Freshwater Biology (1992) 27, 237-247 Plant—herbivore interactions in streams near Mount St Helens G.A. LAMBERTI,* S.V. GREGORY,* C.P. HAWKINS,* R.C. WILDMAN,* L.R. ASHKENAS* AND D.M. DENICOLA* 'Department of Fisheries and Wildlife and ^Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331. U.S.A.. and ^Department of Fisheries and Wildlife and Ecology Center, Utah State University, Logan. UT 84322. U.S.A. Address for correspondence: Dr G.A. Lamberti. Department of Biological Sciences, University of Notre Dame. Notre Dame. IN 46556, U.S.A. SUMMARY 1. In four separate field experiments near Mount St Helens (Washington, U.S.A.) during 1986, the grazing effects of two large benthic herbivores, tadpoles of the tailed frog Ascaphus truei and larvae of the caddisfly Dicosmoecus gilvipes, were investigated using streamside channels and in-stream manipulations. In the experimental channels, abundances of periphyton and small benthic invertebrates declined significantly with increasing density of these larger herbivores. 2. In eleven small, high-gradient streams affected to varying degrees by the May 1980 eruption, in-stream platforms were used to reduce grazing by A. truei tadpoles on tile substrates. Single platforms erected in each tributary and compared to grazed controls revealed only minor grazing effects, and no significant differences among streams varying in disturbance intensity (and, consequently, tadpole density). However, results probably were confounded by high variability among streams in factors other than tadpole abundance. 3. Grazing effects were further examined in two unshaded streams with different tadpole densities, using five platforms per stream. In the stream with five tadpoles m~^, grazing reduced periphyton biomass by 98% and chlorophyll a by 82%. In the stream lacking tadpoles, no significant grazing effects were revealed. Low algal abundance on both platforms and controls, and high invertebrate density in that stream (c. 30 000 m^^) suggests that grazing by small, vagile invertebrates was approximately equivalent to that of tadpoles. 4. The influence of large benthic herbivores on algal and invertebrate communities in streams of Mount St Helens can be important, but reponses vary spatially in relation to stream disturbance history, local environmental factors, and herbivore distributional patterns and abundance. , . flows, which destroyed the coniferous forests within Introduction , , , , , , , ttie blast zone and greatly altered aquatic habitats The cataclysmic eruption of Mount St Helens on (Franklin ef a/., 1985). For over 200 km of streams and 18 May 1980 devastated over 500 km^ of forested rivers, riparian vegetation was burned or buried, landscape in south-west Washington, U.S.A. The sediments were severely scoured by debris flows lateral volcanic blast triggered debris avalanches, or ash, and most aquatic plants and animals were pyroclastic flows, severe windstorms, and mud- killed. Streams near the perimeter of the blast zone, 237 !