Patchiness and depth-keeping of copepods in response to simulated frontal flows

When presented with a fine-scale upwelling or downwelling shear flow in a laboratory flume, 2 tropical copepods from the Red Sea, Acartia negligens and Clausocalanus furcatus, performed a set of behaviors that resulted in apparent depth-keeping and the potential for producing patchiness. Analyses of free-swimming trajectories revealed a behavioral threshold shear deformation rate value of 0.05 s(-1) for both species. This threshold marked the transition to a range of behaviorally relevant shear deformation rate values and triggered statistically significant changes in path kinematics (i.e. relative swimming speed and turn frequency) in the shear layer versus out-of-layer. Gross path characteristics (i.e. net-to-gross displacement ratio, NGDR, and proportional vicinity time, PVT) were also significantly different in the shear layer treatments compared to controls. The vertical net-to-gross displacement ratio (VNGDR) was introduced here to explain a spectrum of depth-keeping behaviors. The mean value of VNGDR significantly increased in the treatments and suggested that the upwelling and downwelling shear layers induced vertical transport with large net vertical displacement. However, histograms of VNGDR revealed a bimodality, which indicated that a sizable portion of the population was also displaying depth-keeping behavior (low VNGDR). Those copepod trajectories not displaying depth-keeping behavior at the scale of the observation (high VNGDR) predominately consisted of copepods attempting to swim against the flow, thereby resisting vertical advection, which is another potential depth-keeping mechanism. Preferential depth-keeping was consistent with in situ acoustic tracking studies and could improve survival by increasing residence time near fronts, which are often coincident with food and mates.