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

BIOLOGICAL METHODS
powerful statistical tools that aid in maintaining
the objectivity of the data evaluation process.
The measures of precision and probability statements
that can be attached to quantitative data
reduce the possibilities of bias in the data evaluation
process and make the results of different
investigators more readily comparable.
The advantages, then, of quantitative methods
are:
• They provide a measure of productivity.
• The investigator can measure precision of
estimates and attach probability statements,
thus providing objective comparisons.
• The data of different investigators may be
compared.
3.1.4 Limitations
Presently, no sampling devices are adequate to
sample all types of habitat; so when quantitative
devices are used, only selected portions of the
environment may be sampled.
Sampling precision is frequently so low that
prohibitive numbers of replicate samples may be
required to obtain meaningful estimates. Sample
processing and analysis are slow and timeconsuming.
In some cases, therefore, time limitations
placed on a study may prohibit the use
of quantitative techniques.
3.2 Qualitative
3.2.1 Definitions and purpose
The objective of qualitative studies is to determine
the presence or absence of forms having
varying degrees of tolerance to contaminants
and to obtain information on "richness of
species." Samples are obtained with the use of a
wide variety of collecting methods and gear,
many of which are not amenable to quantitation
on a unit-area basis. When conducting qualitative
studies, an attempt is usually made to collect all
species present by exhaustive sampling in all
available habitat types.
3.2.2 Requirements
Recognizing and locating various types of
habitats where qualitative samples can be
collected and selecting suitable collecting
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techniques require experience and a high level of
expertise.
When conducting comparative studies of the
macrobenthos, a major pitfall is the confounding
effect of the differences in physical habitat
among the different stations being studied. This
danger is particularly inherent in qualitative
studies when an attempt is made to systematically
collect representative specimens of all
species present at the sampling stations or
reaches of river being compared. Unfortunately,
differences in habitat unrelated to the effects of
introduced contaminants may render such comparisons
meaningless. Minimize this pitfall by
carefully recording, in the field, the habitats
from which specimens are collected and then
basing comparisons only on stations with like
habitats in which the same amount of collecting
effort has been expended.
3.2.3 A dvan tages
Because of wide latitude in collecting techniques,
the types of habitat that can be sampled
are relatively unrestricted. Assuming taxonomic
expertise is available, the processing of qualitative
samples is often considerably faster than
that required for quantitative samples.
3.2.4 Limitations
Collecting techniques are subjective and
depend on the skill and experience of the
individual who makes the field collections.
Therefore, results of one investigator are
difficult to compare with those of another.
As discussed elsewhere, the drift of organisms
into the sample area may bias the evaluation of
qualitative data and render comparisons
meaningless.
No information on standing crop or production
can be generated from a qualitative study.
3.3 Devices
3.3.1 Grabs
Grabs are devices designed to penetrate the
substrate by virtue of their own weight and
leverage, and h,ave spring- or gravity-activated
closing mechanisms. In shallow waters, some of
these devices may be rigged on poles or rods and
physically pushed into the substrate to a