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The composition of epiphytic diatom communities from the cold tailwaters (12°C) of Glen Canyon Dam, Arizona, was analysed after a 2 wk incubation period at 12"C, 18°C and 21°C. There was a significant change in diatom composition between 12°C and 18"C, while no significant changes occurred between 18°C and 21°C. This suggests that a temperature threshold exists between 12°C and 18°C for the diatom flora in the tailwaters of Glen Canyon Dam. At the two higher water temperatures, smaller and closely adnate taxa became more important numerically than larger, upright, cold water stenotherms. The potential importance of this compositional shift in epiphytic diatoms on macroinvertebrate grazers has management implications regarding different release programs from reservoirs.

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