FINE-SCALE FIELD MEASUREMENT OF BENTHIC FLOW ENVIRONMENTS INHABITED BY STREAM INVERTEBRATES

We used hot-film anemometry to quantify fine-scale spatial and tempora l flow variations near the surfaces of stones inhabited by suspension- feeding larval blackflies (Simulium vittatum). We focused especially o n within-stone patterns of covariation between patchy microdistributio ns of larvae and local spatial variations in current speed. Current sp eeds 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 meas ured 2 mm above the bed (the approximate height of larval feeding appe ndages) ranged between 7 and 59 cm s(-1). Current speeds measured 10 m m above the bed were very poor predictors of speeds measured at the 2- mm height. Larval abundance exhibited a significant positive relations hip to current speed at 2-mm height, and within-stone variations in sp eed explained similar to 59% of the variation in abundance. Time serie s of current speed exhibited marked fine-scale temporal heterogeneity, fluctuating by as much as 80 cm s(-1) in <0.1 s. Maximum acceleration s sometimes exceeded 1 x 10(4) cm s(-2), which suggests that the force s tending to dislodge benthic organisms from the bed may be greater th an previous estimates based on assumptions of steady flow. Observed le vels of turbulence were greater than predicted from traditional bounda ry-layer theory. We suggest that much of the turbulence evident on ind ividual stones is not produced by local shear but is inherited from up stream roughness elements that cause flow separation.