Environmental Engineering Reference
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shown that dissolved oxygen plays an important
role in controlling the growth of zooplankton.
Berzins and Pejler (
1989
) suggested that though
some species may be encountered in high abun-
dance at low oxygen values, no true anoxybiosis
ought to exist.
One of the effects of high suspended solid
levels is increased turbidity. Increased turbidity
has been shown to have a variety of infl uences on
biota, affecting characteristics such as ecological
conditions, resource availability, and species
interaction (Hart
1990
). Cottenie et al. (
2001
) from
their study found that differences in zooplankton
communities are strongly related to factors such
as macroinvertebrate densities and turbidity. In the
present study, it was observed that the post-
monsoon season had the highest suspended solid
levels throughout the river and the rotifers were also
present in high. This is in complete agreement
with Telesh (
1995
) who described rotifer diversity
to be inversely proportional to transparency in
highly turbid waters. Transparency in River
Vishwamitri gets highly reduced in the post-
monsoon season when the waters carry heavy
loads of sediments from the surrounding areas.
Telesh (
1995
) also observed that the contribution
of rotifers to total zooplankton biomass was
lower in less turbid waters. He described density
of rotifers to be highest in the turbid section and
low in regions with greater transparency. In the
present study, the levels of crustaceans and cope-
pods were low during the post-monsoon season
(Suresh et al., unpublished). Thus, predation
upon the rotifers is greatly reduced. Threlkeld
(
1979
) also suggested that biotic mechanisms in
the seasonal changes of zooplankton assemblages
involve changes in predation. Increased turbidity
altered predator effi ciency, which might indirectly
impact zooplankton community dynamics. In fact
laboratory experiments illustrated asymmetrical
exploitative competition between rotifers and
Daphnia, leading to Daphnia dominance in zoo-
plankton community (Gilbert
1985
). Hart (
1987
)
reported lower crustacean abundance in years of
high turbidity. McCabe and O' Brien (
1983
)
found
Daphnia pulex
population growth rates
were diminished in the presence of suspended
silt. On the other hand, however, Kirk and Gilbert
(
1990
) observed that inorganic turbidity inhibited
the competitive abilities of Daphnia and this
competitive inhibition may have lead to a decline
of cladocerans, causing a competitive lease of
rotifer population.
In all, however, it can be seen that stations I,
II, and III, which have the highest rotifer diver-
sity, have comparatively low suspended solids in
comparison to stations IV and V. Thus, it would
not be completely right to believe that the rotifer
diversity is directly proportional to the suspended
solids. Pollard et al. (
1998
) observed that turbidity
had a minimum role in regulation of zooplankton
population. They found that rotifer abundance
patterns and species composition as well as rotifer
population dynamics were similar at low and
high turbidity sites. Contrary to all the above
observations, Egborge (
1981
) observed highest
rotifer numbers during periods of high water
transparency.
Gulati et al. (
1992
) indicated that the impor-
tant factors to be examined for changes in zoo-
plankton composition and abundance are its food
and predators. Threlkeld (
1979
) also suggested
that biotic mechanisms in the seasonal changes
of zooplankton assemblages involve changes in
resource availability. Cecchine and Snell (
1999
)
stated that food limitation may be an important
factor in community structuring of rotifers. In
oligotrophic systems, declines in cladoceran pop-
ulations are often associated with decreased total
phytoplankton biomass (Sommer et al.
1986
).
Restrictions associated with lack of optimal food
(Pejler
1977
) or diverse phytoplankton as food
items (Burgis
1974
) are known to be the reason
for low rotifer diversity in low-latitude lakes
(Lewis
1979
; Fernando
1980
). Rotifers feed on
detritus, algae, etc., while some are predatory.
In the present study, most of the recorded rotifers
are herbivorous or detritivorous, suggesting that
the phytoplankton constitute the major source of
food. Any changes in the composition of these
would lead to subsequent changes in the rotifer
community. During the present study, a high
positive correlation (Table
6.17
) was observed
between the chlorophyll-a content and the rotifer
diversity. During the post-monsoon season, the
chlorophyll-a levels were maximum, as was the
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