Sampling, Biomass Estimation and Counts of Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 51

to the required depth, then dropping a weight messen-
ger to trigger closure. The volume of sample (usually
1-2 l) can be brought to the surface and analysed.
Care must be taken to ensure that depth samplers are
completely watertight at the depth being sampled,
and that surfaces in contact with the water sample are
completely inert.
Samples collected from volume samplers are typi-
cally divided into separate aliquots for determination
of a range of parameters - including inorganic nutri-
ent concentrations, soluble organics, phytoplankton
and zooplankton biomass. These water samples can
be very heterogeneous (Wolk et al ., 2004), however,
and care must be taken that they are well-mixed (by
pouring into a large container and stirring) before
subdivision.

weighted end and the tube lowered slowly and ver-
tically into the water. This process has to be carried
out with care in order to collect a column of water in
the tube as undisturbed as possible, representing the
water column of the lake. To ensure a vertical sam-
ple (tube not displaced by current), the tube should
be sufficiently weighted and the boat should be sta-
tionary (attached to a buoy). The top of the tube is
then closed by inserting a rubber bung. With the top
still, the weighted bottom end is raised out of the
water so the tube forms a loop. The weighted end is
placed in a suitable large receptacle and the rubber
bung removed from the other end. The trapped col-
umn of water now flows into the container and totally
mixes. The integrated sample of the lake surface (top
5 m) can then be separated into subsamples to anal-
yse water chemistry, chlorophyll content and species
composition.
Theuseofintegratedsamplingtechniquesprovides
information on the overall phytoplankton population
over a part of the water column (stratified lake) - but
losesdetailsuchasphytoplanktongradientsandstrata
of particular algal species. Comparison of vertical
trawls from different sites on the lake is particularly
useful where horizontal rather than vertical variation
in the lake is being investigated.
Once the lake water sample has been collected,
the phytoplankton can then be analysed in vari-
ous ways - including total biomass, species popula-
tions, biological interactions (e.g. fungal infection),
physiologicalstate,chemicalcompositionandmolec-
ular characteristics.

2.2.3 Integrated sampling

Analysis of the average community structure of the
top part of the water column (epilimnion) of a strati-
iedlake(Fig.2.3b)isnormallyobtainedfromaninte-
grated (composite) sample, using a collecting tube
that is lowered from the boat. In the case of shallow
unstratified lakes, the whole of the water column can
be sampled.
The sampler is typically a flexible plastic tube,
internal diameter approximately 2.5 cm and about
1 m longer than the depth through which the sam-
ple is to be collected, and is weighted at its end
(Fig. 2.6). A long piece of cord is attached to the

Tube coiled
in boat

cord

Figure 2.6 Collection of integra-
ted water sample at lake surface (not
drawntoscale).(A)Weightedlexible
tube is lowered vertically into water.
(B) Tube closed by bung when fully
extended in epilimnion. (C) Weighted
end of tube is raised by attachment
cord. The whole tube is then taken
into the boat, the bung removed and
the lake water poured as a mixed sam-
ple into a collecting vessel.

Flexible
tube
weight

epilimnion

C

A B