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scale of 0 to 100 (0 indicating almost no biomass for chlorophyll). This index can be estimated using sum- mer values of Secchi-disk depth (SO) and summer sur- face values oftota]-phosphorus concentration (TP) and chlorophyll a concentration (ChI). These variables, when transformed to the trophic scale, should have the same or nearly the same value. However, a divergence of one or more variables from this value may indicate a need to further study the relations among various com- ponents of the lake or reservoir (Car]son, 1977). The best variable (SO, TP, or Chi) for determining trophic state may vary from lake to lake and may also vary sea- sonally within a lake. Because variations in the TS] values between SO. TP. and Chi can provide valuable information about a lake, all three variables were used to calculate the trophic-scale values for Lake Henry and Lake Meredith. The trophic-state index was calculated for the three variables using the following equations: TSI (SO) : 10(6- J~:~) ( ]n 48 ) TS] (TP) : ]0 6- InT; TSI (Chi) = 10 (6 _ 2.04 - 0.68 In Chi) In 2 where SO is expressed in meters, TP and ChI are expressed in micrograms per liter, and In is the natural logarithm. The second trophic-classification system used was the fixed-boundary system developed by the Organisation for Economic Co-operation and Deve]op- ment (OECD) (1982). This system attempts to relate specific boundary values for minimum and mean Secchi-disk depth, mean total-phosphorus concentra- tion, and peak and mean chlorophyll a concentration to the descriptive trophic terms oligotrophic, eutrophic, and mesotrophic. The fixed-boundary system is based on opinions of a large group of limnologists from around the world who participated in the OECD pro- gram. SIGNIFICANCE OF WATER-QUALITY VARIABLES The physical, chemical, and bio]ogical relations in lakes are dynamic. The value of anyone property Or constituent is affected by the others. The following sections present a general explanation of the physical, chemical, and biological constituents determined dur- ing the study. Onsite Measurements The onsite measurements made for water tem- perature, dissolved oxygen, pH, specific conductance, and light transparency are easily done and serve as a guide for the collection of other water-quality data. Water temperature affects the physical, chemi- cal, and biological processes that occur in lakes. For example, the solubility of dissolved oxygen is inversely related to temperature; at warmer water temperatures, less oxygen can be dissolved in the water. Temperature also has a direct effect on the density of water. As water temperatures become lesser or greater than 40C, the density decreases. Stratification of lakes occurs when less dense water overlies more dense water. Stratification inhibits the mixing of waters of different densities and may result in a difference in the water quality of the water layers. Dissolved oxygen is necessary for maintaining aquatic life. The dissolved-oxygen concentration is affected by water temperature, barometric pressure. the physical interaction of the water with the atmosphere (aeration). photosynthesis and respiration, and decom- positional processes. Phytoplankton and other plants in the water release oxygen during photosynthesis and consume oxygen during respiration. The concentra- tions of dissolved oxygen may become depleted if res- piration exceeds the rate at which oxygen is replaced. Stratification may prevent transfer of dissolved oxygen from the surface water to underlying water, which may then become anoxic. The solubility of many chemical constituents and the biological activity of many organisms are pH dependent. The pH of lake water commonly is affected by photosynthesis and respiration. The use of carbon dioxide during photosynthesis increases the pH. and the release of carbon dioxide during respiration decreases the pH. Specific conductance is a measure of the ability of water to conduct an electrica] current and generally is used to approximate the dissolved-solids concentration of water. As the concentration of dis- solved solids increases, specific conductance increases. Because of this relation, concentrations of the individ- ual major chemical constituents may be estimated from the specific conductance. Light transparency is the ability of water to trans- mit ]ight and determines the depths at which sufficient light is available for photosynthesis. When light inten- sity is decreased to about I percent of its intensity at the 8 Reconnaissance of Water Quality of Lake Henry and Lake Meredith Reservoir, Crowley County, Southeastern Colorado, April-October 1987