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We tested the recolonization of the phytobenthic community in the tailwaters of Glen Canyon Dam following long‐ and short‐term experimentally induced desiccation. The response of Cladophora glomerata, Oscillatoria spp., miscellaneous phytobenthos species and periphyton was studied over 18 weeks using three treatments: (1) undisturbed control cobbles from the submerged zone; (2) cobbles desiccated and replaced into the submerged zone; and (3) cobbles desiccated and replaced into the varial zone. Periphyton density and compositional response resulting from these treatments were also examined. Desiccation treatments were significantly different in biomass from controls throughout the study. The biomass of desiccated and replaced river cobbles averaged <30% of the controls for C. glomerata and periphyton during the 18‐week recolonization period. In contrast, the biomass of the control Oscillatoria and miscellaneous phytobenthos species averaged only 3 and 50% of that in the desiccation treatments, respectively. Cladophora, the dominant alga, comprised 77% of the phytobenthic biomass. A significant positive relationship between discharge, Cladophora and periphyton biomass was present in all treatments, while there was a negative relationship with discharge for Oscillatoria and miscellaneous phytobenthos species. There was a significant inverse correlation, as well as a pattern of separate biomass dominance, between C. glomerata and Oscillatoria. Cladophora was the dominant phytobenthos species on cobbles below the baseflow and Oscillatoria was dominant on cobbles in the varial zone. In general, the Cladophora biomass decreased under intermittent drying conditions, while Oscillatoria declined under permanently wet conditions. Diatom composition was not significantly different between treatments; however, their density was lower on desiccated cobbles. Diatom density on desiccated cobbles in the submerged and varial zones averaged 69 and 42% of that of the controls, respectively. Recovery and maintenance of benthic resources are hindered by fluctuating flow regimes driven by electricity and irrigation requirements. Repeated desiccation of the phytobenthos has major effects on the bottom‐up interactions in the Colorado River ecosystem. © 1998 John Wiley & Sons, Ltd.

We tested the recolonization of the phytobenthic community in the tailwaters of Glen Canyon Dam following longand short-term experimentally induced desiccation. The response of Cladophora glomerata, Oscillatoria spp., miscellaneous phytobenthos species and periphyton was studied over 18 weeks using three treatments: (1) undisturbed control cobbles from the submerged zone; (2) cobbles desiccated and replaced into the submerged zone; and (3) cobbles desiccated and replaced into the varial zone. Periphyton density and compositional response resulting from these treatments were also examined.Desiccation treatments were significantly different in biomass from controls throughout the study. The biomass of desiccated and replaced river cobbles averaged B30% of the controls for C. glomerata and periphyton during the 18-week recolonization period. In contrast, the biomass of the control Oscillatoria and miscellaneous phytobenthos species averaged only 3 and 50% of that in the desiccation treatments, respectively. Cladophora, the dominant alga, comprised 77% of the phytobenthic biomass. A significant positive relationship between discharge, Cladophora and periphyton biomass was present in all treatments, while there was a negative relationship with discharge for Oscillatoria and miscellaneous phytobenthos species.There was a significant inverse correlation, as well as a pattern of separate biomass dominance, between C. glomerata and Oscillatoria. Cladophora was the dominant phytobenthos species on cobbles below the baseflow and Oscillatoria was dominant on cobbles in the varial zone. In general, the Cladophora biomass decreased under intermittent drying conditions, while Oscillatoria declined under permanently wet conditions. Diatom composition was not significantly different between treatments; however, their density was lower on desiccated cobbles. Diatom density on desiccated cobbles in the submerged and varial zones averaged 69 and 42% of that of the controls, respectively.Recovery and maintenance of benthic resources are hindered by fluctuating flow regimes driven by electricity and irrigation requirements. Repeated desiccation of the phytobenthos has major effects on the bottom-up interactions in the Colorado River ecosystem.

Coarse particulate organic mass varied seasonally and changed from autochthonous to allochthonous organic matter along a 386-km section of the Colorado River below Glen Canyon Dam, Arizona. Dam operations influenced drift components selectively throughout the hydrograph; however, ramping rate had no effect on drift mass. Eddies and pools collected~50% of the main-stem drift with only detritus retained long enough to settle in pools. Coarse particulate organic drift mass was negatively correlated with stream gradient in the Colorado River. Cladophora drift packets were pulverized by rapids, which subsequently increased fine particulate organic matter at downriver sites. Tributary type (spring stream or large desert watershed river) significantly altered the mass and composition of tributary drift; however, tributaries contributed <0.1% of the total organic matter to main-stem drift. Terrestrial insects represented <0.001% of the total invertebrate mass in main-stem drift. As a result of hydraulics and suspended sediment, organic drift mass throughout the river corridor is reach specific. Longitudinal change in drift composition corresponds with a shift in fish species. Résumé : Les masses de particules organiques grossières présentaient des variations saisonnières et leur composition passait des matières organiques autochtones à allochtones le long d'un tronçon de 386 km du fleuve Colorado, en aval du barrage de Glen Canyon (Arizona). Les opérations du barrage modifiaient sélectivement les constituants des matières dérivantes dans tout le bassin hydrographique mais toutefois, le débit de décharge n'avait aucun effet sur les masses dérivantes. Les bassins à tourbillons recueillaient environ 50% des matières dérivantes du courant principal, ainsi que les détritus qui pouvaient être retenus assez longtemps pour s'y déposer. Les masses dérivantes de particules organiques grossières présentaient une corrélation négative avec la pente du Colorado. Les masses dérivantes de Cladophora étaient pulvérisées par les rapides, ce qui augmentait par la suite la teneur en matières organiques fines à des emplacements en aval. Le type de tributaire, soit des ruisseaux de printemps par rapport à des grands cours d'eau à bassin versant désertique, avait une influence significative sur la masse et la composition des matières dérivantes des tributaires. Toutefois, ceux-ci fournissaient moins de 0,1% des matières organiques totales des matières dérivantes du courant principal. Les insectes terrestres représentaient moins de 0,001% de la masse totale d'invertébrés de ce courant. Dans tout le cours d'eau, à cause des caractéristiques hydrauliques et des sédiments en suspension, la masse organique dérivante dépendait de chacun des tronçon. Les changements longitudinaux de la composition de la masse dérivante correspondaient à une variation de la répartition des espèces de poissons. [Traduit par la Rédaction]

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