The effects of grazing on algal assemblages by three different stream herbivores, the mayfly Centroptilum elsa, the snail Juga silicula, and the caddisfly Dicosmoecus gilvipes, were studied during a 48-d experiment in six laboratory streams. Compared with ungrazed control streams, grazing by Centroptilum (500/m2) modified algal community structure slightly but had little effect on periphyton biomass and chlorophyll a. Grazing by Juga (350/m2) reduced periphyton biomass and chlorophyll a by nearly 50%, but increased the rate of primary production by up to 25%. Juga also prevented significant accumulation of cyanophytes and some diatom species. Grazing by Di-cosmoecus (200/m2) reduced periphyton biomass and chlorophyll a to less than 5% of the ungrazed levels, but primary production declined by only 50%. Only adnate algal cells and short filaments persisted on substrates grazed by Dicosmoecus. Algal export rates were increased by all three her-bivores. Modification of algal growth patterns by both consumption and dislodgement, and dampening of temporal fluctuations were key mechanisms by which these herbivores altered periphyton com-munities. Primary production was stimulated by low rates of grazing by Juga in the laboratory streams, possibly as a result of increased light intensity in lower strata of the periphyton or removal of senescent algal cells. Algal assemblages displayed both community-level responses (e.g., biomass, production) and species-level responses (e.g., taxonomic composition) that should be considered in other studies of stream herbivory.
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