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;-i ~. 4) 1 p :~ _, ).." JJ Seasonal and daily variations of dissolved-oxygen concentration in a water body are indirect measures of the biological productivity and oxygen demand of the system. If photosynthetic or respiratory activities are great during thermal stratification, separate biological zones can develop and cause vertical and horizontal variation in chemical characteristics. In lakes, the epilimnion typically is a zone of active biomass (phytoplankton) production, marked by nutrient uptake and daytime oxygen production. Where ample nutri- ents and light conditions exist, zones of oxygen supersaturation may develop. Conversely, overlying biomass or debris transported by inflowing streams may become deposited in the hypolimnion. As organisms die and settle downward, they are decomposed by bacteria and fungi. During decomposition, nutrients are returned to the water and oxygen is consumed. If large quantities of organic matter are present and there is little or no mixing with the oxygen- ated upper water, the dissolved-oxygen concentrations in the water at deeper levels may decrease to zero (anaerobic condition). The quantity of oxygen consumed by indigenous processes in an isolated water sample confined in the dark at 20 oC for 5 days is termed the 5-day BOD. BOD is an estimated measurement of the biological- and chemical-oxygen demand required by the processes of respiration, decomposition, and chemical oxida- tion that occur in the sample. Except for the zero value for dissolved oxygen near the bottom of the reservoir in January 1985, measured concentrations of dissolved oxygen at sites 1 and 2 in the reservoir (tables 11 and 12 in the "Hydrologic Data" section) ranged from 2.3 to 10.1 mg/L. Although concentrations of dissolved oxygen tended to decrease with depth at site 1, concentrations equal to or greater than 4.0 mg/L were maintained from the reservoir surface to a depth of 35 ft. The decrease in dissolved oxygen to concentrations less than 4.0 mg/L at depths greater than 35 ft may indicate that depletion of dissolved oxygen from respiration and decomposition processes occurred near the bottom of the reservoir. The concentration of 0.0 mg/L for dissolved oxygen in January 1985 may have resulted from residual ground water that collected near the bottom of the reservoir. Concentrations determined in the White River at site 3 during 1985-87 ranged from 7.8 to 12.5 mg/L. In June 1987, maximum concentrations of dissolved oxygen within the reservoir were measured during a phytoplankton bloom that drifted at depths generally less than 1 ft. A concentration of 14.7 mg/L dissolved oxygen was measured during additional sampling at the I-ft depth near the center of the reservoir on June 19, 1987 (fig. 8). Measurements of dissolved oxygen within other localized near-surface patches of phytoplankton on June 9 and June 19, 1987, ranged from 14.0 to 18.0 mg/L. These concentrations represented a range of supersaturation of dissolved oxygen from 190 to 240 percent. In addition to the phytoplankton bloom in June 1987, supersaturation of dissolved oxygen from photosynthetic activities occurred occasionally at both reservoir sites (fig. 6; tables 11 and 12 in the "Hydrologic Data" section). Smaller concen- trations and ranges of dissolved oxygen, as compared to the concentrations and ranges of saturation of dissolved oxygen measured within the patches of phyto- plankton, were measured at sites 1 and 2 (fig. 6; tables 11 and 12 in the "Hydrologic Data" section) and at depths greater than 5 ft directly below the phytoplankton bloom in June 1987 (fig. 8). These differences indicate that the greatly supersaturated conditions of dissolved oxygen (190-240 percent) were limited to water that contained the concentrated growths of phytoplankton. 19