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

in the Colorado River just prior to the spring 1995 sample period.
Biological Parameters-In order to define the trophic structure of the Colorado and Gunnison
rivers, primary and secondary producers were quantified in the same locations where physical
and chemical parameters were measured.
Primary producers were quantified by
determining the in-situ standing crop of
periphyton (mg Chl-a/m2) and the
concentration of benthic detritus
(gm/m2) associated with the substrate.
Periphyton biomass as expressed as
river-wide mean concentrations were
significantly different for each sample
period (Figure 12). Initial samples
collected in the Colorado River in the
spring of 1994 had river-wide average
concentrations of 7.2 mg/m2; this
increased to 9.8 mg/m2 by the fall of
that year. However, by spring 1995, the
periphyton densities were reduced to the
lowest levels (4.6 mg/m2) of the entire
study period. In the following sample
period (fall 1995) periphyton densities
had risen to their highest concentrations
(13.4 mg/m). This increase from the
spring to the fall of 1995 was the largest
increase found in the Colorado River
between sample periods.
Figure 12. The mean and 95% confidence interval of the
periphyton chlorophyll values calculated for the river-wide
substrate samples collected in 1994 and 1995 in the Colorado
(above) and Gunnison (below) rivers.
The Gunnison River also had significant differences between sample periods for periphyton
biomass (Figure 12) and had the same temporal pattern as the Colorado River, with the
spring of 1994 having the lowest densities and the fall of 1995 having the highest. In
addition, the fall 1994 and fall 1995 periphyton densities were significantly greater in the
Gunnison River than in the Colorado River.
The concentrations of coarse particulate organic material (detritus) in the substrates of the
Colorado River were not significantly different among sample periods (Figure 13) averaging
21-23 gm/m2. The Gunnison River, although having higher average detritus concentrations
for each respective sample date, was not significantly greater, with the exception of fall 1994.
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