Sampling, Biomass Estimation and Counts of Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 73

appropriate conversion factors) to cell carbon (gC)
or cell biomass (g). Such calculations are poten-
tially very useful, but should be treated with caution,
since they depend on a sequence of approximations -
including mean algal dimensions, conversion of lin-
ear to three dimensions (Fig. 2.17) and variation in
conversion factors from biovolume to biomass and
cell carbon. In practice, algal counts are routinely
converted to biovolumes, which serve as an approxi-
mate index of algal biomass.

cell diameter of 4 μm would have a mean cell volume
of 30 μm 3 . In contrast to this, one cell of Stephan-
odiscus sp., a centric diatom shaped like a short drum,
would have a volume of nearly 16,000 μm 3 . In these
examples, the mean unit volume of one species is
530 times larger than the other so their contribution
to phytoplankton biomass (per cell) would be signif-
icantly different.
The ecological significance of algal size in rela-
tion to biomass is seen in Fig. 2.16, which shows
a lake phytoplankton sample with algae ranging in
size (greatest axial linear dimension) from 100 μm
( Ceratium )to3μm( Synechococcus ). Species counts
from the same sample (Table 2.3) show that Stephan-
odiscus has the highest overall population (78% total
phytoplankton count), but determination of biovol-
umes shows that it occupies only 13% of the total
phytoplankton level.

Species count and biovolume within
phytoplankton populations

Algal size varies considerably from one species to
another and even within the same species at different
stages in its growth cycle. A species of Chlorella ,for
example, whose shape is spherical and has a mean

Sp Sy

P

S 1

R

S

A

N

C

S 2

(a) (b)

Figure 2.16 Taxonomic and size diversity in a mixed phytoplankton sample. (a) Low-power view, showing the pres-
ence of algae belonging to the blue-green algae ( Anabaena ,A; Aphanizomenon , N), green algae ( Sphaerocystis ,Sp),
dinoflagellates ( Ceratium ,C; Peridinium , P), cryptomonads ( Cryptomonas , R) and diatoms ( Stephanodiscus , S). Scale
bar 100 μm. (b) High-power view showing details of some of the smaller phytoplankton - including Stephanodiscus
in valve (S1) and girdle view (S2, pair of cells), and the unicellular blue-green alga Synechococcus (Sy, pair of cells).
Scale bar 25 μm. Considerable variation in size and shape occurs within the sample. The cells have been fixed in iodine
in preparation for cell/colony counts (see Table 2.3). In these fields of view, cell size (greatest axial dimension) ranges
from ∼180 μm( Ceratium )to∼2 μm( Synechococcus ). Cell/colony shape ranges from spherical ( Peridinium )tooval
( Cryptomonas ) to the extended forms of Ceratium (unicell) and Aphanizomenon (colony).