Reservoir-river linkages: Lake Powell and the Colorado River, Arizona

The effect of lotic-lentic interactions on the phytobenthic community was s tudied in the Colorado River below Glen Canyon Dam (GCD) Arizona. Major com positional changes in river phytobenthos occurred following high snowmelt i nflows that filled the Lake Powell ell reservoir and resulted in high, stea dy discharges from GCD. Cladophora glomerata, the previously dominant alga, was partly displaced by a mixed mucilaginous phytobenthic assemblage consi sting of miscellaneous algae, macrophytes, and bryophytes (MAMB). Relations hips between abiotic parameters (discharge conductivity, nutrient concentra tion, and season) and biotic responses (composition and biomass of filament ous and epiphytic algae, and invertebrate biomass) were analyzed from 1995 to 1998. In addition, the response of the phytobenthos to an experimental f lood (1275 m(3)/s) was examined in the tailwaters below GCD. Significant co mpositional changes occurred within 3 mo of lake filling and the onset of e levated discharges in June 1995. Relative phytobenthic composition by bioma ss prior Co June 1995 was: Cladophora glomerata = 915%, MAMB = 35% and cyan obacteria = 5.0% compared to 49.6 47.9, and 2.5%, respectively, after the i nitiation of high discharges. Cladophila and MAMB had an inverse relationsh ip that fluctuated seasonally. Cladophora was positively associated with in creased nutrient concentrations and loner discharge during summer-autumn wh ereas MAMB was positively correlated with deceased nutrients and higher dis charge during winter-spring. Cladophora was positively associated with inve rtebrate biomass and also had greater epiphyton biomass and density than MA MB. The 1996 experimental flood resulted in a significant loss of MAMB biom ass, in contrast to a minimal decrease in Cladophora and invertebrates. int eractions between inflow patterns, reservoir chemistry, and discharge regim e have an important effect on benthic community structure below dams. Resou rce managers need to consider these relationships and regard water resource s above and below impoundments as a connected system when making management decisions.