Macroinvertebrate community structure along gradients of hydraulic and sedimentary conditions in a large gravel-bed river

1. The spatial distribution of macroinvertebrate species was examined in re lation to hydraulic and sedimentary conditions in a large gravel-bed river, the Fraser River, Canada. Mean annual discharge in the Fraser River is 290 0 m(3) s(-1) and annual flood discharge, due to snowmelt in May and June, a verages 8760 m(3) s(-1). 2. Invertebrates were sampled from four water depths (0.2, 0.5, 1.5, 3.0 m) at various levels of discharge that together captured the spatial and temp oral variability of the physical habitat. Several hydraulic (near-bed shear velocity, Boundary Reynolds number, turbulence intensity, depth-averaged v elocity, Froude number, Reynolds number) and substratum variables (mean gra in size, Trask's sorting coefficient, Nikuradse's roughness, percentage of fine sediment, and Shields entrainment function) were measured for each sam ple of macroinvertebrates. Concentrations of fine and coarse particulate or ganic matter were also assessed. 3. The physical habitat was characterized by a major gradient of hydraulic conditions that corresponded positively with increasing water depth and acc ounted for 52% of the total variation in the habitat data. Substratum condi tions and the concentration of organic matter explained 24% of the total va riation in the habitat data. 4. The distribution of invertebrates was correlated significantly with hydr aulic variables and suggests that hydraulic conditions represent a major ph ysical gradient along which the benthic community is organized. The distrib ution of organic matter and substratum texture were also important for some species. The spatial distribution of most species reflected morphological and trophic suitability to particular habitat conditions. 5. Hydraulic stress associated with foraging and maintaining position, as w ell as organic matter retention in coarse substrata, are probable mechanism s affecting the spatial distribution of macroinvertebrates.