Effects of Three Herbivores on Periphyton Communities in Laboratory Streams

Abstract

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.