Sampling, Biomass Estimation and Counts of Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 68

problems arise, it can be useful at this stage to
compare the iodine sample with the fresh sample
(examined previously) or with the formaldehyde-
fixed, trawl-net sample. Check that algae are dis-
persed randomly across the counting area and not
localised to one particular region.

where T is expressed as the number of organisms
(single cell or colonies) per ml of original sample.
If statistical evaluation of phytoplankton species
diversityisrequired,theentirecountingchambermay
be screened to record as many species as possible
within the sample. If possible, at least 400 units (cells
or colonies) of each species should be counted to keep
the counting error at 5. To make species counts, select one square at

random and count all single cells and globular
colonies within the square (see later for filamen-
tous algae). Include all those algae that touch
or overlap sides A-B and A-D of the square
(Fig. 2.13c) but not those that are in contact with
sides B-C and C-D. Further squares for counting
should be selected on an objective basis, without
any bias towards contents. To do this, move five
squaresfromsquare1(theinitialsquare)andcount
from the new square (square 2). Repeat the process
as indicated in Fig. 2.13d. As an alternative, rather
than a fixed interval of five squares, random num-
bers can be used to determine the spacing between
counted squares. McAlice (1971) has shown that a
count of 30 squares can be expected to reveal 90-
95% of the species present. The number of squares
counted should also be sufficient to give a statisti-
cally valid population estimate of major species or
of minor species that are being studied. This will
depend on which species are being investigated,
and variability within the lake - but may exceed
the count of 30 squares noted earlier.

10% (Lund et al . 1958).
Counts of single cells and small colonies (with
defined numbers of cells) are relatively straightfor-
ward, but problems may be encountered with fila-
mentous and large globular (particularly blue-green)
algae, where unit size and cell number are highly
variable.

<

Filamentous algae Although filamentous algae
can simply be recorded as numbers of units (indi-
vidual filaments), a more accurate approach is to
measure filament length. This is because filaments of
these algae may vary considerably in length (within
and between samples), so counting as individuals
becomes meaningless. Filaments may also extend
across more than one side of the counting square,
making enumeration difficult.
To measure filament length, it is necessary to cali-
brate an eyepiece micrometer, which is a small scale
engraved onto a circle of glass. Once this has been
inserted into the eyepiece of the microscope, its scale
becomes visible at the same time as the specimen.
Align the micrometer scale against the filament by
rotating the eyepiece and measure the length of the
filament in eyepiece units (Fig. 2.14a). The eyepiece
units can then be converted to absolute units (μm)
by removing the specimen slide, inserting a stage
micrometer slide, aligning the two scales in the field
of view, then reading off the number of absolute units
equivalent to a block of eyepiece units (Fig. 2.14b).
In practice, rather than converting each filament
to absolute units, the total length of filaments within
each square, and across 30 squares can be summed -
and the final cumulative value (eye piece units) then
converted using the determined conversion factor.

6. Dead algal cells, including algae with no contents
(Fig. 2.18) and the remains of diatom frustules, are
particularly prominent at certain times of year (see
Section 2.6.2a). These should be either ignored or
recorded in the algal counts as a separate category.

7. To calculate the environmental populations ( T )
of individual species from the Sedgwick-Rafter
counts:

If C is the number of organisms counted in N
squares and there is a 10× concentration from the
original aquatic sample,

Large globular algae Large globular algae such
as Microcystis and Gomphosphaeria are frequently
encountered in lake samples, but are difficult to enu-
merate because of their irregular form and variable

T = 1000 C
10 N

(2.3)