Incipient motion and drift of benthic invertebrates in boundary shear layers

The present study aimed to identify threshold shear stresses to cause benthic invertebrates to drift in boundary shear layers. Laboratory experiments were carried out in a small tilting flume, 10 cm wide and 3 m long with a closed recirculation system and a fixed sand bottom of uniform grain size. Benthic invertebrates from different orders (Achaeta, Gastropoda, Crustacea, Ephemeroptera, Odonata, Megaloptera, Trichoptera) were collected from a lowland river (Spree, Germany) and kept in aquariums to adapt to the laboratory conditions. Individuals were placed in the flume at low flow conditions, before gradually increasing the discharge until drift occurred. Photos were taken from the side through the flume walls to capture the behaviour and strategies of the organisms to cope with the flow. Photos against millimeter grid graph paper in the background were taken for each individual in order to quantify the body height and length exposed to flow. Subsequently, velocity and shear stress profiles were measured above the drift location with an Acoustic Doppler Velocimeter (ADV, Nortek Vectrino). Bed shear stresses were determined by extrapolation of the Reynolds shear stress profiles in subcritical flow conditions. For supercritical flow conditions, a simple particle tracking velocimetry was applied to determine velocity profiles. Bed shear stress was then estimated from fitting of a log-law. Results are presented in tabular form, containing the average drift shear stress and drift velocity for 15 taxa commonly found in lowland rivers. Drift shear stresses, which can be interpreted as abiotic ‘absence’ criteria of the species, were in most cases exceeding the conditions of preferential habitats and incipient motion threshold of the sediments. However, it was observed that sudden changes in the angle of flow attack (e.g. by burst events or surface waves) or direct interaction with sediment motion (animals hit by moving grains or erosion of the grains serving as anchorage) were key factors triggering invertebrate drift.