Microdistribution of a torrential stream invertebrate: Are bottom-up, top-down, or hydrodynamic controls most important?

In streams, hydrodynamic forces may influence food web structure by limiting the spatial distribution or diversity of primary consumers. To examine the spatial relationships between organisms and physical drivers, we measured distributions of local bed shear stress (τ_w), periphyton, and herbivorous invertebrates (larvae of the mayfly Epeorus longimanus (Eaton)) on experimentally deployed, submerged stones (diameters ranging from 22 to 33 cm) in a mountain stream in British Columbia. In general, τ_w increased from the upstream to the downstream portion of stones, where there was an abrupt decrease in τ_w due to flow separation. Periphyton density was significantly related to stone surface roughness and topography (i.e., more algae on rougher, higher areas of the substrate). The high-shear regions of the upper, exposed surfaces of the stones were inhabited by high densities of Epeorus larvae (up to 1500 larvae m^− 2); larvae migrated diurnally, with most larvae moving to the underside of stones during the day. Larval density was positively related to stone surface roughness and topography and to a lesser extent with periphyton and τ_w, whereas larvae avoided regions of flow separation. Experimental reversal of the orientation of stones with respect to flow direction indicated that Epeorus larval positioning was a proximate response to near-bed flows, rather than biotic factors such as food availability or predation. Whereas hydrodynamic factors influenced the microdistribution of these primary consumers, the spatial relationship with shear stress was much more complex than anticipated.