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Chapter III Affected Environment and Environmental Consequences 31 river should increase the primary productivity of the river system during the summer months when temperature controls are in operation. When not in operation, releases would return to pre-existing conditions which are typically very low. The cycling and amount of nutrients in the tailwaters and lower reaches below Glen Canyon Dam are not well known. While nutrient levels are often at or below detection limits, the fact that there is high biomass of Cladophora glome rata below the dam suggests that nutrients are may not be a limiting factor. The uptake and cycling of nutrients may be quick enough that there is very little opportunity to sample free dissolved nutrients in the water column of the river. Another hypothesis is that delivery rates oflow nutrient levels are sufficient for Cladophora glomerata. Nutrients may also be provided by sediment inputs from below the dam, even in the Lees Ferry reach. Nutrient levels are important to the ecosystem below Glen Canyon Dam. The existing, post-impoundment, aquatic system is characterized by constant year-round releases of about 80C (460F) water that contain low nutrient and low sediment levels. Discharges of clear, cold water from Glen Canyon Dam have permitted the filamentous green alga Cladophora glomerata to capitalize on the few available nutrients released through the dam and other tributary sources. Cladophora and the diatoms depend on these nutrients and form the habitat and food base for an important community of aquatic invertebrates dominated by the amphipod Gammarus lacustris, midges (Chiromomidae), and other aquatic insects. Cladophora, along with the organisms that live on it, forms the basis of a highly productive food chain below Glen Canyon Dam (USDI 1995). ~0.4 0::::: CI E 0.3 ~ gj 0.2 - '1:: G) 0.1 o o 01/03 ~0.15 0::::: CI E 0.1 ~ CD lU 0.05 CI -<( ~ i3l 0.04 E ~0.03 lU 't:; 0.02 o E 0.01 E -<( Detritus Releases _Without Controls o With Controls 05/03 08/31 12/29 Algae Releases ~Without Controls 8 0 With Controls g E6 o 01/03 05/03 08/31 12/29 Ammonia Releases _Without Controls o With Controls o 01/03 05/03 08/31 12/29 Figure IO-Extremely low nutrient releases from the dam to the downstream environment would increase with temperature controls. Since inception of interim flows, the plant component of the aquatic food base has begun to change. The relative dominance of Cladophora in the Glen Canyon reach may already be