Low-Reynolds-number swimming at pycnoclines
Low-Reynolds-number swimming at pycnoclines Amin Doostmohammadi and Arezoo Ardekani Aerospace and Mechanical Engineering University of Notre Dame
CBET-1066545
Introduction Vertical variations in water density, or “pycnoclines,” occur ubiquitously in aquatic and marine environments, due to gradients in temperature (thermoclines) or salinity (haloclines). Pycnoclines can trigger a wide range of environmental and oceanographic processes. In oceans and lakes, intense biological activity and accumulation of organisms and particles are associated with pycnoclines. For example, formation of phytoplankton blooms is often correlated with stratification, and these blooms can enhance CO2 sequestration or disrupt water supply systems. Stratification can also affect organism migration; some species of euphausiids do not cross thermoclines.
Haloclines can act as a barrier to the vertical migration of dinoflagellates (common motile phytoplankton).
We derive fundamental solutions of low Reynolds number flows in a stratified fluid, including the case of a point force (Stokeslet) and a stresslet. The results show that the appropriate length scale to determine whether stratification affects motion is set by the competition of buoyancy, diffusion and viscosity, and is O (100 μm–1 mm) in aquatic environments (Ardekani. and Stocker, PRL, 2010)
Phytoplankton blooms off Vancouver Island (Steve Roper)
depth
Nutrient concentration contours Homogenous Stratified
Homogenous Stratified
density We explore the role of stratification through numerical simulations of an archetypal low-Reynolds-number swimmer, the “squirmer.” The tangential velocity on the swimmer’s surface is described as
v
sin
2
depth
Results and discussion
sin 2
At pycnoclines organisms are exposed to different environmental conditions compared to the bulk water column, including reduced turbulence, slow mass transfer, and high particle and predator concentrations. Here we show that, at an even more fundamental level, the density stratification itself can affect microbial ecology at pycnoclines, by quenching the flow signature, increasing the energetic expenditure, and stifling the nutrient uptake of motile organisms. Density contours for a pusher
Pusher
density
Density contours for a puller
Flow induced by a squirmer in a stratified fluid and its effects on the density field is shown here. These results demonstrate an unexpected effect of buoyancy on low-Reynolds-number swimming, potentially affecting a broad range of abundant organisms living at pycnoclines in oceans and lakes (Doostmohammadi et al., PNAS, 2012).
Puller
The results suggest that swimming at pycnoclines is a trade-off between a decreased foraging efficiency and a smaller risk of predation. Importantly, however, decreased foraging efficiency might frequently be offset by an increase in nutrient resources, caused by the accumulation of organisms and particles at pycnoclines.
Stratification can significantly affect the swimming speed of microorganisms. We identify the Richardson number—the ratio of buoyancy forces to viscous forces—as the fundamental parameter that quantifies the effects of stratification.
energy expenditure
Conclusion As we dive deeper into the world of motile microorganisms, we are beginning to gain an appreciation for how their physico-chemical environment influences their movement behavior. Here we have shown that a frequent feature of the physical environment— density stratification—can have direct ecological consequences on motility-related traits, including energetic expenditure, nutrient uptake, and the risk of predation. These aspects must be considered when the trade-offs that determine the fitness advantage of motility in the microbial world are evaluated.
motility
detectability
Collaborator: Roman Stocker, MIT
Density stratification
CBET-1066545
Introduction Vertical variations in water density, or “pycnoclines,” occur ubiquitously in aquatic and marine environments, due to gradients in temperature (thermoclines) or salinity (haloclines). Pycnoclines can trigger a wide range of environmental and oceanographic processes. In oceans and lakes, intense biological activity and accumulation of organisms and particles are associated with pycnoclines. For example, formation of phytoplankton blooms is often correlated with stratification, and these blooms can enhance CO2 sequestration or disrupt water supply systems. Stratification can also affect organism migration; some species of euphausiids do not cross thermoclines.
Haloclines can act as a barrier to the vertical migration of dinoflagellates (common motile phytoplankton).
We derive fundamental solutions of low Reynolds number flows in a stratified fluid, including the case of a point force (Stokeslet) and a stresslet. The results show that the appropriate length scale to determine whether stratification affects motion is set by the competition of buoyancy, diffusion and viscosity, and is O (100 μm–1 mm) in aquatic environments (Ardekani. and Stocker, PRL, 2010)
Phytoplankton blooms off Vancouver Island (Steve Roper)
depth
Nutrient concentration contours Homogenous Stratified
Homogenous Stratified
density We explore the role of stratification through numerical simulations of an archetypal low-Reynolds-number swimmer, the “squirmer.” The tangential velocity on the swimmer’s surface is described as
v
sin
2
depth
Results and discussion
sin 2
At pycnoclines organisms are exposed to different environmental conditions compared to the bulk water column, including reduced turbulence, slow mass transfer, and high particle and predator concentrations. Here we show that, at an even more fundamental level, the density stratification itself can affect microbial ecology at pycnoclines, by quenching the flow signature, increasing the energetic expenditure, and stifling the nutrient uptake of motile organisms. Density contours for a pusher
Pusher
density
Density contours for a puller
Flow induced by a squirmer in a stratified fluid and its effects on the density field is shown here. These results demonstrate an unexpected effect of buoyancy on low-Reynolds-number swimming, potentially affecting a broad range of abundant organisms living at pycnoclines in oceans and lakes (Doostmohammadi et al., PNAS, 2012).
Puller
The results suggest that swimming at pycnoclines is a trade-off between a decreased foraging efficiency and a smaller risk of predation. Importantly, however, decreased foraging efficiency might frequently be offset by an increase in nutrient resources, caused by the accumulation of organisms and particles at pycnoclines.
Stratification can significantly affect the swimming speed of microorganisms. We identify the Richardson number—the ratio of buoyancy forces to viscous forces—as the fundamental parameter that quantifies the effects of stratification.
energy expenditure
Conclusion As we dive deeper into the world of motile microorganisms, we are beginning to gain an appreciation for how their physico-chemical environment influences their movement behavior. Here we have shown that a frequent feature of the physical environment— density stratification—can have direct ecological consequences on motility-related traits, including energetic expenditure, nutrient uptake, and the risk of predation. These aspects must be considered when the trade-offs that determine the fitness advantage of motility in the microbial world are evaluated.
motility
detectability
Collaborator: Roman Stocker, MIT
Density stratification
