Buoyancy-dependent Flow Generation by Collectively Migrating Swimmers

Area of Science:

• Fluid dynamics
• Biological oceanography
• Animal behavior

Background:

• Collective vertical swimming by organisms like brine shrimp (Artemia salina) can create large-scale fluid flows.
• These flows are important for mixing and transport in stratified aquatic environments.
• The influence of environmental factors, such as fluid density, on these flows is not fully understood.

Purpose of the Study:

• To investigate how fluid density affects the flow fields generated by vertically migrating brine shrimp swarms.
• To quantify the relationship between swimmer behavior, fluid properties, and induced flow velocity.

Main Methods:

• Simultaneous three-dimensional tracking of individual brine shrimp.
• Particle image velocimetry (PIV) to measure the induced flow field.
• Controlled experiments varying fluid salinity (and thus density) during phototactic migrations.

Main Results:

• Induced velocity increased with buoyancy forcing, directly correlating with the density difference between shrimp and fluid (N(ρs - ρ)).
• Fluid density was a significant predictor of induced velocity, even when controlling for swimmer number, speed, and swarm width.
• A simplified actuator-disk model successfully predicted the first-order dependence of induced velocity on buoyancy and swimmer momentum.

Conclusions:

• Fluid density variations substantially alter the hydrodynamic impact of collective vertical migration in brine shrimp.
• Understanding these density-driven effects is critical for accurate modeling of mixing and transport in marine and freshwater systems.
• Environmental properties play a key role in modulating biological contributions to ocean mixing.