Buoyancy-dependent induced flow by vertically migrating swimmers

Collective vertical swimming may generate aggregate-scale flows that contribute to mixing and transport in stratified environments. The strength of these flows depends not only on swimmer behavior but also on environmental properties. Here we examine how fluid density affects flow generated by vertically migrating swarms of brine shrimp $\textit{Artemia salina}$. Using simultaneous three-dimensional swimmer tracking and particle image velocimetry, we measured swimmer kinematics and the induced flow field during phototactically driven migrations under four controlled salinity conditions. Induced velocity increased with buoyancy forcing and scaled with the parameter $N(\rho_s - \rho)$, where $N$ is the number of swimmers and $\rho_s - \rho$ is the density difference between swimmers and the surrounding fluid ($R^2 = 0.70$, $p = 5.9 \times 10^{-5}$). A multiple regression including swimmer number, swimming speed, fluid density, and the swarm Gaussian width confirmed that density remained a significant predictor of induced velocity after controlling for the other variables ($p = 0.012$, $R^2 = 0.82$). A simplified actuator-disk model captures the first-order dependence of induced velocity on buoyancy forcing and swimmer momentum, suggesting that environmentally driven density variations can substantially modify the hydrodynamic impact of collective vertical migration.