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

Because of the extreme spatial and temporal
fluctuations of salinity in estuaries, simple, rapid
instrumental methods of measurement are more
desirable than slower, more precise chemical
methods (38).
Wide-range, temperature-compensated conductivity
salinometers are recommended for
determining both horizontal and vertical salinity
profiles at high-slack and low-slack tide levels in
the area of estuary or reach of river being
studied.
3.0 SAMPLING METHODS
3.1 QUANTITATIVE
3.1.1 Definitions and purpose
Although the data may be evaluated in various
ways, a quantitative method essentially involves
an estimation of the numbers or biomass
(standing crop) of the various components of
the macroinvertebrate community per unit area
in all or a portion of the available habitats
(including artificially introduced habitats) in the
ecosystem being studied, and provides information
on the species composition, richness of
species and distribution of individuals among the
species.
3.1.2 Requirements
Obtain quantitative estimates by using devices
that sample a unit area or volume of habitat,
such as a Surber square-foot sampler, which in
use presumably collects all organisms enclosed
within the frame of the sampler, or an artificial
substrate sampler having a fixed volume or
exposing a fixed amount of surface.
In the study of macroinvertebrate populations,
the sampling precision is affected by a
number of factors, including: size, weight, and
construction of the sampling device, the type of
substrate, and the distribution of organisms in
and on the substrate. For example, it is expected
that the estimates of standing crop drawn from a
series of samples will be more precise (have a
lower coefficient of variation) when the
community consists of a few species represented
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MACROINVERIEBRAIE SAMPLING
by a large number of individuals, evenly distributed
in the substrate. Conversely, a large coefficient
of variation would be expected if the
fauna consists of a large nUlubcr cf species with
a patchy distribution of individuals. To obtain
the same level of precision at a given level of
probability, a larger number of replicates would
be required in the latter case than in the former.
In general, the smaller the surface area
encompassed by a sampling device, the larger the
number of samples required to obtain a desired
level of precision. Thus, precision can be
increased by collecting larger samples, or by
increasing the numbers of samples collected.
An objective, quantitative approach necessitates
that a measure of the precision of the
estimates be obtained - thus, replicate sampling
in each habitat or stratum selected for study is
an absolute requirement. For measurement of
precision, three replicates are an absolute
minimum. (A series of single samples taken at
discrete points along a transect do not represent
replicate samples of benthic organisms unless it
can be demonstrated that the physical characteristics
of the habitat do not change along the
transect.)
It is preferable, if data are available (or can be
obtained by reconniassance or exploratory
studies), to determine the number of replicates
on the basis of the desired level of precision as
discussed in the Biometrics Section.
3.1.3 Advantages
In addition to providing the same data
obtained from a qualitative study, the standing
crop data generated by a quantitative study provide
a means of comparing the productivity of
different environments; and if a measure of
turnover is available, the actual production can
be computed.
The use of quantitative sampling devices in
carefully chosen habitats is recommended
because they reduce sampling bias resulting from
differences in expertise of the sample collector.
The data from properly designed quantitative
studies are amenable to the use of simple but