Hydraulic habitat preferences of the torrential mayfly Epeorus longimanus (Ephemeroptera: Heptageniidae): the ecological importance of near-bed flows

The larval stage of the mayfly Epeorus longimanus (Ephemeroptera: Heptageniidae) is an inhabitant of torrential stream habitats. It possesses several adaptations to high velocity environments, including a flattened body shape, a tilted head shield, and a "sucker-like" arrangement of abdominal gills. In order to relate the behavioural and morphological adaptations of E. longimanus to its habitat requirements, the distribution of this mayfly and a suite of environmental parameters were measured at a range of spatial scales (i.e. watershed, within-stream, and within-stone scales). Benthic macroinvertebrates were collected at 39 stream sampling sites throughout the lower portion of the Torpy River watershed in eastern British Columbia, in order to (1) identify the particular habitat preferences of larval Epeorus, and (2) examine the community structure of benthic macroinvertebrates in a northern watershed. Several environmental variables (e.g. discharge, stream surface slope, substrate size, pH, conductivity, riparian vegetation, etc.) were measured and related to patterns of faunal abundance using canonical correspondence analysis (CCA); this ordination technique divided the collected invertebrate taxa into four functional assemblages, each with well-defined habitat requirements and trophic relationships. The results of the CCA showed that Epeorus larvae prefer high discharge streams with coarse substrata. To further examine the habitat requirements of this genus, benthic samples and several descriptors of the flow environment of the stream environment (e.g. mean velocity, near-bed velocity, depth, substrate size, Froude number, etc.) were obtained at 50 regularly located sampling sites in two adjacent, high-discharge streams with coarse bed material. Observed patterns of abundance of Epeorus were significantly and negatively related to velocity, depth, channel Reynolds number, and relative roughness. The near-bed hydraulic environment of shallow torrential streams was characterised by measuring velocity profiles, near-bed (U0.002m) and mean (U0.50) velocities, and shear stresses (tw) over the surface of five experimentally deployed and three naturally occurring stones in a high-discharge stream in the Torpy River watershed. The velocity profiles measured above the stones regularly deviated from the "classic" log-normal shape. The profiles were often "wedge-shaped"; velocities were greatest a few millimetres above the bed, and decreased logarithmically below and above this height. Wall shear stresses and near-bed velocities generally increased from the front to the rear of each stone. The daytime and night-time distributions of E. longimanus were recorded and related to shear stress, periphyton biomass, and substrate characteristics (e.g. stone roughness, topography). During the daytime, larvae preferred areas of the stone surface with high shear stress; during the night-time, larvae preferred areas of the stone surface with higher elevation and attached boundary layer flows. Periphyton density was significantly related to stone surface roughness and stone surface topography. A stone reversal experiment suggested that hydrodynamic factors, rather than food (periphyton) availability, proximally influence the microdistribution of E. /ongimanus larvae; however, the precise nature of the forces to which they respond remains unknown. E. longimanus larvae were also found to exhibit a strong diurnal migration, generally migrating to the upper surface of streambed stones at night, and retreating to the underside of the stones during the day. This study represents one of the first detailed examination of the relationship between the distribution of microscale hydrodynamic parameters, (e.g. shear stress, near-bed velocity) and benthic organisms at organism-defined spatial scales. The results demonstrate that flu id dynamics are the proximate factor that determines the microdistribution of benthic organisms in torrential stream environments. Additional research is required to investigate the ecological importance of these small-scale hydrodynamic parameters. In order to understand the behaviour and ecology of benthic stream organisms, models of flow in natural stream channels must be expanded to include patterns of flow at small, organism-defined scales immediately adjacent to the bed.