TROPHIC INTERACTIONS AND BENTHIC ANIMAL COMMUNITY STRUCTURE IN THE COLORADO RIVER, ARIZONA, USA

1. Cladophora glomerata is the dominant filamentous green alga in the tailwaters of the Colorado River, U.S.A., below Glen Canyon Dam, but b ecomes co-dominant with filamentous cyanobacteria, Oscillatoria spp., below the confluence of the Paria River (26 km below the dam) where su spended sediments are elevated. 2. Benthic algal assemblages played an important role in the distribution of the amphipod, Gammarus lacustri s, in the dam-controlled Colorado River through Grand Canyon National Park, Arizona. Cladophora and G. lacustris showed a weak positive rela tionship at ten cobble-riffle habitats in the Colorado River from Lees Ferry (25 km below the dam) to Diamond Creek (362 km downstream), whi le no relationship was found between Oscillatoria and G. lacustris. 3. The relationship between algal substrata and G. lacustris was tested by a series of in situ habitat choice experiments. G. lacustris showed a significant preference for Cladophora (with epiphytes) over Oscilla toria spp., detritus and gravel in treatment pans at Lees Ferry. 4. Ep iphytic diatoms (i.e. food) were the overriding determinant of subtrat um choice by G. lacustris in laboratory experiments. Gammarus chose th e Cladophora/epiphytic diatom community over sonicated Cladophora with few diatoms. The amphipods also chose string soaked in diatom extract over string without diatom extract. 5. Importance of mutualistic inte ractions in aquatic benthic community structure is discussed.