Diversity and Assemblage Structure of Epiphytic Diatoms in the Colorado River Downstream of Glen Canyon Dam Following Reduced Hydropeaking

A major reduction in hydropeaking from Glen Canyon Dam into the Colorado River (Arizona, USA) was initiated in the 1990s to protect downstream riverine habitats. As a result, diel discharge varied significantly less in 2020 than in prior years in the Colorado River tailwaters, a section in which a diverse macrophyte assemblage recently developed. We conducted a year-long study in 2020 to characterize the recent epiphytic diatom assemblage by assessing present-day species richness, taxonomic composition, ecological guilds, and spatial patterns under the current flow regime. We also examined the 2020 assemblage in one section of the river previously studied in the 1980s and 1990s when Cladophora glomerata served as the dominant macrophyte host. We sampled epiphytes from all major macrophytes at 2–5 km intervals over four seasonal periods, identifying 218 diatom species, more than doubling the previously documented species richness. Fifty taxa comprised 99% of valves in the assemblage, with a majority (54%) of high-profile forms, along with 24% low-profile, 14% motile, and 8% planktonic guilds. Hierarchical clustering revealed that diatom species composition and ecological guilds along the 25 km reach varied according to differences in river depth and macrophyte host and varied less with distance from the dam.