Turbulent Lives – May/June 2009
The watery world in which phytoplankton live has been described as being the consistency of honey or molasses for organisms that are so small and slow. It is a world in which the organisms’ movement, ability to flex (in the case of chains), reproduce, eat, escape predation and, ultimately, sink to the sea floor are dependent on the turbulence of their environment.
Most phytoplankton live in a world of low Reynolds numbers, where motion stops as soon as propulsion stops, and the dominant force is friction from the stickiness of water molecules to each other.
Low Reynolds number worlds defy our intuition. For instance, if a semi passes a car in an adjacent lane of the highway, passengers in the smaller vehicle will feel the pull of the wind. But if that truck passes two lanes away, the rush of the wind is indistinguishable. For life at low Reynolds numbers, a diatom or particle experiences substantial fluid motion 100 lanes away, and even more car lengths ahead and behind.
“If you drop a particle and it falls through stagnant water, the object drags substantial volumes of water with it. That’s not what happens at high Reynolds number,” says Jumars. “That’s why it’s important to learn how forces are transmitted through such a continuous medium.”
Jumars and Karp-Boss are testing the hypothesis that diffusing momentum and vorticity on the dissipation scales of turbulence are major contributors to relative motion between water and phytoplankton.
“We know that global warming is going to stabilize the ocean by decreasing the turbulence intensity,” Jumars says. “Understanding what that means to the base of the food web is critical. A signature of climate change is more intense storms at certain places and times. We also have to understand the other extreme.”
Better understanding of biological fluid dynamics will provide insight into the fundamental physics – including motion and behavior of nonspherical shapes – of phytoplankton and other complex particles in turbulent environments.
“The big question is: What processes affect distribution and species composition of phytoplankton?” says Karp-Boss. “We don’t have a good mechanistic understanding of the processes that select for certain species. Hence, our ability to predict who will be there and when is limited.”
The morphological diversity of diatoms in the world is screaming to tell us something, but we don’t yet know what it is, Jumars says.
“We know that their shapes have consequences, but we don’t know how and what,” says Jumars. “They are nature’s art and design, but we don’t understand their function. In design, shape has a function, and that’s our working hypothesis here, too.”
by Margaret Nagle
May – June, 2009
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