Hydraulic microhabitats and the distribution of macroinvertebrate assemblages in riffles

1. The influence of hydraulic conditions on the spatial distribution of macroinvertebrate assemblages was investigated in three riffles in a perennial Australian river. 2. Velocity, depth and variability of substrate roughness were measured at each of 56 macroinvertebrate sampling locations. Complex hydraulic variables (roughness Reynolds number, shear velocity, Froude number) were calculated from combinations of two or three of the directly measured variables. The biological significance of directly measured and complex hydraulic variables was determined by a combination of univariate and multivariate statistical procedures. 3. Macroinvertebrate abundance, number of taxa and community composition were significantly different between the identified roughness Reynolds number, Froude number, velocity and shear velocity microhabitats throughout the studied riffles. 4. Regression analysis showed macroinvertebrate abundance and number of taxa were negatively related to roughness Reynolds number, shear velocity, velocity and Froude number. Depth was negatively related to abundance. In general, the majority of the macroinvertebrate community preferred the areas of riffles with the lowest near-bed turbulence. 5. Roughness Reynolds number explained more of the spatial variation in invertebrate abundance, number of taxa and community composition than the other hydraulic variables, either directly measured or calculated. Of the directly measured variables, velocity had the greatest explanatory power, which was marginally less than roughness Reynolds number and shear velocity. 6. This study demonstrated that small-scale differences in hydraulic conditions created by combinations of velocity, depth and substrate roughness have an important role in the spatial distribution of macroinvertebrate assemblages in riffle habitats.