers explain how patches of photosynthetic plankton form at sea.

Tiny photosynthetic plankton sometimes swim into the watery equivalent of Rod Serling's Twilight Zone: a sharp variation in marine currents that traps billions of the organisms until a shift in wind or tide sets them free by altering the currents.

A research team that includes John O. Kessler of The University of Arizona now explains how some spontaneously developing ocean currents set the stage for swimming single-celled organisms known as phytoplankton to accumulate.    

Adjacent layers of water moving at different speeds produce a "shear" flow that traps the tiny swimmers between the layers, according to the new research.  

Concentrating the organisms requires currents that move horizontally in opposite directions, such as would happen if the wind blows the upper layer one way while the tide moves another.  

The aggregations of the tiny organisms form in the ocean's top 50 yards. The thickness of the patches can range from less than an inch to several yards. The patches can span as much as several miles horizontally and last hours, days or weeks.

The new finding from Kessler and his colleagues at MIT may help predict the occurrence of algal accumulations, including harmful ones such as red tides.   

"These algal blooms come and go and up to now, no one realized that physical, for example, hydrodynamical, mechanisms can cause these organisms to concentrate themselves," said Kessler, a UA professor emeritus of physics. "This is a piece of the puzzle." 

He emphasized that these processes only work on phytoplankton that are swimming.

First author William M. Durham said, "Many species can swim, but this fact is often neglected by researchers because phytoplankton are slow compared to ocean currents."

Phytoplankton are the base of marine food chains. Such concentrations are analogous to watering holes in a savanna, said co-author Roman Stocker. The layers of phytoplankton "draw a wide range of organisms and thus play a disproportionate role in the ecological landscape."

Durham, a doctoral student at MIT in Cambridge, Mass., Kessler, and Stocker, the Doherty Associate Professor of Ocean Utilization at MIT, published their paper, "Disruption of Vertical Motility by Shear Triggers Formation of Thin Phytoplankton Layers," in the Feb. 20 issue of the journal Science.

The National Science Foundation, the U.S. Department of Energy and the MIT Earth Systems Initiative funded the research.

Kessler has been working on the physics of how one-celled organisms move in aquatic environments for more than 20 years.

"From the beautiful swimming patterns made by the algae, I figured out how you could hydrodynamically choreograph the swimming directions of the algae," he said.

About two years ago, he was visiting MIT to give a seminar and talked to Stocker about the biological physics of currents and algae.

As a result, Stocker and Durham began laboratory experiments based on mathematical models extending Kessler's earlier results. Using video-microscopy, Durham and Stocker tracked the movements of individual cells as they become trapped in the layers of shear.

The team's research showed that the swimming cells cannot escape the layers on their own. Once trapped, phytoplankton are at the mercy of the flow and must wait for the shear to decrease before they can swim out of the Twilight Zone.

"Phytoplankton are incredibly small. You would have to stack about 10 back to back to equal the width of a single human hair," Durham said. "Despite their small size, they play an outsized role in the environment. They form the base of the marine food web and cumulatively produce half the world's oxygen."

Because phytoplankton have different shapes and swimming abilities, one species may be able to swim through a layer of shear that will capture another. Therefore, each species could be trapped in a different level of shear, creating an oceanic layer-cake effect – a boon for other organisms that feed on specific species.

When a toxic species of phytoplankton becomes trapped, that can spawn a harmful algal bloom – an explosion in the population of toxic phytoplankton that sickens or kills larger animals that ingest the cells.

Harmful algal blooms occur near coastal areas and are a major source of social and economic concern, because they cause billions of dollars in annual losses to fishing and recreational industries worldwide.

This story is modified from one written by Denise Brehm of the MIT department of civil & environmental engineering.