MACROINVERTEBRATE GRAZERS, VELOCITY, AND BEDLOAD TRANSPORT RATE INFLUENCE PERIPHYTIC ACCRUAL IN A FIELDSCALE EXPERIMENTAL STREAM

We examined influences on periphytic accrual in the St. Anthony Falls Laboratory Outdoor StreamLab in Minneapolis, Minnesota, USA. Macroinvertebrate grazers were excluded from 27 of 65 clay tiles using electric pulses. We examined periphytic biomass accrual as a function of grazer presence, sampling run, and near-bed current velocity using ANCOVA. We found significant (P < 0.01) temporal differences between sampling runs but no significant effect of grazer presence. Along with a strong association between bedload transport rates and mean periphytic biomass (r2 = 0.98), our results suggest that grazers are relatively unimportant in stream systems with high levels of physical disturbance from floods and associated sand bedload. However, the interaction between grazer presence and velocity was marginally significant (P = 0.08). Regression analyses showed no relation between velocity and periphyton in the absence of grazers but a negative relation (r2 = 0.63) when grazers were present, suggesting that mechanical dislodgement of periphyton by grazers may increase with velocity. We conclude that grazers can have subtle effects on periphyton, particularly in streams with high bedload transport rates.