Bruce Chessman, Department of Land and Water Conservation, NSW,
Australia
Peter Gell, University of Adelaide, SA, Australia
Peter Newall, CRC-Freshwater Ecology, Environment Protection
Authority, VIC, Australia
Jason Sonneman, Monash University, VIC, Australia
Introduction
The standard methods presented here for the sampling of diatoms for bioassessment
and biomonitoring of streams are those being used in the collection of
several large data sets across south-eastern Australia (Chessman unpublished
data, Gell unpublished data, Newall unpublished data, Sonneman unpublished
data), covering several hundred sites.
It is expected that most or all of these data sets will be combined
and form the basis of a national diatom data base. Consequently, researchers
using these methods will be able to compare their results with the national
data base and its accompanying environmental data. Similarly, gathering
diatom data using these methods will enable those data to be added to the
data base.
Diatom samples for the data bases have generally focussed on natural
substrata, wherever available. Future additions to the data base may include
artificial substrata.
The Methods
Substratum Selection
Wherever possible, two samples should be taken:
i) hard surface; and
ii) mud/detritus
Hard-surface samples are expected to be the most consistently collected,
are faster to analyse and less likely to be contaminated with dead cells
than the mud/detritus samples. It is expected that hard-surface samples
will be the most readily used in the development of transfer functions
and in general site assessment. Therefore, if it is at all possible to
get a hard-surface sample, it is import ant to do so.
Hard-Surface Samples
The preferable substratum is a rock that is big enough to have remained
stable under most flows, yet small enough to be picked out of the water
(e.g. approx 10-30 cm diameter). The key will be to "make do" where the
preferred situation doesn't exist. So, if there are no rocks of the right
size, use larger or smaller ones. If there are no rocks at all, then the
preferred substrata are logs, othe r woody debris, and macrophytes (in
that order). The type of substratum used should be recorded.
Wherever possible, these samples should be from riffle or run sections
of the streams. It is preferable to sample along the thalweg of the stream
(i.e. the deepest part of the channel where the main current flows). The
sample should be a composite of scrapings from three rocks (or logs, etc.)
at the site, with each individual substratum being at least 20 metres apart
(where practical) from the o thers being sampled. Where it is not practical
to sample from 20 metres apart, the three substrata should be as far apart
as practical and the distance should be recorded.
Diatoms are collected from the hard surface by scraping with a blade
(e.g. from a pocket knife) or from a stick, such as a sharpened ice-cream
stick. When a pocket knife is used, it must be thoroughly washed between
sites. When sharpened ice-cream sticks are used, a new one should be used
for each site.
It is important to scrape from an area of the rock that was exposed
to light when it was in the stream (i.e. the top or side of the rock).
Also, it is preferable to scrape an area that does not have other growths
(such as moss, lichen, or filamentous algae). Sometimes the options may
be limited and a compromise will be needed on some of these preferences.
If there is some detritus on the rock ( or other substratum), the sample
may actually be part "soft surface" community as well as "hard surface"
community. To avoid this contamination of the hard surface sample, shake
any hard surfaces (under the water) prior to sampling. For consistency,
a vigorous shake for 3 seconds is recommended.
It is preferable (if possible) to sample from approximately 15 cm depth.
This measure is also selected purely for maintaining consistency. If the
substratum in the thalweg of the stream is deeper than this, then deeper
is fine. Two exceptions apply:
1. If the substratum is too deep to sample in the thalweg, then sample
where practical; and
2. If the water is too turbid to see the bottom of the stream in the
middle, then keep to the 15 cm rule.
If the stream has no riffles or runs then slow-flowing or still sections
are all that can be sampled. If there are no hard substrata, then there
cannot be a hard-surface sample.
Mud/Detritus Samples
Wherever possible these should be from the edges of pools or protected
areas (such as behind large rocks or logs). The sample is collected using
a pipette. Press the bulb of the pipette, then gradually release it while
dragging the nozzle over the surface of the sediment. This should be done
at three locations in the stream reach. These three locations should be
at least 20 metres apart and ma y be on opposite sides of the stream. A
new pipette should be used for each site (plastic disposable pipettes are
ideal for this).
The preferred depth of sampling is 5 cm - again this is just for consistency
and more shallow is preferable to going deeper.
Two important points to keep in mind when selecting sample points are:
1. The soft-substratum sampling should be done at a point in the stream
that has been inundated for several weeks. In streams where the water depth
fluctuates considerably, it is better to sample a little deeper if this
means that the sample point is more likely to have been inundated for the
required length of time; and
2. Although quiet sections of the stream may offer the most suitable
habitat for detritus accumulation, diatom assemblages from areas that do
not have substantial movement of water over them may reflect the internal
nutrient dynamics of those areas rather than the water quality of the main
stream. Therefore isolated pools or backwaters without significant streamflow
should not be sampled as strea m assessment sites.
As for the hard-surface samples, if there is not a suitable place to
sample, then there cannot be a soft-surface sample from that site.
Warnings
1. Diatoms are often sampled as part of a larger biological monitoring
program. When macroinvertebrates are also being sampled, the associated
kicking and sweeping activity can disturb large areas of the stream site.
It is important to collect the diatom samples from undisturbed areas. Because
diatom sampling requires only three rocks (or other substratum types) and
three sheltered areas it is best to sample the diatoms prior to the invertebrate
sampling. Otherwise, collect the diatom samples upstream of the macroinvertbrate
samples.
2. There is currently some debate as to the best method of preservation
of the diatoms and whether the frustules dissolve in the various preservatives.
Preservatives most commonly used are ethanol (approx 70%) and Lugols iodine
solution. Regardless of which preservative is used, it is best to process
the samples as soon as possible after collection to minimise the risk of
dissolution.
3. Because diatoms are minute and virtually ubiquitous (often occurring
in tap water, for example), and their frustules (shells) are resistant
to decay, it is very easy to contaminate samples with either living or
dead diatoms from elsewhere. Extreme cleanliness is therefore required
during sampling. Dirty hands, used and inadequately washed sampling gear
and containers, and preservative dispens ers that have been dipped into
previous samples are all potential sources of contamination. Procedures
must be followed that prevent contamination from such sources.
DIATOM SAMPLING FIELD NOTES
STREAM AND SITE:
Hard Surface Sample
Detritus Sample
Surface
Type
Rock / Log / Debris / Other (describe) N/A
Depth
15 cm / other (measurement)
5 cm / other (measurement)
Habitat
Riffle / Run / Other (describe)
Pool / backwater / Other (describe) |
The University of Adelaide
