Sampling, Biomass Estimation and Counts of Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 66

Sedimentation chamber - use of inverted
microscope

1972) and upright (Hamilton et al ., 2001) micro-
scopes. Sedimentation chambers typically consist of
two main parts - a tower section (for holding the sedi-
menting plankton) and a settling chamber (contained
within a base plate) to retain the sedimented algae
for making counts (Fig. 2.12b). The tower section is
positioned on top of the base plate with the cen-
tral column directly above the settling chamber. A
known volume (range 5-100 ml) of iodine-preserved
phytoplankton suspension is poured into the col-
umn and allowed to settle for 24 h, leading to the
deposition of algae onto a coverslip at the base of
the settling chamber. The tower section can then be
slid across the base plate, voiding the surplus liq-
uid, and a coverslip finally placed on top of the
settling chamber to limit evaporation. Sedimented
phytoplankton can then be observed, identified and
counted.

The combined use of a sedimentation chamber and
inverted microscope allows a small volume of iodine-
preserved sample to sediment and be analysed in
a single operation. Cells that have collected at the
bottom of the chamber are immediately viewed
and counted from beneath. The essential differences
between an inverted light microscope and a normal
one is that the objective lenses face upwards, the con-
denser has a greater working distance and the slide
is placed on the stage with the objectives beneath
(Fig. 2.12a)
Originally designed by Utermohl, sedimentation
chambers have been widely used in phytoplank-
ton studies (Rott,1981) and have been subsequently
modified in various ways for both inverted (Evans,

(a)

Condenser

Figure 2.12 Combined use of
inverted microscope and phytoplank-
ton sedimentation chamber. (a) Inve-
rted microscope: Substage objectives
allow close optical access to algae
that have sedimented to the bottom
of the settling chamber. (b) General
view of sedimentation chamber,
composed of tower section and set-
tling chamber base plate. (c) Detailed
view of settling chamber after algal
sedimentation and removal of the
tower section. Sedimented algae
lie on the lower coverslip (fixed in
place by a retention ring) and can
be observed from below using the
inverted microscope.

Objectives

Top coverslip

(b)

Sedimented algae

Bottom coverslip

Tower
section

View from
below

(c)

Settling chamber
base plate

Settling chamber