Modeling the magnetic signature of diel vertical migrations of zooplankton

Diel vertical migration (DVM) of zooplankton can cause velocity fluctuations and a respective increase in the dissipation rate of turbulent kinetic energy dependent on zooplankton concentration (Dean et al. 2016). In this work, we used a 3D non-hydrostatic computational fluid dynamics (CFD) model (ANSYS Fluent) with Lagrangian particle injections (proxy for migrating organisms) to simulate the effect of turbulence generation by DVM. We tested a range of organism concentrations. The simulation at the extreme (10,000 organisms/m3) concentration of zooplankton showed an increase in dissipation rate of turbulent kinetic energy by two to three orders of magnitude during DVM over background turbulence (10-8 W kg-1). At the low (1000 organisms/m3) concentration, almost no turbulence above the background level was produced by DVM in the model. Seawater is an electric conductor; as a result, the motion of seawater in the magnetic field of the Earth induces electrical currents and, consequently, secondary magnetic fluctuations. Therefore, the turbulence produced by DVM can have a measureable magnetic signature. We have applied a magnetohydrodnamics (MHD) module to the CFD model to test this hypothesis. The MHD model results indicate that DVM of the extreme concentration of zooplankton create a magnetic signature on the order of 0.1 nT, which is relatively small but are well within the range of modern magnetometers.