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

(2) or comparable enclosure filled with 30, 5 to
8-cm-diameter rocks or rock-like material is
recommended for use by EPA biologists.
To reduce the number of organisms that
escape when the samplers are retrieved, the
multiple-plate sampler and the rock-filled basket
sampler should be enclosed by a dip net constructed
of 30-mesh or finer grit bolting cloth.
Artificial substrate samplers, to a great extent,
depend on chance colonization by drifting or
swimming organisms; and, thus, the time of
exposure may be critical to the development of
a relatively abundant and diverse community of
organisms. Adequate data are currently unavailable
to determine the optimum exposure period,
which is likely to differ in different bodies of
water and at different times of the year. Until
more data become available, adoption of a
6-week exposure period (2) is provisionally
recommended as standard. If study time limitations
reduce this period, the data must be
evaluated with caution and, in no case, should
data be compared from samplers exposed for
different time periods (43).
In deeper waters, artificial substrate samplers
should be suspended from floats and should be
well up in the photic zone so that periphytic
growths can develop and provide food for
grazing forms of macroinvertebrates. Unless the
wa t er is exceptionally turbid, a 1. 2-meter
(4-foot) depth is recommended as standard. If
the water is less than 2.5 meters deep, the
sampler should be suspended from a float halfway
between the water surface and the stream
bed.
In some situations, artificial substrate
methods are the best means of conducting
quantitative studies of the ability of an aquatic
environment to support a diverse assemblage of
macroinvertebrate organisms. Advantages of the
method are:
• The confounding effects of substrate differences
are reduced.
• A higher level of precision is obtained than
with other sampling devices (Table 3).
• Quantitatively comparable data can be
obtained in environments from which it is
virtually impossible to obtain samples with
conventional devices.
MACROINVERTEBRATE ARTIFICIAL SUBSTRATES AND DRIFT NETS
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• Samples usually contain negligible amounts
of extraneous material, permitting quick
laboratory processing.
Limitations of artificial substrate samplers are:
• The need for a long exposure period makes
the samplers unsuited for short-term survey
studies.
• Samplers and floats are sometimes difficult
to anchor in place and may present a
navigation hazard.
• Samplers are vulnerable to vandalism and
are often lost.
• Samplers provide no measure of the
condition of the natural substrate at a
station or of the effect of pollution on that
substrate, including settled solids.
• Samplers only record the community that
develops during the sampling period, thus
reducing the value of the collected fauna as
indicators of prior conditions.
Two other objections often made to the use
of artificial substrate samplers are that they are
selective to certain types of fauna and the data
obtained do not provide a valid measure of the
productivity of a particular environment. The
validity of the latter objection depends on study
objectives and may be of minor consequence in
many pollution-oriented studies. The selectivity
of artificial substrate samplers is a trival objection,
since all currently available devices are
selective. The selectivity of conventional
sampling devices other than artificial substrates
is directed toward those organisms that inhabit
the types of substrate or substrates for which a
particular type of sampler is designed.
3.3.5 Drift nets
Nets having a 15 by 30-cm upstream opening
and a bag length of 1.3 m (No. 40 mesh
netting) are recommended for small, swift
streams. In large, deep rivers with a current of
approximately 0.03 meters per second (mps),
nets having an opening of 0.093 m 2 are recommended
(2). Anchor the nets in flowing water
(current not less than 0.015 mps) for from I to
24 hours, depending on the density of bottom