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

Abstract

In general, stream food webs consist of algae (periphyton; primary producers growing on rocks) that are consumed by grazing invertebrates, which are in turn preyed upon by a variety of predators. Many invertebrate grazers avoid predators by hiding under rocks during the daytime when visual predators like fish are active, or by seeking high‐velocity microhabitats where invertebrate predators cannot access them. We examined the food web in a mountain stream in the Rocky Mountains by placing marked rocks in the streambed and measuring the distributions of local bed shear stress (force per unit area across the bottom; τ w ), periphyton, and herbivorous invertebrates. Grazing mayfly larvae ( Epeorus longimanus (Eaton)) were the only invertebrates (grazer or predator) found in large numbers on the upper surface of stones. τ w increased from the upstream to the downstream portion of stones, and large numbers of Epeorus larvae (up to 1500 larvae per square meter) migrated to these areas nightly. More periphyton was found on rougher and higher areas of the stones. Larval density was positively related to stone surface roughness and topography and to a lesser extent with periphyton and τ w . Reversing the stones in the streambed revealed that Epeorus larvae responded to near‐bed flows, rather than to periphyton or predators. Hydrodynamics can have important effects on stream ecosystems and their food webs. 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.