8067

• UCREFRP Catalog Number: 8067
• Author: Nance, E. L.
• Title: Planktonic and Benthic Invertebrate Densities in Three Backwater Habitats on the Lower Green River Final Report.
• USFW Year: 1997.
• USFW - Doc Type: Grand Junction, CO.
• Copyright Material: NO

In general, samples were counted in their entirety under a dissecting microscope. Single
eggs, egg masses, algae, or diatoms were not counted. For those samples containing more
than 400 organisms, subsamples were taken using a Hensen-Stempel pipette. These
samples were concentrated into 50 ml of water and stirred to avoid a vortex. Three, Iml
or 2ml aliquots were then taken for each sample and all organisms counted. These
subsample counts were then averaged by taxa and multiplied by the number of aliquots
per 50 m1 sample to obtain an overall density estimate. Finally, each of the five vertical
tows and five benthic samples were averaged to obtain a fmal density estimate per
sampling date. Density estimates were converted to number of organisms per liter for the
zooplankton samples, and numbers per square centimeter for the benthic samples.

RESULTS

Plankton

Zooplankton density estimates ranged from a low of 0.57 per liter on the second sample
date (08May), to a high of 124 per liter on the fmal sample date (12 June) in Millard
Canyon (Table 1). Average densities remained low (< 20 per liter) in both the Anderson
and Holeman backwaters. Densities never rose above 20 organisms per liter in Anderson
and rose to an average of only 26 organisms per liter on the 12 June sample in Holeman
Canyon.

SAMPLE DATE MILLARD ANDERSON HOLEMAN
01 May 6.17 + 1.19 2.40 + 0.65 6.60 ::!: 2.99
08 May 0.57 + 0.33 5.80 + 3.88 5.20 ::!:3.65
15 May 8.44 + 2.04 3.63 + 1.12 3.39 ::!:2.08
22 May 29.02 + 26.92 7.60 ::!: 2.22 6.24 ::!: 1.10
29 May 30.85 ::!: 36.18 6.82 + 3.23 6.83 + 1.86
05Jun 53.74 ::!: 30.94 7.64 + 2.59 14.9::!: 3.33
12 Jun 124.07 ::!: 29.82 19.23 + 5.81 26.23 ::!:10.98
10 Jul 95.75 + 111.57

Table 1 : Average zooplankton densities (organismsIL ::!: SD) from five vertical tows.

The percent composition of dominant food items by taxa, sample date and site are
depicted in Figures 1-3. The Copepoda category is represented by combining the adult
and copepodid life stages. Nauplii, which are exclusively Copepoda, were abundant
enough to warrant their own category. Minor taxa never comprised more than 9% of the
average total of five tows in Millard Canyon with the exception of the 8 May sample,

5