The vertical distribution of hydropsychid larvae and pupae (Trichoptera: Hydropsychidae) in stream substrates

Vertical stratification of benthic samples at three stations on the Credit and Humber rivers revealed significant between-layer differences in the physical structure of riffles and abundances of larvae and pupal cases of Cheumatopsyche Wallengren and four species of Hydropsyche Pictet (Trichoptera: Hydropsychidae). Rock size and total rock surface area decreased from the top layer to the bottom (surface areas of pebbles excluded). Larvae of all five instars of H. sparna Ross were significantly more abundant (P < 0.05) in the top layer than in the bottom, but pupal cases were more numerous in the middle and bottom layers. Similar patterns were found for larvae and pupal cases of Hydropsyche bronta Ross, Hydropsyche slossonae Banks, and Cheumatopsyche. Pupal cases as well as larvae of Hydropsyche morosa Hagen were more numerous in the top layer. Depth and current conditions influenced sedimentation. Mean current velocities and total accumulation of fine material were greater at downstream stations. Significant variation in deposition of fine and coarse materials was found within the riffle at each station, suggesting that local streambed conditions govern sediment deposition at the microhabitat level.