HERBIVORY UNDER DIFFERENT FLOW REGIMES - A FIELD EXPERIMENT AND TEST OF A MODEL WITH A BENTHIC STREAM INSECT

An experiment was conducted in an open-canopy mountain stream to exami ne the effects of different densities of the grazing caddisfly Agapetu s boulderensis (Trichoptera: Glossosomatidae) on algal biomass and ass emblage structure at two current velocities. Unglazed ceramic tiles we re colonized by algae in grazer-free troughs for 30 d at slow (approxi mately 20 cm s-1) and fast (approximately 40 cm s-1) velocities before transfer to open troughs of similar velocity, where they were exposed to grazer densities ranging from ca 0.7 to 2.7 times natural streambe d densities. At low velocity, algal biomass (chl-a and particulate C) and absolute densities of bacillariophytes, chlorophytes and cyanophyt es declined as grazer density increased. Grazers also significantly al tered algal assemblage structure by reducing the relative abundance of cyanophytes (mostly Anabaena cf. affinis) and chlorophytes (Ulothrix zonata) and increasing the relative abundance of some bacillariophytes (Cocconeis placentula, Fragilaria pinnata, F. vaucheriae, Melosira it alica). The algal assemblage was effectively converted from a ''late-s uccessional'' to an ''early-successional'' stage by grazers. By contra st, at high velocity, initial algal density was relatively low and gra zers were less effective in inducing changes. As grazer density increa sed, total algal cell abundance (mostly bacillariophytes) declined, al though the relative abundances of individual diatom species remained u nmodified under increasing grazer densities. Grazers essentially maint ained the ''early-successional'' nature of the algal assemblage under high velocity conditions. Further, for both current velocities, we tes ted a model that predicts algal biomass as a function of grazer densit y and time. Independent estimates of current-dependent grazing rate an d algal growth rate were used in the model to generate predictions of particulate C along a gradient of grazer density at two current veloci ties. Algal biomass predicted by the model compared favorably with an empirical fit of the data under low and high velocities. These results suggest that the role of grazers in structuring algal assemblages may vary spatially with local current velocity in heterogeneous stream sy stems.