Environmental Engineering Reference
In-Depth Information
(Osborne et al.
1976
) or in habitats having very
favourable physico-chemical variables. In fact,
it is a general phenomenon that when the envi-
ronmental stress in any particular habitat increa-
ses, the community in that system would be
dominated by a fewer species (species that are
more resistant to the existing set of physico-
chemical and biological variables), which are
often referred to as the opportunistic species. The
magnitude of the index of dominance is usually
high in stressful habitat.
The mathematical expression of
richness
index is as follows:
Richness index
ð
d
Þ¼
S
N
p
where S = number of species and N = total number
of individuals of all the phytoplankton species.
Shannon
-
Weiner (
1949
) Species Diversity
Index
It is the most commonly used index in ecological
studies owing to the fact that it is dimensionless
and independent of sample size and expresses the
worth of each species. It is expressed as follows:
Index of Similarity
Two phytoplankton communities (like the one in
the Indian Ocean and the other in the Paci
c
Ocean), geographically wide apart from each,
may also show similarity with respect to their
species composition. The similarity arises due to
the closeness of the two aquatic ecosystems with
respect to environmental characteristics such as
salinity, nutrient load and transparency.
The index of similarity between two sampling
stations is calculated according to the expression:
Index of similarity
ð
S
Þ¼
2C
A
þ
B
Species diversity index
ð
H
Þ¼
p
i
log
e
p
i
X
S
n
i
N
log
e
n
i
N
ðÞ¼
i
¼
1
where
p
i
= importance probability for each spe-
cies,
n
i
= importance value for each species and
N
= total of importance values.
The value of H
has several ecological explana-
tions. Higher diversity signi
es longer food chains
and more cases of symbiosis (mutualism, com-
mensalisms, etc.) and greater probabilities for
negative feedback control, which reduces oscilla-
tion and hence increases stability. The value of H
where
A
= number of species present at sampling
station
= number of species present at
sampling station
'
X
'
,
B
is
also a unique indicator of environmental stress. As
the sensitive species gradually shift or are elimi-
nated from a habitat with the increase of the mag-
nitude of environmental stress, the species diversity
index has been claimed as an effective statistics for
predicting the change in environment (Wilhm and
Dorris
1968
;CairnsandDickson
1971
). Thus,
decrease in the value of H
= number of species
common to both the sampling stations.
'
Y
'
and
C
Richness Index
The index reflects the suitability of a particular
habitat for successful thriving and growth of dif-
ferent phytoplankton species. The value of the
index usually decreases, when environment
becomes unfavourable or stressful due to intrusion
of some foreign matters that have adverse effect on
the community, e.g., pollutants or any biological
organisms that may be a parasite or predator to the
existing group of species. Zooplankton often
graze on phytoplankton making the environment
highly stressful for phytoplankton. There are also
reports on sharp decrease of phytoplankton due to
oil pollution (Mitra
2000
).
ects an increase in the
magnitude of environment stress on the species/
community, and the gradual restoration of the
environment quality is indicated by an increment in
the value of H through recruitment of new species.
re
fl
5.1.2 Chemical Method
The standing stock of phytoplankton can also be
assessed in the laboratory by chemical method
