Environmental Engineering Reference
In-Depth Information
Nitrate Nitrogen
Stations IV and V showed high levels of nitrate
nitrogen in comparison to stations I and II. An
increase in the level of nitrate nitrogen was
observed from station I to station V (Table
6.15
).
By and large, the highest values were obtained
during the post-monsoon season at all the stations
(Table
6.15
). The winter season showed the lowest
values of nitrate nitrogen at all the stations.
that rotifers from India follow a marked periodicity.
George (
1961
) attributed a summer periodicity to
the rotifers in Delhi waters. Chacko and Rajagopal
(
1962
) found that rotifers were dominant in
the month of May and August. Michael (
1968
)
observed different peaks in slightly different
periods during his 2-year study period.
Dhanapathi (
1997
) observed a bimodal curve of
rotifer abundance from two ponds in Andhra
Pradesh. In their study on the seasonal dynamics
of rotifers of the river Yamuna in Delhi, Arora and
Mehra (
2003
) reported to no seasonal variation in
the species diversity. During the present study,
a remarkable alteration in the rotifer community
was observed with change in the various seasons.
A less obvious change was observed on a monthly
basis (Table
6.17
), and hence only the seasonal
data compiled by combining the monthly data
has been discussed here. In the present study,
rotifer diversity, richness, and equitability were
found to be highest during the post-monsoon
season. Similar results were obtained by
Fernando and Rajapaksa (
1983
), who found
rotifers in high numbers both during the dry and
rainy seasons in tropical lakes. Green (
1960
) and
Duncan and Gulati (
1981
) found high rotifer
numbers during the fl ushing periods or fl ood
cycle, while Robinson and Robinson (
1971
) and
Burgis (
1974
) found that rotifer numbers were
highest during warm dry months and lowest
during the cold period. This is only partially in
agreement with the results obtained in the present
study wherein too the rotifer numbers in the winter
season were low.
The seasonality of rotifers can be ascribed to a
number of climatological and biological factors
(Mengestou et al.
1991
). Herzig (
1987
) from an
intensive study of the Rotifera from temperate
lakes observed that some central factors such as
physical, chemical limitations, food and mechanical
interference, competition, predation, and parasitism
regulate rotifer succession. Many studies have
been conducted to fi nd the causative factors for
the seasonal variations. Studies conducted by
Sarma et al. (
2011
) revealed that a wide range of
physicochemical factors infl uence the seasonal
variation in zooplankton abundances including
Biological Oxygen Demand
Post-monsoon season recorded the lowest values
of BOD at all the stations, while summer had
the highest values (Table
6.16
). The upstream
stations I and II had lower BOD values as com-
pared to the downstream stations (Table
6.16
).
Highest BOD value was recorded at station V in
the month of May, while the lowest value was
recorded at station I in August.
Discussion
Understanding rotifer community structure and
the factors affecting its diversity, abundance, and
richness is very complex. Contradictory reports
exist on various factors that could be affecting it.
Even the question of whether or not seasonality
exists in the rotifer community is riddled with
contradictions. Pennak (
1955
) from his observations
concluded that there is no seasonal periodicity
in North American Rotifers. Wesenberg-Lund
(
1908
,
1930
) has shown that these seasonal
variations are not very marked in Danish waters.
Mengestou et al. (
1991
) based on their study of
rotifer dynamics in Ethiopia did not observe a
consistent seasonal pattern or generalized scheme
of succession in Rotifers. In a long-term study
across four Polish lakes, Steinberg et al. (
2009
)
observed a relatively stable species composition
among these lakes within years and within the
lakes between years, but they also report varia-
tion in the species abundance patterns that seem
to be most affected by season. Nayar (
1965
)
based on his study concluded that periodicity of
occurrence cannot be assigned to a particular sea-
son. However, there are few reports that conclude
Search WWH ::
Custom Search