Microflow regimes and the distribution of macroinvertebrates around stream boulders

Abstract

1. An acoustic doppler velocimeter (ADV) was used to collect velocity data in three dimensions in front of and in the wakes of selected boulders in a riffle, where macroinvertebrates were also sampled.2. Shear stress was calculated from the covariance of downstream and vertical velocity fluctuations.3. Velocities, shear stress and turbulence measured at three points above the bed in the front and wake regions of riffle boulders were consistent with predicted patterns of flow around isolated roughness elements. There was a significant difference in mean velocities, but not near‐bed velocities between front and wake regions. At 5 mm above the bed, velocities in both front and wake regions were greatly reduced, relative to the mean velocity (at 0.4 × total depth). Flows at the bed were directed upstream as part of the vortex formed at the front of the boulder and as part of the recirculation occurring in the wake.4. Shear stress was highest in the vertical plane in the wake of the boulders. Turbulence intensity and turbulent kinetic energy were greatest in the wake region, in both the down‐stream and cross‐stream directions. However, these results were not statistically significant. Large standard errors associated with measurements within wakes indicated that wake flows are highly variable. Further research is needed to determine the most appropriate methods of analysing differences associated with such variable flow regimes.5. The benthic macroinvertebrate fauna was significantly richer and more abundant in the wakes than at the front of boulders. Classification and ordination revealed the presence of distinct front and wake benthic communities. These differences could not be attributed to velocity differences within the benthic microhabitat, because near‐bed velocities were greatly reduced in both the front and wake regions. The fauna may be responding to the highly variable turbulent regime of the wake and associated effects, such as increased deposition of particulate organic matter and exchange of dissolved gases.6. Despite the complex flow patterns inherent in natural streamflow, a large roughness element (boulder) appears to predominate in determining microflows within in its immediate surroundings. The design of some benthic sampling programs (e.g. for biomonitoring) may need to incorporate hydraulic microhabitat considerations.7. The fine‐scale field measurements obtained with the ADV indicate that experimental determination of the role of turbulence in the ecology of stream benthos is now both possible and desirable.