Biophysical Interactions in the Straits of Florida: Turbulent Mixing Due to Diel Vertical Migrations of Zooplankton

摘要

Diel vertical migrations (DVM) comprise the largest animal migration on the planet and are a phenomenon present in all bodies of water on Earth. A strong sound scattering layer undergoing DVM was observed in the Straits of Florida via a bottom-mounted Acoustic Doppler current profiler (ADCP) Workhorse Longranger 75 kHz (Teledyne RD Instruments) located at the 244 m isobath. ADCP average backscatter showed a clear periodicity corresponding with sunrise and sunset times indicating the presence of a nocturnal DVM. Analysis of the ADCP backscatter data indicated zooplankton swimming velocities were faster during sunrise than sunset times. In several cases the zooplankton swimming velocity appeared to be faster at the beginning of the descent, after which the swimming velocity decreased. Analysis of ADCP velocity data indicated a measureable decrease in the northward component of the current velocity field during migrations (sunrise and sunset) compared to three hours prior. This was presumably associated with an increase in drag due to turbulent friction associated with DVM. A non-hydrostatic computational fluid dynamics (CFD) model with injection of Lagrangian particles was utilized to simulate the effects of DVM on the velocity field and turbulence signature of the Florida Current. A domain simulating a section of the Florida Current was created and zooplankton were represented by particle injection with a discrete phase model. The model was run with and without particles, holding all other parameters the same, for comparison. Idealized temperature stratification and velocity profiles were set for both summer and winter conditions to observe seasonal differences. For each case, velocity and turbulence with particles were compared to results without particles to confirm the changes in profiles were due to the zooplankton (Lagrangian particles). In several cases there was an observable change in average x-velocity profiles due to the injection of particles into the domain. In all cases there was an observable increase in subgrid turbulent viscosity in the wake of the injected particles. This effect was much stronger in the winter case, most likely due to stratification of the water column which gave a near critical Richardson number. These results indicated that DVM does in fact have an effect on the velocity profile and turbulence signature in a strong current under certain conditions and that there was a seasonal difference due to stratification profiles.