Hydraulic and geomorphic effects on mayfly drift in high-gradient streams at moderate discharges

Abstract

We investigated relationships between geomorphic and hydraulic factors and invertebrate drift in high-gradient streams. We measured drift density of a highly mobile mayfly (Baetis bicaudatus) into and out of 12 stream reaches in western Colorado, as well as benthic density and abiotic variables within those reaches, during a time of moderate discharge. Multiple regression analysis indicated that drift propensity (drift density/benthic density), a measure of drift standardized by the benthic density of the source population, was significantly related to Reynolds number, a dimensionless ratio of fluid inertial forces to viscous forces, and Shields number, a dimensionless ratio of shear stress to submerged particle weight that quantifies flow competence. Drift propensity was positively correlated with Reynolds number, but counter to our hypothesis that stronger hydraulic forces would be associated with higher drift, mayflies in reaches with greater flow competence (Shields number) showed lower propensities to drift. Further, immigration ratio (drift in/drift out of each reach) increased significantly with Shields number, indicating that more individuals drifted into than out of reaches with higher flow competence. Although we hypothesized that more hydraulically rough stream reaches (i.e. those with greater flow resistance) would be more favourable to benthic invertebrates and would thus have lower drift, neither drift propensity nor immigration ratio were related to flow resistance. In high-gradient streams at discharges below the range of incipient motion of bed particles, mayfly drift behaviour may be influenced by hydraulic forces, but the relationships we observed are not indicative of passive, abiotically driven drift. Copyright  2008 John Wiley & Sons, Ltd. KEY WORDS drift; mayfly; ecogeomorphology; Reynolds number; Shields number; high-gradient streams; fluvial geomorphology; Baetis Received 7 January 2008; Accepted 15 April 2008

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