Turbulence ‘unmixes’ motile phytoplankton [with video]
Patchiness in the distribution of tiny plants known as phytoplankton promotes many important ecological interactions in the marine food web. Our study reveals that turbulent fluid motion, normally synonymous with mixing, triggers intense small-scale patchiness in the distribution of motile phytoplankton.
HFSP Long-Term Fellow William Durham and colleaguesauthored on Fri, 15 November 2013
Nearly all life in the ocean depends on microscopic organisms called phytoplankton, which compose the bottom of the marine food chain and cumulatively produce half of the world’s oxygen. The spatial distribution of phytoplankton is notoriously patchy over a wide diversity of length scales, with regions of enhanced cell concentration spanning from a few centimeters to hundreds of kilometers wide. While phytoplankton patches at scales larger than a kilometer are usually sustained by localized phytoplankton growth, the mechanisms responsible for the formation of the smallest phytoplankton patches are more elusive. However, ocean sampling suggests a key ingredient for patchiness may be cell movement: motile phytoplankton, which propel themselves using whip-like flagella, are often found to be much more patchily distributed at centimeter scales than phytoplankton that cannot swim.
Our paper presents a new biophysical mechanism by which motility drives centimeter scale phytoplankton patches through its interaction with turbulence, the disordered fluid motion that is ubiquitous in the ocean. While turbulence is normally associated with mixing, for example it mixes patches of cream with coffee, we find that turbulence reorients motile cells such that they swim towards specific regions of the flow, forming patches. We test this hypothesis using laboratory experiments, which reveal that the motile phytoplankton Heterosigma akashiwo, forms strong patchiness when it swims within a vortex, the simplest constituent of turbulence. However, when the same phytoplankton are killed, they no longer form patches, highlighting the essential role of motility. To understand how phytoplankton respond to realistic marine turbulence we used a supercomputer to simulate millions of cells as they swim through a constantly evolving flow field composed of many superimposed vortices. We find that motile cells form patches that are more than ten times more concentrated than observed for non-motile cells, suggesting that this mechanism radically affects the centimeter scale distribution of phytoplankton.
Figure: Supercomputer simulations reveal that non-motile phytoplankton cells remain randomly distributed in a turbulent flow (left), while motile cells form dense patches (right, shown in blue) where the local cell concentrations are increased more than ten-fold.
Most ecological interactions in plankton are limited by the rate at which organisms encounter one another. This new ‘unmixing’ mechanism – which dramatically reduces the distance between individuals – likely has profound consequences on the marine ecosystem. Patchiness can increase the rate at which phytoplankton are consumed by predators, thus this mechanism may impact the amount of energy transferred up the marine food chain and, ultimately, affect populations of larger organisms like fish. On the other hand, patchiness is not always detrimental for phytoplankton: to survive harsh winter conditions phytoplankton need to reproduce sexually, and finding a mate is much easier in a dense patch. Intriguingly, because our mechanism relies on active motility, it opens the possibility that motile phytoplankton can actively tune their spatial distribution by altering their motility, for example, to rapidly accumulate and find mates during times of sexual reproduction or remain randomly distributed during times when there are many predators in the local vicinity. Future experiments may reveal how the interaction between motility and turbulence affect the survival of phytoplankton populations, and ultimately the productivity of the marine ecosystem that they support.
Turbulence drives microscale patches of motile phytoplankton. Durham, W. M., Climent, E., Barry, M., De Lillo, F., Boffetta, G., Cencini, M., Stocker, R. (2013) Nature Communications. 4.