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Chapter III Affected Environment and Environmental Consequences 32 declining as other algae (e.g., Chara spp. and other filamentous, muscilagenous green algae) and submerged aquatic plants become established on sediment deposits from canyon wall "pour overs". It is important to note that substrates for these plants differ. Cladophora grows on rock and cobble, while Chara and other vascular aquatic plants grow best in sand or silt substrate. Higher minimum flows have permitted algae and other aquatic plants to become established above minimum reliable flow levels (5,000 cfs). The prolific growth of Cladophora, and recently Chara spp. and other aquatic plants, has resulted in a high flow zone below the dam becoming established as an important production area that feeds immediate downstream reaches. Drift from Lees Ferry reach is quickly pulverized by rapids and probably contributes very little to higher trophic levels. Drift in lower reaches originates within those reaches with particulate organic matter in the form of plant debris and aquatic invertebrates in the current as drift. Much of the drift that feeds fish and other aquatic organisms is Cladophora (either dead from desiccation or scoured loose by waterflow) and invertebrates forced to move to avoid drying. Drift is also entrained and settles to the bottom in eddies and backwater areas where it is fed on by organisms and recycled. Haden et al. (1997) investigated three main topics for Reclamation as part of a research project conducted by Northern Arizona University (NAU). The fIrst study objective was to identify the mass and structure of the existing benthic community above and below Lake Powell. The Green and Colorado Rivers above Lake Powell were used as an analogue to pre- dam conditions. They found that the composition of primary producers and consumers were dramatically different. Differences in community structure were attributed to both food source availability (detritus vs algae) and temperature. Although macroinvertebrate composition was different between the two systems, total macro invertebrate mass was not significantly different in a comparison of the most productive reaches (above and below Lake Powell) during their July 1996 survey. Haden's second study objective was to evaluate how Gammarus lacustris would react to warmer water temperatures. The post-dam food base is dependent upon Gammarus lacustris and midges. Their laboratory experiments compared growth and survivorship at 100C and 200C over a 30-day incubation period in experimental circulation chambers. The report concluded that survivorship at both temperatures were very similar. Growth rates were somewhat lower at 200C. Though statistically significant, the report noted the growth difference between the two temperatures were small and might be attributed to experimental error or temperature driven changes in phytobenthos that caused a reduction in epiphyton food base. The study did not evaluate the effects of higher nutrient loading that would be expected from the temperature control alternative, though higher nutrient loading should increase productivity. Haden's final study topic was to evaluate the possibility of other macroinvertebrate populations becoming established below Glen Canyon Dam as a result of warmer releases. Low temperatures have been speculated to be a barrier to many other macroinvertebrates. Haden's report states that colonization by other macro invertebrates would probably be limited by several factors: lack of a specific food source, competition with introduced species of invertebrates, predation by fish, and specific temperature requirements during the entire life cycle.