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<br />ZS6G <br /> <br />Zooplankton <br /> <br />Zooplankton are the animal part of the plankton and are an important <br />source of food for fish. Some zooplankton are strong swimmers, while others <br />have weak powers of locomotion. A daily, vertical migration of zooplankton is <br />common (Britton and others, 1975). Zooplankton feed on bacteria, algae, and <br />other zooplankton. Zooplankton grazing on phytoplankton may limit algal <br />populations. <br /> <br />Zooplankton densities (organisms per cubic meter) in Pueblo Reservoir are <br />shown in figure 21. Zooplankton densities ranged from a few thousand organ- <br />isms per cubic meter at transects 1 and 2 to about 80,000 organisms per cubic <br />meter at transect 5 during August. <br /> <br />SUMMARY AND CONCLUSIONS <br /> <br />Pueblo Reservoir is the farthest upstream, main-stem reservoir con- <br />structed on the Arkansas River and drains an area of 4,669 mi2, of which about <br />94 percent drains into the Arkansas River upstream from the reservoir. This <br />reconnaissance study provides a better understanding of the reservoir and <br />quality of water entering Pueblo Reservoir and makes a preliminary assessment <br />of some of the water-quality characteristics of the reservoir. <br /> <br />During the 1985 sampling period, Pueblo Reservoir was stratified, and <br />underflow from the Arkansas River occurred that resulted in stratification <br />of the water in the reservoir with respect to specific conductance. Concen- <br />trations of dissolved solids decreased markedly below the thermocline during <br />June. Later in the summer, circulation deepened the thermocline, and <br />dissolved-solids concentrations increased substantially below the thermocline. <br />The variations in specific conductance that occurred in the reservoir gen- <br />erally coincided with variations in specific conductance measured at station <br />07097000, Arkansas River at Portland. During the 1985 water year, the maximum <br />daily mean specific conductance at Arkansas River at Portland was 540 ~S/cm <br />and occurred during January 1985; the minimum daily mean specific conductance <br />of 140 ~S/cm occurred during June 1985. <br /> <br />Dissolved-oxygen profiles made during the summer of 1985 indicated that <br />substantial depletion of dissolved-oxygen concentrations occurred in the lower <br />strata of Pueblo Reservoir. The minimum dissolved-oxygen concentration of <br />0.1 mg/L occurred during August near the reservoir bottom at transect 7 <br />(nearest the dam). During fall and winter, much smaller changes in disso1ved- <br />oxygen concentrations occurred between the reservoir surface and reservoir <br />bottom because of mixing and decreased biological activity in the reservoir. <br />Mixing caused dissolved oxygen to be transported throughout the water column. <br /> <br />The pH of Pueblo Reservoir typically ranged from about 7.5 to 9 during <br />the 1985 sampling period. The pH was largest near the reservoir surface as <br />the result of photosynthesis and smallest near the bottom where organic matter <br />was decomposing. During fall and winter, less change in pH occurred with <br />depth because of mixing and decreased biological activity in the reservoir. <br /> <br />46 <br />