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<br />2S~0 <br /> <br />Dissolved Oxygen <br /> <br />Dissolved oxygen is an important constituent of water because oxygen is <br />essential to the metabolism of most aquatic organisms and is necessary for <br />aerobic decomposition of organic matter. The dissolved-oxygen concentration <br />in water is inversely related to water temperature and is affected by photo- <br />synthesis, respiration, physical interaction of water with the atmosphere <br />(aeration), and oxygen-consuming waste loads. The concentrations of dissolved <br />oxygen in Pueblo Reservoir also are affected by the concentrations of <br />dissolved oxygen input by the Arkansas River. Photosynthetic organisms use <br />carbon dioxide from the water to synthesize carbohydrates and release oxygen. <br />As a result, the water becomes supersaturated with dissolved oxygen when <br />photosynthetic oxygen production is substantial. Dissolved oxygen is used for <br />respiration by aerobic bacteria, plants, and animals, and dissolved-oxygen <br />concentrations become depleted if respiration exceeds the rate of oxygen <br />supply. <br /> <br />In a stratified lake, the thermocline impedes transfer of dissolved <br />oxygen from the epilimnion to the hypolimnion. Hypolimnetic water, as a <br />result of respiration and decomposition of organic matter, may lose all the <br />oxygen gained during the last mixing period. If anaerobic conditions result, <br />large quantities of nutrients and trace elements may be released from the <br />bottom sediments into the water column. <br /> <br />If large quantities of decomposable organic matter are introduced into <br />a lake, oxygen depletion can occur throughout the lake, possibly causing the <br />death of aquatic organisms, primarily fish, because they have large oxygen <br />requirements. Different types of fish require varying concentrations of <br />dissolved oxygen to survive. Cold-water fish, such as trout, require larger <br />concentrations of dissolved oxygen (more than 5 mg/L) than do warm-water fish, <br />such as carp and catfish (more than 4 mg/L) (U.S. Environmental Protection <br />Agency, 1976). Because Pueblo Reservoir is a cold-water fishery, the Colorado <br />Department of Health (1982) established a water-quality standard for dissolved <br />oxygen of 6.0 mg/L during most of the year and 7.0 mg/L during the spawning <br />season. <br /> <br />Selected profiles of dissolved-oxygen measurements made during June, <br />September, and December 1985 (fig. 13) indicate changes in dissolved-oxygen <br />concentration with depth. Small increases in dissolved-oxygen concentrations <br />occasionally occurred near the thermocline and may have been caused by inflow <br />of Arkansas River water that contained relatively large dissolved-oxygen con- <br />centrations or because of photosynthetically produced oxygen by phytoplankton <br />that accumulated in this layer of increased density. During June, July, and <br />August, dissolved-oxygen concentrations measured near the reservoir surface <br />were between 7 and 8 mg/L; at the reservoir bottom, dissolved-oxygen concen- <br />trations ranged from 6 mg/L at transect 1 to less than 2 mg/L at transect 7 <br />(fig. 14). This indicates that substantial depletion of dissolved-oxygen <br />concentrations occurred in the lower strata of the reservoir, especially <br />between transects 3 and 7. The dissolved oxygen minimum of 0.1 mg/L occurred <br />at the reservoir bottom at transect 7 during August 1985. During September, <br />October, and December, the reservoir was well oxygenated, and much smaller <br />changes in dissolved-oxygen concentrations occurred between the reservoir <br />surface and reservoir bottom as a result of mixing and decreased biological <br />activity in the reservoir. Mixing caused dissolved oxygen to be transported <br />throughout the water column. <br /> <br />30 <br />