WSP11975

• File Number: 8146.100
• Description: Pueblo Dam - Project Description
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 1/1/1986
• Author: US DoI BoR
• Title: Physical Chemical and Biological Characteristics of Pueblo Reservoir Colorado 1985 - 1986
• Water Supply Pro - Doc Type: Report/Study

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Demands are placed on nutrient concentrations
in the euphotic zones of temperate-zone reservoirs
(such as Pueblo Reservoir) by phytoplanlcton during
intense periods of growth or blooms, which typically
begin in the spring and continue into fall (Goldman and
Home, 1983), The demand on nutrient concentrations
generally decreases substantially during the colder
months, typically October through April, when sun-
light and water temperatures decrease. During the
summer in biologically productive reservoirs, when
thennal stratification is well established in the reservoir
and dissolved-oxygen concentrations become very low
or depleted in the hypolimnion, mobilization of nutri-
ents from the bottom sediments occurs, The nutrients
released from the bottom sediments may be discharged
from the reservoir in deep-water withdrawals or redis-
tributed throughout the water column following fall
turnover.

In 1986 through 1989, the median concentration
of dissolved inorganic nitrogen at sites 3B, 5C, and 7B
ranged from 0.07 to 0.10 mg/L near the surface and
from 0.19 to 0.30 mg/L near the bottom during May
through September (fig. II). Inorganic nitrogen con-
centrations in water samples collected from near the
reservoir surface did not vary much between the
upstream and downstream parts of the reservoir and
generally were smaller than the dissolved inorganic
nitrogen concentrations in samples collected near the
bottom of the reservoir during May through September
probably because of biological uptake of nitrogen in
the epilimnion, underflow of nitrogen from the Arkan-
sas River, and bottom-sediment releases of ammonia.
The effect of bottom releases of nitrogen to the
hypolimnion is most evident at site 7B, where inor-
ganic nitrogen concentrations were substantially larger
than at sites 3B and 5C. During October through April,
the median dissolved inorganic nitrogen concentrations
ranged from 0.17 to 0.24 mg/L near the surface and
near the bottom (fig. II). There is little variation
between the surface and bottom concentrations of dis-
solved inorganic nitrogen during October through
April, because (I) the reservoir generally is well mixed
following fall turnover, (2) the reservoir is well oxy-
genated, thus, decreasing the flux rate of nitrogen from
the bottom sediments, and (3) the biological uptake of
nitrogen in the euphotic zone is decreased during the
colder months.

In 1986 through 1989, the median concentration
of total phosphorus at sites 3B, 5C, and 78 ranged from
0.015 to 0.04 mg/L near the reservoir surface and from
.032 to .053 mg/L near the reservoir bottom during
May through September (fig, 12). During October
through April, median concentrations of total phospho-
rus ranged from .013 to .031 mg/L near the reservoir

surface and from .012 to ,039 mg/L near the reservoir
bottom (fig. 12). Total phosphorus concentrations typ-
ically are larger during May through September than
during October through April. The seasonal variation
probably is caused by larger phosphorus concentrations
in the inflow during May through September and
releases of phosphorus from the bottom sediments dur-
ing the summer. Throughout the year, concentrations
of total phosphorus decrease in a downstream direction
probably because of sedimentation of the particulate
matter on which phosphorus is sorbed and uptake of
dissolved phosphorus by phytoplankton. Concentra-
tions of total phosphorus typically are larger near the
reservoir bottom than near the surface probably
because of underflow of phosphorus from the Arkansas
River, settling of particulate matter, phosphorus
releases from the bottom sediments, and photosyn-
thetic demands for dissolved orthophosphate by phy-
toplanlcton in the euphotic zone.

Dissolved orthophosphorus data collected in
1987 and 1989 indicate concentrations near the reser-
voir surface decrease downstream from the upper part
of the reservoir (site 38, fig, 13). The decrease proba-
bly results from uptake and removal of most of the
incoming orthophosphorus by phytoplankton in the
euphotic zone in the upstream end of the reservoir,
This is consistent with the concept of longitudinal
chemical and biological zonation in a reservoir
reported by IGmmel and others (1990), which suggests
that the largest phytoplankton production occurs just
downstream from the "plunge point" in a reservoir
and decreases toward the dam end of a reservoir.
Dissolved-orthophosphate concentrations near the res-
ervoir bottom changed very little throughout the reser-
voir. The lack of variation in dissolved-orthophosphate
concentration near the reservoir bottom probably is the
result of the lack of biological uptake of nutrients in the
hypolimnion and the flux of phosphorus from the bot-
tom sediments replenishing the phosphorus that sorbs
to particulate matter and settles from the water column.

Nutrient loading to Pueblo Reservoir was ana-
lyzed for 1986 and 1987 (table 5). Total-nitrogen and
total-phosphorus loads were computed for two periods:
(I) the more biologically active period, May through
September, and (2) the colder, less biologically active
period, October through April. Data for the computa-
tion of nutrient loads in the reservoir inflow and Oul-
flow were collected at Arkansas River at Portland
(station 07097(00) and Arkansas River above Pueblo
(station 070994(0), respectively. Loads were com-
puted as the product of total streamflow and the median
nutrient concentration for each period. Loads were
computed for the two periods to account for any sea-

.30 Physical. Chemical, and Biological Characteristics ot Pueblo Reservoir, Colorado, 1985-89