CEE research could help predict harmful algal blooms like red tide
February 19, 2009
By Denise Brehm
Civil & Environmental Engineering
Not far beneath the ocean’s surface, tiny phytoplankton swimming upward in a daily commute toward morning light sometimes encounter the watery equivalent of Rod Serling’s Twilight Zone: a sharp variation in marine currents that traps billions of these single-celled organisms and sends them tumbling head over heels until a shift in wind or tide alters the currents and sets them free.
Scientists are aware of these thin layers of single-celled creatures and their enormous ecological ramifications, but until now, they knew little about the mechanisms responsible for their formation.
The explanation by researchers in MIT’s Department of Civil and Environmental Engineering 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, a well-known example of which is red tide. The work also opens new perspectives on other phenomena, like predatory feeding by larger organisms at these ecological hotspots.
“Phytoplankton are incredibly small. You would have to stack about 10 back to back to equal the width of a single human hair,” said PhD student William Durham, co-author on a paper appearing in the Feb. 20 issue of Science. “But 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. Many species can swim, but this fact is often neglected by researchers because phytoplankton are slow compared to ocean currents. However, we have shown that their motility can play a crucial role by concentrating them into dense assemblages, known as thin layers.”
In the Science paper, Durham, Professor Roman Stocker and University of Arizona physics Professor John Kessler explain how adjacent layers of water moving at different speeds produce a “shear” flow that traps the phytoplankton as they swim into it. These layers form in the top 50 meters of the ocean and can be anywhere from a few centimeters to a couple of meters thick, span several kilometers horizontally and last hours, days or weeks.
“Our research pinpoints a mechanism for the formation of these thin layers of phytoplankton, which 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,” said Stocker, the Doherty Assistant Professor of Ocean Utilization at MIT.
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.
And when a toxic species of phytoplankton gets trapped in a thin layer, that layer can spawn a harmful algal bloom — an explosion in the population of toxic phytoplankton