8250

• UCREFRP Catalog Number: 8250
• Author: Lamarra, V. A.
• Title: Longitudinal Variation in the Trophic Structure of the Upper Colorado River.
• USFW Year: 1999.
• USFW - Doc Type: Logan, Utah.
• Copyright Material: NO

25 shows the river-wide averages for detritus densities in riffles and runs in the Colorado and
Gunnison rivers. No significant differences were found between the detritus levels in riffles
and runs in the Colorado River for the spring of 1994 and 1995. However, riffles were found
to be significantly greater in fall 1994 than in fall 1995. In both years, the amount of detritus
in riffles increased between the spring and fall periods while decreasing within run habitats.
The highest mean concentration of detritus occurred in riffles and the lowest concentrations
occurred within runs in fall 1994.
In the Gunnison River, detritus levels were highly variable (note the large 95% confidence
intervals in Figure 25) with mean values greater than the Colorado River for both runs and
riffles. However, these higher mean values were not significantly different from the mean
Colorado River levels.
Densities of secondary consumers were estimated by measuring the standing crop of
macroinvertebrates within the benthic substrate. These results are presented in Figure 26 for
the Colorado and Gunnison rivers. Riffles had significantly higher biomass densities
(expressed as a dry weight per area) when compared to run habitats. The highest densities
occurred in spring 1994 for both habitat types while the lowest densities occurred in spring
1995. The temporal pattern between the two habitat types was almost identical except during
fall 1994. Although both habitats experienced decreases relative to the spring 1994 densities,
the decrease was much greater in runs (1.9 gm/m2 to 0.43 gm/m2) versus riffles (4.1 gm1m2
to 2.9 gm/m2). The biomass decrease in runs was significant (P<0.05) level while the
decrease in riffles was not significant.
COLORADO RIVER-RIFFLES COLORADO RIVER-RUNS
Mean Periphyton Chlorophyll-a Mean Periphyton Chlorophyll-a
100 100

r

R 10 R 10 -
U V

y
1 1
Spring 1994 Fall 1990 Spring 1995 Fall 1995 - Spring 1994 Fall 1994 Spring 1995 Fall 1995
PERIOD PERIOD
GUNNISON RIVER-RIFFLES GUNNISON RIVER-RUNS
Mean Periphyton Chlorophyll-a Mean Periphyton Chlorophyll-a
too 100
-

E E
W 10 - P 10
O O

1 t
Spring 1994 Fall 1994 Spring 1995 Fall 1995 - Spring 1994 Fall 1994 Spring 1995 Fail 1995
PERIOD PERIOD
Figure 24. The seasonal distribution of periphyton chlorophyll for riffles and runs in the Colorado River (above) and the
Gunnison River (below).
24