Sampling, Biomass Estimation and Counts of Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 77

of unit biovolume and species count. The use of phy-
toplankton biovolume as an index of overall phy-
toplankton biomass complements the different, more
direct bulk analysis techniques noted earlier (see Sec-
tion 2.3), and avoids some of the pitfalls associated
with each of these methods. A further advantage of
biovolume calculation is that total changes in P b can
be related to individual algal species and genera,
thus providing insight into phytoplankton dynamics
within the lake ecosystem.
If biovolume determinations are valid, then
changes in P b (e.g. with seasonal progression)
should parallel bulk determinations of phytoplankton
biomass. In many situations (as shown in Fig. 2.8)
there does appear to be a good correlation, with
total biovolumes showing corresponding increases to
chlorophyll- a concentration during spring and sum-
mer algal blooms, and an inverse relationship to Sec-
chi depth.

Margalef index ( d ) This index (Margalef, 1958)
avoids the complication of sample size by incorpo-
rating the number of individuals ( N )inthesample.
The index thus provides a measure of the number of
species relative to the overall sample size, where

d = ( S − 1)∕log e N (2.10)

Species evenness/dominance

The evenness of species occurrence is also an impor-
tant measure of biodiversity. Considering two hypo-
thetical samples (I and II), each composed of four
species (A, B, C and D):

Sample I. Total 100 individuals; species A (25), B
(25), C(25) and D (25).

Sample II. Total 100 individuals; species A (94), B
(2), C (2) and D (2).

2.5.5 Indices of biodiversity

Determinations of species counts (see Section 2.5.3)
and biovolumes (see Section 2.5.4) can be used to
estimate biodiversity within mixed-species algal pop-
ulations (Sigee, 2004). Most estimates of algal diver-
sity are based on species counts, which can be used to
generatebioindicesofthreemaintypes-speciesrich-
ness, species evenness/dominance and a combined
index of species diversity.

Sample I would be regarded as more diverse than
sample II, though species richness is the same. Sam-
ple I has a greater evenness of species occurrence,
but a lower dominance of any species. Sample II is
the converse.
Species evenness may be determined as Pielou's
evenness index ( J ′ ):

J ′ = H ′ (observed)∕ H ′ max (2.11)

Species richness

where H ′ max is the maximum possible diversity that
would occur if all species were equally abundant.
Dominance is the converse of evenness and can
be determined using the Simpson index ( D ), which
relates the number of individuals in each species ( n i )
to the total number of individuals in the sample ( N ):

This index relates to the total number of species
present in the population sample - the greater the
number of species the greater the measure of biodi-
versity. Species richness may be assessed in two main
ways:

Total number of species ( S ) Species richness
is often determined simply in relation to the total
numberofspecies.Themajorproblemwiththisindex
is that the value S may depend on sample size - the
bigger the sample the more species there are likely
to be.

D = ∑ ( n i ∕ N 2 )

(2.12)

Comparison of indices for species evenness and dom-
inance over a number of population samples shows a
clear inverse relationship.