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2S~J pH The pH is a measure of the hydrogen ion activity and is important because the solubility of many chemical constituents, including trace elements, and the biological activity of many organisms are pH dependent. Because the pH may affect the suitability of water for various uses, the Colorado Department of Health (1982) established an acceptable pH range of 6.5 to 9.0 for Pueblo Reservoir. The pH of water in Pueblo Reservoir is altered by photosynthesis and res- piration and by the pH of water entering Pueblo Reservoir from the Arkansas River. The pH typically ranged from about 7.5 to 9 during the 1985 sampling period. The consumption of carbon dioxide during photosynthesis increases the pH of water, whereas the release of carbon dioxide during respiration de- creases the pH. During the summer (June), the pH values were larger near the reservoir surface as a result of photosynthesis and smaller near the bottom where organic matter is decomposing. During September at transect 3 and to a lesser extent at transect 5 (fig. 15), the pH increased below the thermocline, possibly as a result of inflow from the Arkansas River. During fall and winter, fewer variations in pH occurred with depth because of mixing and decreased biological activity in the reservoir. Chemical Constituents The presence and concentrations of various chemical constituents determine the chemical quality of water in lakes. The chemical constituents discussed in this report include nitrogen, phosphorous, major chemical con- stituents, and trace elements analyzed by the U.S. Geological Survey Denver analytical laboratory using methods described by Fishman and Friedman (1985). Nitrogen and Phosphorus Nitrogen and phosphorus species, commonly referred to as major nutrients, often are the plant nutrients most likely to limit phytoplankton growth. Thus, as nutrient concentrations increase, lake productivity increases during a process known as eutrophication. Application of fertilizers and discharge of municipal wastewater increase nutrient loads to many lakes; however, nutrient loadings to the Arkansas River upstream from Pueblo Reservoir are small because of the limited quantity of agricultural and municipal wastewater discharges. During the summer of 1985, total-nitrogen concentrations in the reservoir ranged from less than 0.2 to 1.2 mg/L and had an average concentration of about 0.6 mg/L. The predominant nitrogen species was organic nitrogen, which is not as readily utilized by algae as nitrate, and ammonia. Organic nitrogen comprised about 60 percent of the total nitrogen. The cumulative concen- trations of nitrite, nitrate, and ammonia generally were less than 0.3 mg/L. Total-inorganic-nitrogen concentrations of water samples collected near the reservoir surface ranged from less than 0.02 to 0.6 mg/L (fig. 16). The average total-inorganic-nitrogen concentration near the reservoir surface was about 0.2 mg/L. Total-inorganic-nitrogen concentrations near the reservoir 33