Effect of gravity-induced fluid inertia on the accumulation and dispersion of motile plankton settling weakly in turbulence

Abstract

We investigate the effect of gravity-induced fluid inertia on motile plankton cells settling weakly through isotropic turbulence using direct numerical simulations (DNS). Gyrotaxis arises from the gravity-induced fluid inertial torque, leading to upward migration of the settling elongated micro-organisms when their swimming speed exceeds the set- tling speed. Preferential sampling and small-scale fractal clustering of plankton cells are studied over a wide range of swimming speeds and aspect ratios. It is found that orientation fluctuation induced by the effect of the fluid inertia and preferential alignment with turbulent strain are the most important factors affecting the statistics, which are responsible for determining the optimal shape. For strong gyrotaxis, the organisms tend to form noticeable clusters in the vertical direction. An investigation of the dispersion reveals that the fluid inertial effects contribute to the enhancement of the long-time vertical dispersion of the organisms by increasing their root-mean-sqaured (RMS) velocity. Our results show how the fluid inertial effects can influence clustering and dispersion statistics of the organisms in turbulence, which turns out to provide an environment conducive to their survival.

Monitoring Bioindication of Plankton through the Analysis of the Fourier Spectra of the Underwater Digital Holographic Sensor Data

“…In accordance with this self-organization of the flow, the pulsation component generates large relatively ordered vortex structures absorbing the suspension, including plankton, due to a change in the direction of the velocity vector. There are quite a few publications on the interaction of mesozooplankton and these vortices [53][54][55][56][57][58][59][60][61], which show that this interaction is non-linear and affects such plankton processes as nutrient absorption, grazing, predation, and reproduction. However, it is well known that the fine and mesoscale turbulent structure of water masses combined with plankter grazing by predators leads to a characteristic spot-like nature of plankton spatial distributions, which is called the mosaic or fractal structure of biocenoses [62,63].…”