LOCAL-EFFECTS OF A SEDENTARY GRAZER ON STREAM ALGAE

1. Larvae of the sedentary aquatic caterpillar Petrophila confusalis ( Walker) construct silken retreats around which they feed; outside thes e clearly demarcated grazed areas, stream algae are exposed to a varie ty of mobile grazers. Comparisons of the algal community inside and ju st outside grazed zones were made for third- and fifth-instar Petrophi la in the South Fork of the Eel River, California. 2. Densities of bot h the filamentous macroalga Cladophora and diatom assemblages were sig nificantly reduced within the grazed areas during both larval instars. Grazing of diatoms was taxonomically non-selective. 3. In spring, the grazed zones were relatively large (mean = 22.7 cm(2)) and visibly in creased epilithic spatial patchiness. 4. Per cent composition of diato m assemblages inside and outside the grazed areas differed during the third instar but not during the fifth instar. During the third instar, the grazed zone contained more Synedra ulna (a common immigrant) and less Gomphonema olivaceum (a late-successional species).