8141A

• UCREFRP Catalog Number: 8141A
• Author: Wydoski, R. S. and W. T. Helm.
• Title: Effects of Alterations to Low Gradient Reaches of Utah Streams.
• USFW Year: 1980.
• USFW - Doc Type: \
• Copyright Material: NO

1971), this loss is due mainly to dehydration, rather than the loss of soluble
lipids as when specimens are preserved in alcohol. This was more desirable
in the present study as dry, rather than wet, weights were used in weight
calculations.

In the laboratory, invertebrates were sorted from detritus by hand. The
samples obtained from larger sieve sizes (5-10 or 14) were sorted under an
illuminated magnifier (2x) in their entirety. Samples from sMaller sieve sizes
(14 or 18-60) were mechanically sub-sampled (Waters 1969b). The sub-sampler
has been shown to give statistically random samples (Appendix C ; Table 3;
Elliott 1971). Sub-sa~ples were sorted until the total number of invertebrates
examined was ~50 (Elliot 1971, Cummins 1975). The samples from the smallest
sieve sizes (35-60) were sorted under a dissecting microscope (10-30x).
Identification was made to family, or to genus when possible.
I

The total lengths of all invertebrates from one sampling station were
measured with an ocular micrometer. The data provided a means of analyzing the
efficiency of size sorting by the sieves (Appendix E; Figures 1-3) and allowed
comparison with data from the literature based on total lengths.

Composite samples, by taxa and sieve size, were used for the determination
of average wet, dry (850C, 12 hr), and ash (5000C, 2 hr) weights of the insects
(Winberg 1971). Further calculations based on weight were made from linear
regressions of log normalized ash free weights (Appendix F; Table 1 ); sieve
sizes with inadequate sample sizes were omitted from these regressions. A
weighted mean ash free dry weight for each steve size, regardless of taxa, was
calculated from the same data for use in estimates of mean total biomass stand-
ing crop and total production (Appendix F ; Figures 1-4).

Estimates of the mean density (number of individuals of each taxa per m2)
were made by sieve size for each date at each site. Each sample was treated
equally, and corrected for sample area and number of sub-samples. Estimates
of the2mean standing crop (gm/m2) were calculated from estimates of density
(no./m ) and average weights.

Estimates of the mean total standing crop (n~mber of individuals per m2
by sieve regardless of taxa, and totals regardless of taxa) were also
made for each date at each site. In these cases, the total number of insects
in each sample was treated as the sample attribute, and the means were calculated
as above. r1ean total biomass was calculated by treating the total biomass of
each sample (calculated as above) as the sample attribute.

Total production estimates following Hynes. method (Hynes and Coleman 1968,
Hamilton 1969, Waters and Crawford 1973) were made from mean standing crops and
average weights using data for total invertebrates.

A Shannon diversity index was determined following the description of
Wilhm and Dorris (1968). Diversity index values were calculated for each date
at each site using mean standing crop estimates.

When seasonal values are used in comparisons, the seasons are based on the
water year and are:

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