Biological field and laboratory methods for measuring the quality of ...

sieve is then partially submerged in water and
agitated until all fine materials have passed
through. The sieve is agitated preferably in a tub
of water.
A variation of this technique is to place the
original sample in a bucket or tub, add screened
water, stir, and pour the slurry through a U. S.
Standard No. 30 sieve. Only a moderate amount
of agitation is then required to completely clean
the sample. Since this method requires considerably
less effort, most biologists probably prefer
it.
In both of the above methods, remove all the
larger pieces of debris and rocks from samples
collected, clean carefuly, and discard before the
sample is stirred or agitated.
The artificial substrate samplers are placed in
a bucket or tub of screened water and are
dismantled. Each individual piece of substrate
material is shaken and then cleaned gently under
water with a soft brush (a soft grade of toothbrush
is excellent), examined visually, and laid
aside. The water in the bucket or tub is then
poured through a U. S. Standard No. 30 sieve to
remove the fines.
4.2 Preservation
Fill sample containers no more than one-half
full of sample material (exclusive of the preservative).
Supplemental sample containers are used
for samples with large volumes of material.
Obtain ample numbers and kinds of sample
containers before the collection trip: allow two
or three I-liter containers per grab sample, a
I-liter container for most artificial substrate
samples, and 16-dram screw-cap vials for miscellaneous
collections.
Preserve the sample in 70 percent ethanol. A
70 percent ethanol solution is approximated by
filling the one-half-full bottle, containing the
sample and a small amount of rinse water, with
95 percent ethanol. Do not use formalin.
4.3 Labelling
Make sample labels of water-resistant paper
and place inside the sample container. Write all
information on the label with a soft-lead pencil.
Where the volume of sample is so great that
several containers are needed, additional
13
MACROINVERTEBRATE SAMPLE PROCESSING
external labels with the log number and
notations such as I of 2, 2 of 2, are helpful for
identifying sample containers in the laboratory.
Minimum information required on the sample
label is a sample identification (log) number.
The log number identifies the sample in a bound
ledger where the name of water body, station
number, date, sampling device used, name of
sample collector, substrate characteristics,
depth, and other environmental information are
placed.
4.4 Sorting and Subsampling
For quantitative studies, sort and pick all
samples by hand in the laboratory using a lowpower
scanning lens. To pick organisms
efficiently and accurately, add only very small
amounts of detritus (no more than a heaping
tablespoon full) to standard-sized (25 X 40 X 5
cm), white enamel pans filled approximately
one-third full of water. Small insects and worms
will float free of most debris when ethanol­
-preserved samples are transferred to the waterfilled
pan.
Analysis time for samples containing
excessively large numbers of organisms can be
substantially reduced if the samples are subdivided
before sorting. The sample is thoroughly
mixed and distributed evenly over the bottom of
a shallow tray. A divider, delineating one-quarter
sections, is placed in a tray, and two opposite
quarters are sorted. The two remaining quarters
are combined and sorted for future reference or
discarded (57). The aliquot to be sorted must be
no smaller than one-quarter of the original
sample; otherwise considerable error may result
in estimating the total numbers of oligochaetes
or other organisms that tend to clump. The same
procedure may be followed for individual
taxonomic groups, such as midges and worms,
that may be present in large numbers.
Numerous techniques other than hand-picking
have been proposed to recover organisms from
the sample, including sugar solutions, salt solutions,
stains, electricity for unpreserved samples
in the field, bubbling air through sample in a
tube, etc. The efficacy of these techniques is
affected both by the characteristics of the substrate
material and the types of organisms. No