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.~,~..- ~~~, Concentrations of major chemical constituents near the reservoir surface at transect 7 and near the reservoir bottom at transect 2 for the summer of 1985 are shown in figure 17. Between July and August, concentrations of calcium, magnesium, sodium, bicarbonate, chloride, and sulfate more than doubled near the reservoir bottom, whereas concentrations of these constit- uents changed very little near the reservoir surface at transect 7. The variations in ion concentrations at transects 2 and 7 were consistent with changes in specific conductance that occurred with depth and demonstrate the effects of chemical stratification for specific ions. The changes in concen- trations of specific ions probably result largely from variations in concen- trations of the ions in the Arkansas River upstream from the reservoir. Trace Elements Trace elements occur in relatively small concentrations and usually are expressed in micrograms per liter. Concentrations of selected trace elements analyzed from water samples collected during the summer of 1985 near the reservoir bottom at transect 2 and near the reservoir surface at transect 7 are summarized in table 3. These data indicate that most of the trace ele- ments are suspended rather than dissolved; the trace elements are sorbed to the sediment particles (and organic particulates) and, therefore, are trans- ported with sediment entering the reservoir. As the sediment settles to the reservoir bottom, trace elements are removed from the water. Reducing condi- tions commonly occur in the bottom sediments where the water filling the void spaces between sediment particles contains little or no dissolved oxygen. These conditions result in dissolution and mobilization of trace elements which then migrate upward. As trace elements reach the surface of the reser- voir sediments, the trace elements are oxidized by dissolved oxygen in the reservoir water and precipitate on the upper sediment particles (Britton and Wentz, 1980). If dissolved oxygen becomes depleted in the lower strata of the reservoir, reducing conditions may cause large concentrations of trace elements to be released quickly to the reservoir water. During August and September 1985, concentrations of total recoverable iron were 4,100 ~g/L and 2,800 ~g/L near the reservoir bottom at transect 2. The recommended criterion for aquatic life is 1,000 ~g/L (U.S. Environmental Protection Agency, 1976). Dissolved-manganese concentrations also exceeded public water-supply standards near the reservoir bottom at transect 2 during July and September. Concentrations of other trace elements were less than established water-quality standards for Pueblo Reservoir (Colorado Department of Health, 1982). Biological Constituents Lakes contain a large variety of organisms. During 1985, the distribu- tion and abundance of phytoplankton and zooplankton were measured. Plankton is the community of suspended or floating organisms that drift with water currents. The plant portion of the plankton are called phytoplankton and commonly are referred to as algae. Zooplankton are the animal part of the plankton. 37