Simultaneous field measurements of ostracod swimming behavior and background flow

Zooplankton swimming near the substratum experience boundary layer flow that is characterized by steep velocity gradients and turbulence. How do small swimming organisms navigate flows at this interface to forage and interact with mates? To address this question, we collected field measurements of the swimming behavior of the marine ostracod Paravargula trifax near complex living substrata, which were exposed to two conditions: slow “ambient flow” and faster “experimental flow.” Ostracod trajectories and background flow were recorded simultaneously using a self-contained underwater velocimetry apparatus (SCUVA). Particle image velocimetry (DPIV) produced instantaneous velocity vector fields in which the ostracods were swimming. Mean velocities, local shear stresses, turbulence intensity, and boundary shear velocity (u_*) were greater in the experimental flow treatment. In slow ambient flow (u_rms = 0.39 ± 0.13 [mean ± SD] cm s ^− 1), ostracod swimming tracks were more tortuous and swimming angles corrected for background flow were randomly distributed compared with tracks in faster flow (u_rms = 3.49 ± 0.50 cm s ^− 1), indicating decreased maneuverability in rapidly flowing, turbulent water. Modeled, passive neutrally buoyant particles moved at substantially slower speeds, and their tracks were less tortuous than those of the ostracods, thus illustrating the importance of behavior as well as environmental flow in determining ostracod trajectories. Frequencies of encounters by ostracods with the benthos and with other ostracods were not different between treatments. However, in the experimental flow treatment, interactions with other ostracods occurred more frequently in the boundary layer than in the free stream, suggesting that microhabitats in the boundary layer may allow for enhanced mating encounters.