Turbulence - copepod interaction: Acartia tonsa behavioral response to Burgers' Vortex
Young, David Louis
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The purpose of this study is to quantify the effect of finescale turbulence on copepod behavior in order to shed light on the influence of turbulence on copepod distribution. Specifically, the study will examine the behavioral response of the marine copepod Acartia tonsa to a steady state Burgers' vortex intended to mimic the characteristics of a turbulent vortex (Jumars et al. ) that a copepod is likely to encounter in the coastal zone. A laboratory apparatus was constructed to create a Burgers' vortex with size and strength consistent with turbulence vortices in the coastal zone (and relevant to the marine copepod species). The radius, circulation, and axial strain of the Burgers' vortex were specified to match typical dissipative vortices corresponding to two turbulence intensity levels. The levels are described by Webster et al.  as Level 2 (ϵ = 0.009 cm²/s³) and Level 3 (ϵ = 0.096 cm²/s³), which span an apparent behavior transition in copepods [Yen et al., 2008]. Tomographic particle image velocimetry (Tomo - PIV) was performed to calibrate the device and verify that it produces the desired vortex characteristics, as well as to provide a three dimensional velocity vector field to compare with behavioral assays. The laboratory apparatus, dubbed the "Burgers' Vortex Apparatus", accurately reproduces the appropriate vortex characteristics of the Turbulence Level 2 and 3 vortex cartoons. Copepod behavioral assays were conducted with Acartia tonsa. When exposed to these vortices, Acartia tonsa did not exhibit a meaningful behavioral response to the Level 2 vortices, but drastically altered their swimming behavior in the presence of Level 3 vortices. In the presence of a Turbulence Level 3 vortex, Acartia tonsa increased relative swim speed, decreased turn frequency, increased the angle of alignment with the vortex axis, increased net-to-gross displacement ratio, and increased escape acceleration (relative to control). These alterations in swimming kinematics all served to move the animal away from the vortex core.