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

Abstract

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. BRIDGES is a recurring feature of J-NABS intended to provide a forum for the interchange of ideas and information between basic and applied researchers in benthic science. Articles in this series will focus on topical research areas and linkages between basic and applied aspects of research, monitoring policy, and education. Readers with ideas for topics should contact Associate Editors, Nick Aumen and Marty Gurtz. Benthic scientists usually focus their research within 1 type of aquatic ecosystem. However, junctions between lotic and lentic ecosystems are often physically, chemically, and biologically complex, and are important gateways to the matter and energy dynamics within either or both of the adjoining systems. A symposium on Lotic–Lentic Interactions, held on 5 June 1998 at the 46th Annual NABS Meeting in Charlottetown, Prince Edward Island, highlighted the research being done on this topic. Two of the papers from the symposium dealt with restoration and management projects and, as such, link basic and applied benthic research. These 2 papers are presented below. A. D. Steinman and B. H. Rosen present an overview of restoration efforts for the Kissimmee River/Lake Okeechobee hydroscape in south Florida in the context of historical land-use changes in the watershed. These changes have accelerated P loading and eutrophication in Lake Okeechobee. The effects are demonstrated by watershed and water-quality models. E. P. Benenati et al. examine linkages between the Lake Powell Reservoir and the downstream Colorado River. These authors show that the Colorado River benthic community is strongly affected by discharges from the Lake Powell Reservoir, caused by snowmelt pulses or experimental flooding. The 2 papers illustrate the need to carefully consider adjoining ecosystems in studies of lotic and lentic systems, and in management and restoration scenarios. A. Hershey A. D. Steinman Nick Aumen, nick_aumen@nps.gov Marty Gurtz, megurtz@usgs.gov Co-editors