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<br />effluent at site 6 (about ]40 times) compared to site 3
<br />(about 40 times), Furthermore, some of the effects of
<br />photosynthesis at site 6 possibly were caused by
<br />discharge of sewage outfall from SSRWWTP, much
<br />smaller sewage discharges at Hayden and Mi]ner, and
<br />agricu]tural effluent.
<br />Downstream from site 6, the effects of photo-
<br />synthesis decreased to Yampa River near Maybell
<br />(site 8) and further to Yampa River above Little Snake
<br />River (site 9), Values of pH decreased slightly to 8,74
<br />at both sites, Dissolved oxygen concentrations
<br />became less oversaturated (120 percent at site 8 and
<br />105 percent at site 9), and the river became only
<br />slightly undersaturated with CO2 (87 percent of satu-
<br />ration at site 8 and 86 percent at site 9),
<br />Apparently, respiration plus oxidation of plant
<br />material and organic matter (approximately the reverse
<br />of equation I; Drever, 1982) were more dominant than
<br />photosynthesis at Yampa River at Deerlodge Park (site
<br />II) and at Yampa River at mouth (Echo Park) (site 12),
<br />This conclusion is based on the smaller pH value
<br />measured at site II (8,51) and site] 2 (8,60), which
<br />resulted from oversaturation of CO2 (189 percent at
<br />site 11 and 137 percent at site 12), The apparently
<br />abrupt shift to dominance of respiration plus oxidation
<br />at site II compared to relative balance between photo-
<br />synthesis and respiration plus oxidation at sites 8 and 9
<br />possibly resulted (I) from inflow of nutrient-poor
<br />organic matter from the Little Snake River (site 10),
<br />(2) from arrival of water with larger concentratious of
<br />oxidizable organic matter, possibly flushed from the
<br />land surface by rain in the basin on the previous day,
<br />(3) from a reduction in aquatic-plant biomass and
<br />productivity between sites 9 and 11, or (4) from a
<br />combination of these factors, Slight oversaturation of
<br />the river water with dissolved oxygen (109 percent at
<br />site II and 103 percent at site 12) indicates that less
<br />photosynthesis occurred at sites II and 12 than at
<br />upstream sites.
<br />Except for dissolved nitrogen as ammonia plus
<br />organic nitrogen, concentrations of dissolved nutrients
<br />(table 3) were small at Yampa River and tributary sites
<br />during August ]6-19, 1999, Downstream from Yampa
<br />River below Stagecoach Reservoir (site I), concentra-
<br />tions of dissolved ammonia were less than 0,02 mg/L
<br />as N, and concentrations of dissolved nitrite plus
<br />nitrate were less than 0,05 mgfL as N, Concentrations
<br />of dissolved nitrogen as ammonia plus organic
<br />nitrogen generally decreased from 0,36 mgfL as N at
<br />Yampa River below Stagecoach Reservoir (site I) to
<br />
<br />0.18 mg/L as N at Yampa River at mouth (site 12),
<br />These relations imply that most of the dissolved
<br />nitrogen in the Yampa River during this sampling
<br />consisted of organic (reduced) nitrogen, Downstream
<br />from Yampa River above Elk River (site 3) concentra-
<br />tions of dissolved phosphorus were less than
<br />0,05 mg/L as P, and concentrations of dissolved ortho-
<br />phosphate were less than 0,01 mg/L as p, Bacterial
<br />mineralization (oxidation) of reduced nitrogen and
<br />phosphorus provided most of the nutrients for photo-
<br />synthesis downstream from sites 3 and 6,
<br />Calculations using PHREEQC indicate that
<br />samples from all Yampa River sites, the Williams Fork
<br />(site 7), and the Little Snake River (site 10) were
<br />substantially oversaturated with calcite (CaC03)
<br />(fig, 5), This relation implies that these waters could
<br />not dissolve calcite and possibly were precipitating it.
<br />Only water from the Elk River (site 4) was undersatu-
<br />rated with calcite,
<br />Geochemica] calculations indicate that most of
<br />the variation in pH between Yampa River sites was
<br />caused by differences in Peo2 (the effective partial
<br />pressure of CO2 gas on the solution) and degree of
<br />oversaturation with calcite, The geochemical reaction
<br />model PHREEQC was used to simulate the effect of
<br />allowing the samples collected from Yampa River sites
<br />to equilibrate with ambient P CO2 (0,00033 times
<br />ambient atmospheric pressure), The resulting pH
<br />values from the simulations are in the narrow range
<br />from 8,55 to 8,80 (fig, 6) with a relative distribution
<br />very similar to that of a]kalinity concentrations (fig, 4),
<br />Once the dominating effects of undersaturation and
<br />oversaturation with CO2 were removed, pH was
<br />large]y a function of, and was proportional to, alka-
<br />linity,
<br />Simulated equilibration with atmospheric Peo
<br />had the greatest effect on the sample from site 3 2
<br />(which was most undersaturated with dissolved CO2
<br />because of photosynthesis), decreasing the pH value
<br />by 0,5] unit. The simulated equilibrium also decreased
<br />the pH of water from site 5 (by 0,15 unit) and from site
<br />6 (by 0.21 unit), Water from these sites also was
<br />undersaturated with CO2 (fig, 5), Simulated equilibra-
<br />tion with atmospheric CO2 substantially increased the
<br />pH value of samples from sites that were substantially
<br />oversaturated with CO2 (fig, 5): site I (by 0.34 unit);
<br />site 11 (by 0,23 unit); and site 12 (by 0,11 unit), Values
<br />for pH at sites 8 and 9 were decreased slightly (by 0,06
<br />unit) by the simulation because they were close to
<br />equilibrium with atmospheric CO2 (fig, 5),
<br />
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<br />INTERPRETATION OF DATA COLLECTED FOR THIS STUDY 11
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