Effects of regulation on the phytobenthic community in the Colorado River below Glen Canyon Dam

This study investigated the effects of Glen Canyon Dam (GCD) operations and the presence of the reservoir, Lake Powell, on the downstream benthic community. Specific factors of regulation examined were: (1) effects of changing flow regimes including fluctuating flows; steady, high-volume discharge; and an experimental flood release, (2) downstream effects of modifications in reservoir water quality including decreased conductivity and nutrient concentrations, and (3) effects of elevated water temperature. The biomass and composition of individual components of the phytobenthic community were tested and observed under the above regulation factors.; Fluctuating discharge regimes caused periodic exposure and desiccation to the algal food base. The phytobenthic community, dominated by Cladophora glomerata could not maintain stable biomass in undisturbed treatments and desiccated algae were unable to recolonize >35% of the control biomass during fluctuating flow regimes. Desiccation-tolerant Oscillatoria spp. successfully recolonized and dominated in the varial zone. However, Oscillatoria contributed less potential energy than other foodbase components and had less associated diatom and macroinvertebrate density due to the exposed habitat, thus is of little trophic significance to the Colorado River foodweb.; Declines in conductivity and nutrient concentrations in Lake Powell in concert with high, steady releases below GCD resulted in an unprecedented decrease in the dominant alga, Cladophora glomerata, in conjunction with an invasion of a mixed phytobenthic group (MAMB). Seasonal and opposite fluctuation patterns occurred between Cladophora and MAMB. Cladophora was positively associated with higher nutrient concentrations and lower discharge while MAMB was positively associated with lower nutrient concentrations and higher flows. Cladophora also had significantly greater associated epiphyton and macroinvertebrate biomass than MAMB.; Experimental incubations showed significantly less epiphytic biomass in 20°C Colorado River water as compared to epiphyton in 10°C river water. Gross primary production estimates on Cladophora were 3-fold lower in the 20°C water. Net primary production estimates indicated that Cladophora in the 20°C water was dead at the end of a 30-day incubation period.