Environmental Engineering Reference
In-Depth Information
Cleaning diatoms and mounting them in Naphrax ©
means that identification and counts are made from
permanent prepared slides, rather than from volume
or sedimentation chambers (Section 2.5.3). It is nor-
mal to use a ×10 eyepiece and a ×100 oil immersion
objective lens on the microscope for this purpose.
Before counting, it is always desirable to scan the
slide at low magnification to determine which are the
dominant species.
Attached diatoms can be found on a variety of sub-
strates including sand, gravel, stones, rock, wood and
aquatic macrophytes (Table 2.6). The composition of
the communities that develop is in response to water
flow, natural water chemistry, eutrophication, toxic
pollution and grazing.
Various authors have proposed precise protocols
for the collection, specimen preparation and numeri-
cal analysis of benthic diatoms to ensure uniformity
of water quality assessment (see below). More gen-
eral aspects of periphyton sampling are discussed in
Section 2.10.
Numerical analysis
Sample collection
Diatoms on the slide are identified to either genus
or species level and a total of 100-500+ counted,
depending on the requirements of the analysis being
used. Once the counting has been completed and the
results recorded in a standardised format, the data
may be processed.
In recent times, there has been developed a dual
approach to analysis of periphytic diatoms in relation
to water quality:
The sampling procedure proposed by Round (1993)
involves collection of diatom samples from a reach
of a river where there is a continuous flow of water
over stones. About five small stones (up to 10 cm
in diameter) are taken from the river bed, avoiding
those covered with green algal or moss growths. The
diatom flora can be removed from the stones either in
the field or back in the laboratory. As an alternative
to natural communities, artificial substrates can be
used to collect diatoms at selected sample sites. These
overcome the heterogeneity of natural substrata and
consequently standardise comparisons between col-
lection sites, but presuppose that the full spectrum of
algal species will grow on artificial media. Dela-Cruz
et al . (2006) used this approach to sample diatoms
in south-eastern Australian rivers, suspending glass
slides in a sampling frame 0.5 m below the water
surface (see Fig. 2.27). Slides were exposed over
a 4-week period to allow adequate recruitment and
colonisation of periphytic diatoms before identifying
and enumerating the assemblages.
evaluationoftheentirediatomcommunity(Section
3.4.3), often involving multivariate analysis
determination of numerical indices based on key
bioindicator species (Section 3.4.4)
Individual studies have either used these appr-
oaches in combination (e.g. Dela-Cruz et al ., 2006)
or separately.
3.4.3 Evaluation of the diatom community
Specimen preparation
The term 'diatom community' refers to all the diatom
species present within an environmental sample.
Species counts can either be expressed directly
(number of organisms per unit area of substratum) or
as a proportion of the total count. Evaluation of the
diatom community in relation to water quality may
either involve analysis based on main species or a
more complex statistical approach using multivariate
techniques.
The diatoms are then cleaned by acid digestion, and
an aliquot of cleaned sample is then mounted on a
microscope slide in a suitable high refractive index
mounting medium such as Naphrax © . Canada balsam
should not be used as it does not have a high enough
refractive index to allow resolution of the markings
on the diatom frustule.
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