Settling and Swimming in Stratified Fluids: Rigid Particles to Self-Propelled Organisms

Microorganisms play pivotal functions in the trophic dynamics and biogeochemistry of aquatic ecosystems. Their concentrations and activities often peak at localized hotspots, an important example of which are pycnoclines, where water density increases sharply with depth due to gradients in temperature or salinity. 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. We identify the Richardson number−the ratio of buoyancy forces to viscous forces−as the fundamental parameter that quantifies the effects of stratification. In addition, the results of our direct numerical simulations of the sedimentation of particles show that the presence of vertical density gradients in the water column can substantially affect the settling dynamics of a particle, interaction between pair of particles, and settling rates and microstructure of suspension of particles. Moreover, the role of stratification on reorientation of elongated particles at density interfaces is scrutinized.