Environmental Engineering Reference
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rotifer diversity. And the lowest chlorophyll-a levels
were encountered in the winter season. The summer
months showed a moderate chlorophyll-a level
and concomitantly moderate rotifer diversity
(Table
6.13
). Mishra and Saksena (
1998
) have
also observed a high positive correlation between
rotifer number and total phytoplankton population.
It is evident from the results that stations I, II, and
III have higher chlorophyll-a content as com-
pared to stations IV and V; similarly the rotifer
diversity at these stations is also low as compared
to stations I, II, and III throughout the year.
Yet another reason for low rotifer diversity down-
stream could be attributed to the fact that the
cyanophytes are disproportionately high at these
stations (Dhuru et al.
2003
). It has been stated
that blue-green algae are not edible as they are
toxic to rotifers (Fulton and Pearl
1987
).
Threlkeld (
1979
,
1986
) has attributed the decline
in rotifer community in mesotrophic and eutro-
phic systems, to the replacement of palatable
forms of phytoplankton with the less palatable
fi lamentous cyanophytes. Moreover, fi lamentous
cyanophytes, at high densities, are reported to
affect the zooplankton adversely by mechanical
interference with its fi ltering mechanism
(Webster and Peters
1978
; Porter and Orcutt
1980
). Apart from food, availability of proper
shelter is also an important factor determining the
community structure of plankton.
Factors affecting the phytoplankton community
would also indirectly affect the rotifer dynamics.
In most freshwaters, phosphorous and nitrogen
are limiting nutrient for phytoplankton growth
(Plath and Boersma
2001
). Even in marine
waters, zooplankton substantially mediates the
recycling of nutrients such as phosphorous and
nitrogen that directly infl uences the phytoplankton
therein (Trommer et al.
2012
). Phosphate is an
important nutrient, which controls plant growth
(Hynes
1978
). Tebutt (
1992
) and Dean and
Lund (
1981
) mention that phosphorous occur in
sewage effl uents due partly to human excretion
and partly due to their use in synthetic detergents.
Consequently in Vishwamitri River, the values of
phosphate increases as sewage gets dumped into
the river from station III onwards. This can be
seen clearly in Table
6.14
, wherein the phosphate
values are lowest at station I and gradually
increase from there onwards. The highest values
are found at station V. This trend is seen during all
the seasons. The lowest total reactive phosphate
levels were encountered during the post-monsoon
season, while the highest values during summer.
Thus, it would be expected that phytoplankton
diversity and consequently rotifer diversity would
be highest in the downstream stations in the
summer season. This is, however, not the case.
Both the phytoplankton levels (Suresh et al.,
unpublished) and the rotifer diversity in the
downstream stations are low. This could probably
be due to the very low dissolved oxygen content
in this stretch of the river.
In case of nitrate nitrogen, the highest values are
seen at the downstream stations, while low values
in the upstream stations (Table
6.15
). On basis of
the seasons, the highest values are seen during
the post-monsoon, while the lowest during the
winter season (Table
6.15
). Accordingly high
rotifer diversity is seen during post-monsoon
season and low during winter. However, as far as
the stations are concerned where high nitrate
nitrogen values are present (downstream stations),
the rotifer diversity is not correspondingly high.
This could again be attributed to low DO levels at
these stations.
Water pollution also affects the rotifer com-
munity. Archibald (
1972
), Verma et al. (
1984
),
and Kulshreshtra et al. (
1989
) observed that the
species diversity is high in clean waters and low
in polluted waters. Banerjea and Motwani (
1960
)
reported an appreciable fall in the rotifer species
just below the effl uent outfall and further reduc-
tion in the septic zone of Suvaon stream.
However, Prabhavathy and Sreenivasan (
1977
),
Gannon and Stemberger (
1978
), Sampath et al.
(
1979
), and Mishra and Saksena (
1998
) found
that rotifer population was enhanced by increased
load of pollution. Similarly Venkateswarlu and
Jayanti (
1968
) recorded high counts of rotifers at
polluted stations of Sabarmati River in compari-
son to clean stations. In River Vishwamitri, the
sewage pollution begins from station III, and as is
evident from the data, this station on the whole
has a greater diversity of rotifers throughout the
year. However, towards station IV and station V,
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