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005.; Table 4. Light-transparency measurements for all sites at Lake Henry Sampling site (1Ig.2) "HEW2 Secchl-dlsk deplh1 (Ieel) 1.5 .5 .5 .5 1.5 .5 .5 1.5 .5 .5 1.5 .5 .5 .5 .5 .5 1.5 .5 .5 1.5 .5 .5 Sampling dale. 1987 HNW 05-0t 06-29 08-t9 10-06 05-0t 06-29 08-t9 05-Ot 06-29 08-t9 05-01 06-29 08-19 05-01 06-29 08-t9 05-0t 06-29 08-t9 05-01 06-29 08-19 HSE HEWI HEW) HNE HSW ILight lransparency represents the deplh LO the nearest 0.5 fool 01 which the Secchi disk was nOI visible. 2HEW2 was the only site measured on October 6. 1987. Chemical Constituents During the sampling period, total-nitrogen con- centrations at site HEW2 in Lake Henry ranged from 0.86 to 1.4 mg/Land averaged about 1.1 mg/L(table 16, "Supplemental Data" section at the back of this report). The principal nitrogen form was organic nitrogen, which averaged about 86 percent of the total nitrogen. Organic nitrogen is less biologically available than are the inorganic forms of nitrogen (ammonia, nitrite. and nitrate). The concentrations of total-inorganic nitrogen (ammonia plus nitrite plus nitrate) were less than 0.2 mglL throughout the sampling period. The Colorado Department of Health (1987) has established a water-quality standard for aquatic life of 0.5 mg IL of total nitrite and 0.06 mg/L of un-ionized ammonia for Lake Henry. In greater concentrations, these two forms of nitrogen can be toxic to fish. Total- nitrite concentrations at site HEW2 ranged from 0.003 to 0.012 mg/L, as nitrogen. Un-ionized ammonia is calculated as a percentage of total ammonia, given water temperature and pH (Willingham. 1976). The percentage of total ammonia that would exist as un- ionized ammonia increases as water temperature and pH increase. Calculated un-ionized-ammonia concen- trations ranged from 0 to 0.008 mglL. During the sampling period. total phosphoms concentrations in Lake Henry ranged from 0.030 to 0.106 mg/L and averaged about 0.066 mglL. The bio- logically available form of phosphorus is orthophos- phate, which averaged about 26 percent of the total phosphorus. According to Ryding (1988). the biologically- available nutrients are beller approximated by dissolved-inorganic nitrogen and dissolved onhophos- phate as phosphorus. Concentrations of dissolved- inorganic nitrogen less than 0.02 mg/L indicate possi- ble nitrogen limitation, and concentrations of dissolved orthophosphate as phosphorus less than 0.005 to 0.010 mg/L indicate possible phosphorus limitation (Ryding, 1988). During the sampling period, concen- trations of dissolved-inorganic nitrogen, averaged for the two sampling depths at site HEW2, ranged from less than 0.034 to 0.2 I 9 mglL (table 5). Concentrations of dissolved onhophosphate as phosphorus. averaged for the two sampling depths at site HEW2, ranged from less than 0.00 I to 0.037 mg/L. The concentrations of dissolved-inorganic nitrogen and dissolved onhophos- phate as phosphorus listed in table 5 indicate that phosphorus was potentially limiting on May I, June 29, and August 19. On October 6. neither nitrogen nor phosphorus was potentially limiting. The ratio of nitrogen to phosphorus also may be used to determine which nutrient may limit algal growth in a lake. A mass ratio of dissolved-inorganic nitrogen to dissolved onhophosphate as phosphorus of 7.2:1 has been used as a boundary for defining the lim- iting nutrienl. If the ratio is less than about 7: I. nitro- gen may be the limiting nutrient; if the ratio is greater than 7: I, phosphorus may be limiting (Ryding, 1988). The ratios listed in table 5 indicate potential phospho- rus limitation in Lake Henry on May I. June 29. and August 19 and potential nitrogen limitation on October 6. WATER-QUALITY CHARACTERISTICS OF LAKE HENRY AND COMPARISON TO WATER-QUALITY STANDARDS 15