Experimental study of sediment entrainment and dispersal of aquatic insect larvae

In stream ecosystems, flow increases that mobilize sediment are a primary form of disturbance for organisms that inhabit the streambed. A mechanistic understanding of hydrologic disturbance is needed to make generalizations about how the magnitude of flow events are related to impacts on the biota, or about what kinds of hydrological and geomorphic information are needed to describe the fluvial disturbance regime.; Three series of laboratory flume experiments were conducted to document the relationship between sediment entrainment and the dispersal of aquatic insect larvae. Larvae were placed in the gravel bed of the flume and flow strength was varied to produce a range of transport from entrainment of only finer particles at the lowest flows to complete mobilization of all surface particles at the highest flow rates. Several families of insects were used to assess the influence of differences in organism morphology and behavior on patterns of dispersal. The effects of bed surface texture and the magnitude of fine sediment transport on dispersal was evaluated by varying the fine sediment content of the bed surface.; Flow-driven dispersal of the surface-dwelling larvae scaled with proportional streambed entrainment, whereas dispersal of subsurface larvae was limited over the range of flows that entrain the bed surface. As flow strength increases, dispersal is increasingly dominated by displacement from the streambed. At intermediate flows crawling and burrowing can be important mechanisms of dispersal and may allow mobile species to avoid displacement.; Removal of fine sediment from the bed surface reduced dispersal, whereas adding sand to the bed in increased dispersal of larvae. These effects were more pronounced for the surface-dwelling taxa than for the subsurface larvae, and dispersal of ephemerellid larvae was increased dramatically by adding sand to the bed. Increases in dispersal with fine sediment content were due to reduced availability of interstitial refugia on the bed surface and reflect increases in flow exposure and the probability of displacement. The experiments demonstrate a direct relationship between sediment entrainment and dispersal, suggesting that streambed entrainment is a meaningful metric of disturbance and that understanding population dynamics during flow events requires understanding patterns of sediment transport...