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

Abstract

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.

Behavioral response of the freshwater cyclopoid copepod Eucyclops serrulatus to hydropeaking and thermopeaking in a laboratory flume

“…The experimental setup and methodology are described in detail in Sidler et al (2017). Here we briefly restate the main features.…”

“…The cameras were mounted above the flume at different angles and in a planar configuration (Sidler et al 2017). They recorded at a frame rate of 100 Hz and were focused on an investigation volume that was approximately 15 cm (x) £ 15 cm (y) £ 15 cm (z).…”

“…Rundle and Hildrew 1990;Robertson et al 1995;Di Lorenzo et al 2015). We have observed in a previous laboratory study that this species preferentially moves at the surface of the sediment and performs very frequent excursions within the interstices of the sediment bed and into the free water column (Sidler et al 2017). These behavioral features, i.e.…”

“…Previous laboratory measurements suggest that Eucyclops displays strong swimming abilities to maintain position at times of high flow (Richardson 1992). We have recently observed that this species (a) performs very frequent incursions within the interstices of the sediment bed where flow velocity is negligible, and (b) swims actively against the flow to reduce downstream transport (Sidler et al 2017). In this study, we test for the hypothesis that these two behavioral traits render copepods less vulnerable to high flow velocities and less prone to drift entry.…”

“…In our cultures, they were fed with a mixture of Paramecium, Chilomonas paramecium and the algae Scenedesmus. Paramecium and C. paramecium were grown on wheat grains in soya bean infusion (Kumazawa 2000;Park et al 2005;Sidler et al 2017). Scenedesmus was grown in autoclaved water and in Conway medium, at 18 C and under a 12L:12D light cycle.…”