Characterization of the Hydraulic Habitat of Brachycentrus occidentalis, a Filter-Feeding Caddisfly

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Abstract
Local velocity and flow profiles (50 cm in length) were measured in accelerating transitional-velocity zones in two 20-40-m long contrasting reaches of cobble and shale substrate in Wilson Creek, Manitoba, to characterize the observed hydraulic habitat of individual Brachycentrus occidentalis larvae (instars IV and V). Both instars in the cobble reach were in local habitats with faster water-column velocities, shallower depths, steeper water-surface slopes, and higher Froude numbers than randomly available locations in similar transitional-velocity zones. Values for hydraulic variables were mostly higher for instar IV habitat. Random conditions in the shale reach differed from those in the cobble reach, but differences between larval (instar V) habitat and random locations were again significant. Although larvae in the shale reach were at slower velocities and shallower depths than larvae in the cobble reach, they selected identical hydraulic regimes in terms of water-surface slope and Froude number. At both sites, Froude number was the best discriminator between larval and random locations. Microhabitats selected by B. occidentalis larvae coincided with zones of converging streamlines (average Froude number: 0.6) which may expose filter feeders to higher seston delivery rates. Additional observations for comparison showed that filter-feeding Simulium vittatum larvae selected positions with high Froude numbers also (average: 0.7), whereas Glossosoma intermedium larvae were in locations that differed from random only in depth. The profiling device used to map patches of local flow conditions allowed larval habitats to be distinguished from random locations within accelerating transitional-velocity zones, which are a subset of the full range of complex local flow zones in boulder-cobble stream beds.

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