Macroinvertebrate distribution in a reach of a north temperate eastern Canadian river: Relative importance of detritus, substrate and flow

The importance of microhabitat variables to stream macroinvertebrates can be difficult to assess due to the large number and intercorrelation of relevant environmental variables. In this study, substrate, detritus, and hydraulic variables were evaluated in patches arranged in a grid across a run of the West River, an agricultural river in Prince Edward Island, Canada, and related to spatial variation in macroinvertebrate densities. Samples were collected in August and November, 2001, then densities of individual taxa were compared to patch characteristics (substrate and detritus size composition, algal cover, shear velocity, mean velocity, Froude number, depth) and to location of the patch in the stream reach. Patterns were compared using redundancy analysis and variance partitioning. Detritus variables were more important than sediment or hydraulic variables in explaining variations in invertebrate densities, but these were intercorrelated with hydraulic and substrate variables. Contrary to literature reports, densities of several taxa were highest in the patches with the finest substrates, but these were also the patches with the highest amounts of deposited detritus. Simuliium (Diptera, black flies) was the only taxon to show a strong positive relationship to hydraulic parameters. Physical patch variation explained only 37-50% of the variation in overall macroinvertebrate densities, and physical variation had a stronger effect in autumn than in summer. Three taxa were investigated in detail to evaluate responses in representative deposit-feeding taxa. Patch variation explained 50-80% of variation in densities of Optioservus (Coleoptera: Elmidae) and leuctrid/capniid stoneflies (Plecoptera), but only about 20-50% of the variation in Antocha (Diptera: Tipulidae)