Fluid Transport by Aggregations of Small Swimming Organisms

Diel vertical migration of zooplankton has been proposed to affect global ocean circulation to a degree comparable to physical phenomena. Almost a decade after shipboard measurements showed high kinetic energy dissipation rates in the vicinity of migrating krill swarms, the hypothesis that biogenic mixing is relevant to ocean dynamics and local fluid transport has remained controversial due to the inability to directly measure the efficiency of this biological process. In situ field measurements of individual swimming jellyfish have demonstrated large-scale fluid transport via Darwinian drift, but it has remained an open question how this transport mechanism is manifested in smaller species of vertically-migrating zooplankton that are sufficient in number to be accountable in the dynamics. The goals of the present study are, first, to devise and implement experimental instruments and develop methodologies to investigate this biological process in a laboratory setting and, second, to determine whether efficient fluid transport mechanisms become available during vertical collective motion and, if so, analyze how energy is distributed within the flow. By leveraging the phototactic abilities of zooplankton, a multi-laser guidance system was developed to achieve controllable vertical migrations of A. salina concurrently with laser velocimetry of the surrounding flow. Measurements show that the hydrodynamic interactions between neighboring swimmers during vertical migration result in the development of a pronounced jet opposite to animal motion. In non-stratified fluid, this hydrodynamic feature is shown to trigger a Kelvin-Helmholtz instability that results in the generation of eddy-like structures with characteristic length scales much larger than the individual size of the organisms. Experiments in a thermally stratified water column also display the presence of a downward jet despite the strong stable stratification. Furthermore, overturning regions larger than the size of an individual organism are observed adjacent to the migrating aggregation, suggesting an alternate energy transfer route from the small scale of individual swimmers to significantly larger scales, at which mixing can be efficient via a Rayleigh-Taylor instability. The computed velocity spectrum is consistent with these findings and displays energy input at scales larger than the body length of a single swimmer. The mixing efficiency, inferred from the spectral energy distribution with and without stratification, matches experimentally achieved mixing efficiencies via a Rayleigh-Taylor instability within a stable stratification. According to our findings, biogenic mixing does have the potential to redistribute temperature, salinity and nutrients effectively. We propose the employment of laser control to examine additional species as well as alternative oceanic environments and interrogate its effect on the efficiency of biogenic mixing