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

BIOLOGICAL METHODS
fauna and hydrologic conditions. Place the top
of the nets just below the surface of the water to
permit calculation of the flow through the nets
and to lessen the chance for collection of
floating terrestrial insects. Do not permit the
nets to touch bottom. In large rivers, maximum
catches are obtained 0.3 to 0.6 meter above the
bottom in the shoreline zone at depths not
exceeding 3 meters.
Drift nets are useful for collecting macroinvertebrates
that migrate or are dislodged from
the substrate; they are particularly well-suited
for synoptic surveys because they are lightweight
and easily transported. Thousands of
organisms - including larvae of stoneflies,
mayflies, caddisflies, and midges and other
Diptera, may be collected in a sampling period
of only a few hours. Maximum drift intensity
occurs between sunset and midnight (55). Elliot
(14) presents an excellent synopsis of drift net
methodology.
3.3.6 Photography
The use of photography is mainly limited to
environments that have suitably clear water and
are inhabited by sessile animals and rooted
plants. Many estuarine habitats, such as those
containing corals, sponges, and attached algal
forms, fall in this category and can be photographed
before, during, and after the introduction
of stress. The technique has been used with
success in south Florida to evaluate changes
brought about by the introduction of heated
effluents.
The technique for horizontal underwater
photos using scuba gear involves placing a photographically
identifiable marker in the habitat to
be photographed and an additional nearby
marker on which the camera is placed each time
a photograph is taken. By this means, identical
areas can be photographed repeatedly over a
period of time to evaluate on-site changes in
sessile forms at both affected and control
stations. Vertical, overhead photos may also be
taken under suitable conditions.
3.3.7 Qualitative devices
The investigator has an unlimited choice of
gear for collecting qualitative samples. Any of
12
the qualitative devices discussed previously, plus
hand-held screens, dip nets, rakes, tongs, post
hole diggers, bare hands, and forceps can be
used. For deep-water collecting, some of the
conventional grabs described earlier are normally
required. In water less than 2 meters deep, a
variety of gear may be used for sampling the
sediments including long-handled dip nets and
post-hole diggers. Collections from vascular
plants and filamentous algae may be made with
a dip net, common garden rake, potato fork, or
oyster tongs. Collections from floating debris
and rocks may be made by hand, using forceps
to catch the smaller organisms.
In shallow streams, short sections of common
window screen may be fastened between two
poles and held in place at right angles to the
water flow to collect organisms dislodged from
upstream materials that have been agitated.
4.0 SAMPLE PROCESSING
4.1 Sieving
Samples collected with grabs, tubular devices,
and artificial substrates contain varying amounts
of finely divided materials such as completely
decomposed organic material, silts, clays, and
fine sand. To reduce sample volume and
expedite sample processing in the laboratory,
these fines should be removed by passing the
sample through a U. S. Standard No. 30 sieve.
Sieves may range from commercially constructed
models to homemade sieves framed
with wood or metal. Floating sieves with
wooden frames reduce the danger of accidental
loss of both sieve and sample when working over
the side of a boat in deeper waters. A good sieve
contains no cracks or crevices in which small
organisms can become lodged.
If at all possible, sieving should be done in the
field immediately after sample collection and
while the captured organisms are alive. Once
preserved, many organisms become quite fragile
and if subjected to sieving will be broken up and
lost or rendered unidentifiable.
Sieving may be accomplished by one of
several techniques depending upon the reference
of the individual biologist. In one technique, the
sample is placed directly into a sieve and the