Herbivory under Different Flow Regimes: A Field Experiment and Test of a Model with a Benthic Stream Insect

Abstract

An experiment was conducted in an open-canopy mountain stream to examine the effects of different densities of the grazing caddisfly Agapetus boulderensis (Trichoptera: Glossosomatidae) on algal biomass and assemblage structure at two current velocities. Unglazed ceramic tiles were colonized by algae in grazer-free troughs for 30 d at slow (∼ 20 cm s −1 ) and fast (∼ 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 streambed densities. At low velocity, algal biomass (chl-a and particulate C) and absolute densities of bacillariophytes, chlorophytes and cyanophytes declined as grazer density increased. Grazers also significantly altered 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 italica). The algal assemblage was effectively converted from a «late-successional» to an «early-successional» stage by grazers. By contrast, at high velocity, initial algal density was relatively low and grazers were less effective in inducing changes. As grazer density increased, total algal cell abundance (mostly bacillariophytes) declined, although the relative abundances of individual diatom species remained unmodified under increasing grazer densities. Grazers essentially maintained the «early-successional» nature of the algal assemblage under high velocity conditions. Further, for both current velocities, we tested a model that predicts algal biomass as a function of grazer density and time. Independent estimates of current-dependent grazing rate and algal growth rate were used in the model to generate predictions of particulate C along a gradient of grazer density at two current velocities. 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 systems