Light and a grazing mayfly shape periphyton in a Rocky Mountain stream

To examine the combined effects of light and grazers in structuring periphyton in a subalpine stream, we conducted a field experiment in St. Louis Creek (elevation 3000 m), a 2nd-order Rocky Mountain stream (Colorado, USA). Quarry tiles were placed in the stream for 60 d to colonize with periphyton and then moved into 36 stream channels in which light and grazers were manipulated. Light treatments were high (100% of ambient), intermediate (similar to 40% of ambient), and low light (similar to 5% of ambient). A grazing mayfly, Rhithrogena robusta, was maintained inside channels at high (288/m(2)), low (96/m(2)), or zero densities. After 22-23 d of exposure, mayfly grazing significantly reduced algal biovolume under all light regimes, although species assemblages differed between light treatments. As light levels changed, some algae showed different responses to grazing. Hydrurus foetidus (Chrysophyta), for example, was more abundant on grazed substrata relative to non-grazed controls under high light, but it declined in abundance when grazed under intermediate light. Light and grazers also had an interactive effect on periphytic biomass; as light increased, grazers caused greater depletions in periphytic ash-free dry mass. Results suggest that, in subalpine streams, light may be instrumental in establishing periphytic structure and in modifying the impact that grazers have on algae.