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'2,8 ',j"7 10,oao 5,000 0 z 0 2,000 U w (fj c: 1,000 W <l. <65, 000 f- <00. 000 w 500 w 150.000 u. ~ 100.000 '" ::J u 200 Z 50,000 ;:' 100 0 ..J U. f- ::J 50 0 ] o 60 120 RESIDENCE TIME, IN DAYS -265,000- RESERVOIR STORAGE, IN ACRE-FEET 180 240 300 360 Figure 6, Relation of residence time, outflow, and reservoir slorage in Pueblo Reservoir. Water Transparency Water transparency is the capability of water to transmit light and determines the depth of the euphotic zone where sufficienllight is available for photosyn- thesis, Light penetration within the reservoir is limited by suspended inorganic and organic particulate mailer. An approximation of the depth of the euphotic zone and the capability of water in Pueblo Reservoir to transmitlighl was made using a Secchi disk. The depth of the euphotic zone in Pueblo Reservoir can be approximated by multiplying the Secchi-disk depth by two, which is consistent with the relation reported by Moss (1980). The relation between the euphotic zone and Secchi-disk depth is based on a comparison of Secchi-disk measurements 10 measured compensation- zone depths made on July 1, 1986, May 27 and 28, 1987, and July 28 and 29,1987. The compensation zone is the depth of water al which oxygen production by photosynthesis equals the oxygen consumption by respiration, which usually occurs when the light inlen- sity is decreased to about 1 percent of the intensity al the surface. In Pueblo Reservoir, Secchi-disk de'pths increase in a downstream direction throughout the year (fig, 8) and are consistently larger near the dam than near the inflow. The largest water transparencies and euphotic- zone depths occur throughout Pueblo Reservoir from December to late April, a period that corresponds to the minimum annual sediment load to Pueblo Reservoir and the minimum annual biomass. The largest Secchi- disk depth (9.75 m) was recorded at transect 7 during March 1986 (Edelmann and others, 1991). The capability of the reservoir to transmit light and the depths of the euphotic zone are diminished from lale April or May through November (fig. 8). From lale April or May through August, water trans- parency is suppressed because of increased sediment loads 10 Pueblo Reservoir and an increase in biomasS. During this period, the depth of the euphotic zone gen- erally (less than 50 percent of the time) is less than about 2 m in the upstream part of the reservoir as shown by site 3B in figure 8, Jess than about 4 m in the middle part of the reservoir as shown by site 5C, and less than about 5 m in the downstream part of the reservoir as shown by site 7B, 18 PhYllcll, Chemlcll, Ind Blologlcll Charlcterlstlcl 01 Pueblo Re..rvolr, Colorado, 198!H19