Morphology alters fluid transport and the ability of organisms to mix oceanic waters

Abstract

Mixing in the ocean is opposed by the stratification of fluid, such that density of seawater increases with greater depth. The mechanisms by which mixing occurs have been attributed largely to physical processes that include atmospheric forcing, tides, and internal waves. Biogenic mixing, another potential source of mixing in the ocean, may generate significant transport of fluid during diel vertical migrations of organisms. Biogenic mixing is not limited to the near-surface or to regions of rough bottom topography, as are other physical mixing processes, and may contribute significantly to the energy budget of mixing in mid-ocean. "Fluid drift", a mechanism first described by Charles Galton Darwin, has been identified as a mechanism that allows for long-distance, vertical transport of fluid by the smallest of swimming organisms. However, little is known about how fluid drift varies with morphology and behavior of swimming organisms. We conducted numerical simulations of theoretical and experimentally measured flows of swimming medusae (Phyllorhiza sp.), and compared the volume of the drift induced by these flows. Our numerical simulations of fluid drift showed that morphology coupled with swimming behavior alters the transport of fluid both spatially and temporally. Given empirical velocity field data, the methods presented here allow us to systematically compare fluid transport across taxa, and enable us to deduce the potential of swimming organisms to influence fluid transport.