Watery trap stalls phytoplankton’s commute

February 2009

PROBLEM

Thin layers of phytoplankton that form in the top 50 meters of the ocean are analogous to watering holes in a savanna — localized areas of concentrated resources that draw a wide range of organisms and thus play a disproportionate role in the ecological landscape. These layers of single-celled, photosynthetic organisms can be a few centimeters to several meters thick; span tens of kilometers horizontally; and last hours, days or weeks. When a toxic species of phytoplankton forms a thin layer, that layer can generate a harmful algal bloom — an explosion in the population of toxic phytoplankton that sickens or kills the larger animals that ingest the cells. Because these tiny creatures form the base of the marine food web and cumulatively produce half the world’s oxygen, the thin layers have enormous ecological ramifications. But until now, scientists knew little about the mechanisms responsible for their formation.

APPROACH

Roman Stocker of MIT’s Department of Civil and Environmental Engineering studies the interplay of motile microorganisms and fluid flow by creating microhabitats in his laboratory and using video-microscopy to record the cells. Many species can swim, but their motility is often disregarded because the plankton are slow compared to ocean currents. In recent work, Stocker and Ph.D student William Durham explored the connection between the movements of motile phytoplankton and formation of thin layers using experiments and mathematical modeling.

FINDINGS

Stocker and Durham demonstrated that thin layers can form when the vertical migration of phytoplankton is disrupted by hydrodynamic shear generated by tidal currents, wind stress or internal waves. In their quest for sunlight, many phytoplankton exhibit a natural tendency to swim upwards towards the ocean’s surface.  However, these new findings show that this vertical motility can be thwarted in regions where the shear exceeds a critical value.  Shear, in this case strong variations in horizontal water velocity, causes the cells to tumble end over end, trapping them at depth. As phytoplankton attempt to transverse a zone of enhanced shear on their daily commute to the morning light, they could be snared by the flow, confining the billions of cells to a region only a few tens of centimeters in depth. 

Using video-microscopy, Durham and Stocker tracked the movements of individual cells as they become trapped in the layers of shear. They also mathematically modeled the movements of the swimming cells and proved that they cannot escape the layers until the shear decreases. Because motile phytoplankton have different morphologies and swimming abilities, one species may be able to swim through a layer of shear that will capture another. This means that each species could be trapped in a different level of shear, creating a sort of oceanic layered-cake effect, a boon for zooplankton or young fish that feed on specific species.

IMPACT

Red tides and other forms of harmful algal blooms are a major source of social and economic concern, particularly near coastal areas, as they create billions of dollars in annual losses to fishing and recreational industries worldwide and they are occurring more frequently.

The explanation by Stocker and Durham of how these common, startlingly dense layers of photosynthetic phytoplankton form, moves the scientific community a step closer to being able to predict harmful algal blooms. The work also opens new perspectives on other phenomena, like predatory feeding by larger organisms at these ecological hotspots.

MORE

A paper on this work by Stocker, Durham and physics professor John Kessler of the University of Arizona appeared in Science on Feb. 20, the same day that Stocker’s promotion to associate professor was announced. Articles about the research also appeared in Science News and other media.

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