Effects of three herbivores on periphyton communities in laboratory streams

The effects of grazing on algal assemblages by three different stream herbivores, themayfly Centroptilum elsa, the snail Juga silicula, and the caddisfly Dicosmoecus gilvipes, were studiedduring 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 effecton periphyton biomass and chlorophyll a. Grazing by Juga (350/m2) reduced periphyton biomassand chlorophyll a by nearly 50%, but increased the rate of primary production by up to 25%. Jugaalso 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 ungrazedlevels, but primary production declined by only 50%. Only adnate algal cells and short filamentspersisted 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 dampeningof 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 laboratorystreams, possibly as a result of increased light intensity in lower strata of the periphyton or removalof 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 inother studies of stream herbivory.
Key words: herbivory, stream, periphyton, benthic algae, grazing, Centroptilum, Juga, Dicosmoecus,primary production, chlorophyll a, export.