LIGHT AND A GRAZING MAYFLY SHAPE PERIPHYTON IN A ROCKY-MOUNTAIN STREAM

To examine the combined effects of light and grazers in structuring pe riphyton in a subalpine stream, we conducted a field experiment in St. Louis Creek (elevation 3000 m), a 2nd-order Rocky Mountain stream (Co lorado, USA). Quarry tiles were placed in the stream for 60 d to colon ize with periphyton and then moved into 36 stream channels in which li ght and grazers were manipulated. Light treatments were high (100% of ambient), intermediate (similar to 40% of ambient), and low light (sim ilar to 5% of ambient). A grazing mayfly, Rhithrogena robusta, was mai ntained inside channels at high (288/m(2)), low (96/m(2)), or zero den sities. After 22-23 d of exposure, mayfly grazing significantly reduce d algal biovolume under all light regimes, although species assemblage s differed between light treatments. As light levels changed, some alg ae showed different responses to grazing. Hydrurus foetidus (Chrysophy ta), for example, was more abundant on grazed substrata relative to no n-grazed controls under high light, but it declined in abundance when grazed under intermediate light. Light and grazers also had an interac tive effect on periphytic biomass; as light increased, grazers caused greater depletions in periphytic ash-free dry mass. Results suggest th at, in subalpine streams, light may be instrumental in establishing pe riphytic structure and in modifying the impact that grazers have on al gae.