We examined the impact of the 1996 test flood released from Glen Canyon Dam (GCD) on the aquatic food base in the Colorado River through Grand Canyon National Park, Arizona, USA. Benthic scour and entrainment of both primary and secondary producers occurred at all study sites along the 385-km river corridor. The majority of the organic drift occurred within the first 48 h of the test flood with the arrival of the hydrostatic wave. Recent macrophyte colonizers (Chara, Potamogeton, and Elodea) of fine sediment in the tailwaters were scoured from the channel bottom, with recovery to pre-flood estimates within 1-7 months depending on taxa. Macroinvertebrates and filamentous algae recovered within three months depending on taxa. The test flood removed suspended particles from the water column and increased water clarity, which enhanced benthic recovery. The testflood hydrograph was designed primarily as an experiment in sand transport and occurred during a period of sustained high releases from GCD starting in June 1995 due to aboveaverage inflow into Lake Powell. We discuss the implications of the hydrograph shape, pre-and post-riverine conditions, and the slow response time of biological resources for design of aquatic ecosystem experiments.

Dual stable isotope analysis in the regulated Colorado River through Grand Canyon National Park, USA, revealed a food web that varied spatially through this arid biome. Down-river enrichment of delta13C data was detected across three trophic levels resulting in shifted food webs. Humpack chub delta13C and delta15N values from muscle plugs and fin clips did not differ significantly. Humpback chub and rainbow trout trophic position is positively correlated with standard length indicating an increase in piscivory by larger fishes. Recovery of the aquatic community from impoundment by Glen Canyon Dam and collecting refinements for stable isotope analysis within large rivers are discussed.

The effect of lotic-lentic interactions on the phytobenthic community was studied in the Colorado River below Glen Canyon Dam (GCD), Arizona. Major compositional changes in river phytobenthos occurred following high snowmelt inflows that filled the Lake Powell reservoir and resulted in high, steady discharges from GCD. Cladophora glomerata, the previously dominant alga, was partly displaced by a mixed mucilaginous phytobenthic assemblage consisting of miscellaneous algae, macrophytes, and bryophytes (MAMB). Relationships between abiotic parameters (discharge, conductivity, nutrient concentration, and season) and biotic responses (composition and biomass of filamentous and epiphytic algae, and invertebrate biomass) were analyzed from 1995 to 1998. In addition, the response of the phytobenthos to an experimental flood (1275 m3/s) was examined in the tailwaters below GCD. Significant compositional changes occurred within 3 mo of lake filling and the onset of elevated discharges in June 1995. Relative phytobenthic composition by biomass prior to June 1995 was: Cladophora glomerata = 91.5%, MAMB = 3.5%, and cyanobacteria = 5.0%, compared to 49.6, 47.9, and 2.5%, respectively, after the initiation of high discharges. Cladophora and MAMB had an inverse relationship that fluctuated seasonally. Cladophora was positively associated with increased nutrient concentrations and lower discharge during summer-autumn, whereas MAMB was positively correlated with decreased nutrients and higher discharge during winter-spring. Cladophora was positively associated with invertebrate biomass and also had greater epiphyton biomass and density than MAMB. The 1996 experimental flood resulted in a significant loss of MAMB biomass, in contrast to a minimal decrease in Cladophora and invertebrates. Interactions between inflow patterns, reservoir chemistry, and discharge regime have an important effect on benthic community structure below dams. Resource managers need to consider these relationships and regard water resources above and below impoundments as a connected system when making management decisions.

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The concentration and composition of drifting fine particulate organic matter (FPOM) in regulated rivers may be influenced by dam operations and site of collection within the river channel. We examined FPOM composition and biomass in the Colorado River, Arizona below Glen Canyon Dam along a 350 km reach during 15 collection mps over four years. Lotic zooplankton and detritus components were positively correlated with distance downriver from the dam, increased discharge, and near-shore habitats versus mid-channel locations. Lentic zooplankton also increased at higher discharges and in near-shore habitats but was negatively correlated with distance downriver. There is evidence the near-shore habitat provides a more stable environment than the mainstem for invertebrates which may enhance rearing and development of lentic zooplankton.