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--:?..~ -- ... -"iO:J..; 0.025 0.039 I 7 Median and 75th Percentile ............. Vl ::J 0: u)~ ::Jo. O:Vl 00 ::C::C 0.0. VlVl ~<( 0.0: OW ::c!:: >--' 0:0: Ow co. wVl >:l' -'<( 00: Vl<:J Vl- --' c" :l' ;;:; 0.020 0.Q15 0.010 0.005 level of Detection ______J__ 11 l' 38 l' -,,- -Median 5C SITES o 0.03 I 0.045 ~ 7. EXPLANATION SURFACE 0.03 MAXIMUM GREATER THAN I GRAPH SCALE 7 MAXIMUM 75TH PERCENTILE MEDIAN 25TH PERCENTILE MINIMUM 11 NUMBER OF MEASUREMENTS BOTTOM 0.045 MAXIMUM GREATER THAN I GRAPH SCALE ~ MAXIMUM 75TH PERCENTilE MEDIAN 11 11 78 25TH PERCENTILE MINIMUM 11 NUMBER OF MEASUREMENTS Figure 13, Dissolved-orthophosphorus concentrations in Pueblo Reservoir near the reservoir surface and near the reservoir bottom, 1987 and 1989. Table 5, Estimated loads of total nitrogen and total phosphorus in Pueblo Reservoir inflow and outflow, 1986 and 1987 Constituent load, In ton8 Total nRrogen Total phosphorus Inflow Outflow Inflow Outflow May - September 528 486 189 32 October - April 479 162 54,6 8.6 Tolal 1.007 648 243,6 40,6 growth-limiting variables for phytoplankton (Goldman and Home, 1983; Wetzel. 1983; Ryding and Rast, 1989), Dissolved-orthophosphate concentrations and the sum of dissolved ammonia and dissolved nitrite- plus-nitrate concentrations for water samples collected near the surface of Pueblo Reservoir in 1987 and 1989 were evaluated to determine if phosphorus or nitrogen concentrations limited phytoplankton growth in the reservoir. Absolute concentrations of biologically available nitrogen and phosphorus were first reviewed to determine if either nutrient had been depleted to algal growth-limiting levels. If the sum of dissolved nitrite- plus-nitrate and dissolved ammonia as nitrogen is less than 0,020 mgIL, then nitrogen is likely to be a limiting nutrient. If dissolved-orthophosphate concen- trations are less than about 0.010 mgIL, then phospho- rus is likely to be a limiting nutrient (Ryding and Rast, 1989), After reviewing the absolute concentrations of biologically available nutrients, mass ratios of the bio- logically available concentrations (N:P) were evalu- ated, Based on analysis of the phosphorus and nitro- gen content of plankton cells, Redfield and others (1963) determined that, on average, phytoplankton uti- lize these nutrients in a mass ratio of7.2N: IP. If the ratio of the biologically available nitrogen and phos- phorus (N:P) in the waterbody is less than about 7.2, nitrogen is a potentially limiting nutrient; if N:P is greater than about 7.2, phosphorus is a potentially lim- iting nutrient. If N:P is about 7.2, then both nutrients or some other factor may be limiting phytoplankton growth. Biologically available nitrogen is the sum of dissolved ammonia, nitrite, and nitrate, while biologi- cally available phosphorus is dissolved orthophospho- rus. Although N:P may vary between species, it generally is agreed upon that a mass ratio of biologi- cally available nutrients of about 7-8N: I P is a "reason- CHEMICAL CHARACTERISTICS 33