Document Type


Publication Date



University of Nevada, Las Vegas


The response to periodic desiccation of periphyton communities developed in eddy environments or in areas exposed to variable, direct current was examined. Algal communities were incubated on clay tiles for between 2 and 12 weeks in the Colorado River, directly below Hoover Dam from October, 1983 - January, 1984. Mean daily discharge from the dam decreased over this period. To assess the influence of these discharge changes on community dynamics, samples collected over the first 12 weeks of the investigation were compared to samples of the same age collected over the final 8 weeks.

The small diatom taxon Achnahthes spp., a composite of A., minutissima Kutz. and A., microcephala (Kutz.) Grun., was the numerical dominant in all collections, accounting for 50-80% of the diatoms identified from a given sample. Relative abundances of other diatom taxa varied with . community age, current regime, and desiccation frequency.

Undesiccated algal communities sheltered from direct current developed more rapidly and exhibited both higher biomass and diatom densities than communities established in fast current. Diatom species diversity on undesiccated tiles did not differ among current regimes.

Low frequency desiccation (LFD) reduced biomass and diatom densities in eddy environments while enhancing biomass accumulation and cell accrual on fast current tiles. These responses weakened with age, disappearing in 8, 10, and 12 week old assemblages. Diatom species diversity increased with LFD in all current regimes. Differences in LFD reactions were attributed to higher initial mucilage concentrations in communities exposed to direct current and to increased community mucilage production in response to desiccation. The presence of filamentous cyanophytes also enhanced diatom colonization and diversity.

Hoover Dam discharge reductions resulted in lower biomass and cell densities in undisturbed eddy communities, and increases in these 2 parameters in assemblages exposed to the ambient river current. Species composition was more affected by discharge changes in eddy environments than in fast current habitats.

Periphyton communities developed on tiles removed from direct current were less resilient and exhibited lower resistance to perturbation than those communities established under more physically rigorous conditions. Both colonization potential and community desiccation resistance appear to be strongly influenced by mucilaginous material produced within the periphyton community itself.


Algae; Diatom community; Colorado River; Hoover Dam; Periphyton communities


Bacteriology | Biology | Botany | Environmental Sciences | Fresh Water Studies | Plant Biology | Plant Sciences | Terrestrial and Aquatic Ecology