Drift and settlement of stream insects in a complex hydraulic environment

1. The hydraulic and geomorphic characteristics of stream patches are often associated with distinctive assemblages or densities of stream invertebrates, and it is routinely presumed that these patterns reflect primarily species-specific habitat requirements. An alternative hypothesis is that such patterns may be influenced by constraints on movement, such as the results of departure and settlement processes. We describe a manipulative experiment that examined how the hydraulic environments created by topographic bedforms influenced the drift behaviour and potential settlement sites for two species of mayfly (Baetis rhodani and Ecdyonurus torrentis). These species are common in the drift and often co-occur in streams, but differ in their small-scale distribution patterns, body shape and movement behaviour. 2. Flume experiments were carried out to determine how the hydraulic environments conditioned by a step bedform influence the behaviour of mayflies in the drift (swimming, posturing, tumbling), and the consequences of those behaviours (drift distance and time), compared to drift over a plane bed. The ramped step in the flume mimicked step bedforms that are common in coarse-grained, high-gradient streams. In contrast to the plane bed, a zone of recirculating flow was created downstream of the step, above which flow was faster and more turbulent. Uniform flows are used in most flume studies of drift; our approach is novel in recreating a complex hydraulic environment characteristic of stream channels. 3. Both species had some behavioural control over drift, and drift distances and times were shorter for live larvae than for dead larvae over the plane bed. The step had no impact on drift time or distance for live Baetis, but dead larvae were trapped in the flow separation eddy and drift time increased accordingly. Some Ecdyonurus also became trapped in the eddy, but live larvae drifted farther than dead larvae, and farther over the step than the plane bed. 4. Whilst in the drift, larvae altered their behaviour according to the ambient hydraulic environment, but in a species-specific manner. Over the plane bed, Baetis had occasional swimming bursts, but primarily postured (maintained a stable body orientation), whereas Ecdyonurus spent roughly equal time posturing and swimming. In the more turbulent flows generated by the step, Baetis spent proportionately more time swimming, whereas Ecdyonurus spent more time posturing and often tumbling as body orientation became unstable. 5. In a high-gradient stream, Baetis was more abundant close to steps than in plane bed patches with less complex flow, whereas the opposite pattern held for Ecdyonurus. Thus, the small-scale distribution patterns of these species within streams correspond to their drift behaviours and ability to access various hydraulic patch types in our flume. These results are consistent with the hypothesis that constraints on movement and settlement may be important driver of distribution patterns within streams.