Marine copepod behavior responses in and near internal waves

The objective of this study is to provide insight to the bio-physical interaction and the role of biological versus physical forcing in mediating organism distributions in and near internal waves. A laboratory-scale configuration is presented with a density jump of 1 σ_t. Theoretical analysis of the two-layer system provided guidance to the target forcing frequency needed to generate a standing internal wave with a single dominant frequency of oscillation.  The results show a close match to the target wave parameters.  Marine copepod (mixed population of Acartia tonsa, Temora longicornis, and Eurytemora affinis) behavior assays were conducted for three different physical arrangements: (1) no density stratification (i.e. control), (2) stagnant two-layer density stratification, and (3) two-layer density stratification with internal wave motion.  Digitized trajectories of copepod swimming behavior indicate that in the control (case 1) the animals showed no preferential aggregation.  In the stagnant density jump treatment (case 2) copepods preferentially moved horizontally, parallel to the density interface.  In the internal wave treatment (case 3) copepods demonstrated loopy, orbital trajectories near the density interface.  Noted differences with simulated trajectories and a consideration of the potential hydrodynamic cues indicate that copepod behavior response has a substantial influence on the swimming trajectories in the internal wave region.