Examining behavior of a cruise swimming copepod in a Burgers' vortex

Copepods continuously encounter small-scale turbulent fluid motions in their habitat, which can be conceptualized as a random size and orientation distribution of stretching vortices. Any change in the turbulence intensity affects the copepod’s vertical distribution in the water column, and the response is species specific. The purpose of this study is to examine the interaction of individual copepods (Temora longicornis) with small-scale turbulent fluid motions, which are ideally described by Burgers’ vortex model. The vortex is generated in the laboratory with its axis aligned in either the horizontal and vertical directions to examine the directional swimming response of T. longicornis, which depends on its mechanosensory setae architecture and its orientation with respect to gravity. The turbulence intensity is classified into 4 levels, corresponding to target turbulent dissipation rates of 0.002 (level 1) to 0.25 (level 4) cm²/s³ in the coastal environments. The swimming behavior of T. longicornis is assessed in the presence of the vortex treatment and compared to the control level (no flow). The results show that T. longicornis exhibited a minimal response to level 1 and level 2 intensities, but significantly changed its swimming behavior in higher turbulent intensities.