Three-dimensional tracking of the motion of benthic copepods in the free water and inside the transparent sediment bed of a laboratory flume

Benthic organisms in regulated rivers are exposed to sharp variations in environmental conditions. Variations in, e.g., shear stress and water temperature can result in rapid alteration in invertebrate drift. Catastrophic drift is associated with hydrodynamic thresholds. Active drift results from deliberate behaviors and occurs, e.g., in response to changes in temperature. Cyclopoid copepods constitute a substantial part of the meiobenthos. The relationships between hydraulic conditions or temperature and copepod drift, and between their behavior and drift entry, transport and exit, remain not well defined. We present a laboratory flume fitted with a transparent sediment bed that enables studying the behavior of benthic invertebrates both in the free water and inside the sediment. By means of three-dimensional particle tracking velocimetry, we reconstructed the trajectories of many copepods (Eucyclops serrulatus) and we quantified their velocity and vertical distribution under statistically steady state flow conditions. Copepods were able to swim in the entire flume and moved readily in the sediment bed. They moved by successive jumps and often against the flow. This behavior may contribute to drift avoidance in natural streams. Copepods were preferentially found within the interstices of the sediment bed and within a narrow layer at its surface, in agreement with the epibenthic nature of this species. Our setup enables the detailed observation of fine-scale behavioral processes that could not be approached in field studies or through theoretical approaches. It offers the opportunity to better understand the impacts of physical conditions on the behavioral processes that drive invertebrate drift in streams.