Fine‐scale field measurement of benthic flow environments inhabited by stream invertebrates

We used hot‐film anemometry to quantify fine‐scale spatial and temporal flow variations near the surfaces of stones inhabited by suspension‐feeding larval blackflies ( Simulium vittatum ). We focused especially on within‐stone patterns of covariation between patchy microdistributions of larvae and local spatial variations in current speed. Current speeds were sampled at 256 Hz for heights between 1 and 10 mm above the bed. Profiles of current speed exhibited complex shapes, and boundary‐layer thicknesses ranged from <1 to >5 mm. Average current speeds measured 2 mm above the bed (the approximate height of larval feeding appendages) ranged between 7 and 59 cm s −1 . Current speeds measured 10 mm above the bed were very poor predictors of speeds measured at the 2‐mm height. Larval abundance exhibited a significant positive relationship to current speed at 2‐mm height, and within‐stone variations in speed explained ~59% of the variation in abundance. Time series of current speed exhibited marked fine‐scale temporal heterogeneity, fluctuating by as much as 80 cm s −1 in <0.1 s. Maximum accelerations sometimes exceeded 1 × 10 4 cm s −2 , which suggests that the forces tending to dislodge benthic organisms from the bed may be greater than previous estimates based on assumptions of steady flow. Observed levels of turbulence were greater than predicted from traditional boundary‐layer theory. We suggest that much of the turbulence evident on individual stones is not produced by local shear but is inherited from upstream roughness elements that cause flow separation.