National University Journal of Science
Vol. 1, No. 2, 2014
The Impacts of Dredging on the Recovery Rate of Zooplankton
Abundance and Benthic Composition in Recently Dredged Harda
Baor in Meherpur
A.K.M. Nazrul Kabir1* and M. Niamul Naser2
Abstract: The present investigation was carried out in recently dredged Harda
baor of Meherpur district during September 2006 to August 2008. Among
zooplankton a total of 63 taxa (7 Protozoans, 36 Rotifers, 14 Copepods and 6
Cladocerans) and 74 taxa (11 protozoans, 44 Rotifers, 13 Copepods and 6
Cladocerans) while among benthic fauna a total of 16 taxa (2 chironomids, 10
oligochaetes and 4 moluscs) and 18 taxa (3 chironomids, 13 oligochaetes and 2
molluscs) were identified during first and second sampling period, respectively.
In the present investigation zooplankton abundance showed less population
during the first sampling year (Mean ± SE: 488 79.33 indiv/l) than second
sampling year (Mean ±SE: 1231241.69) while average value of total benthos
abundance were recorded as (Mean ±SE: 44781.82) and (Mean ± SE:
38960.81) indiv/m2 in the first and second sampling year, respectively.
Zooplankton and benthos diversity indices (species richness (SR); ShannonWeiner index (H`) and evenness (J`) were also found to be fluctuated throughout
the study period. The present study revealed that the recovery of zooplankton
population occurred very quickly than benthic population. The recovery of
benthic population may be required several years. The complete dredging of
Harda baor showed negative impacts on productivity.
Key words: Dredging impacts, recovery rate, zooplankton abundance, benthic composition, Harda Baor
Introduction
Zooplankton is a source of food for fish fry and for some species of mature fish. In
addition, species diversity, abundance and biomass of zooplankton determine production
of fish in the aquatic ecosystem while benthic macro-invertebrates constitute an important
intermediate link between phytoplankton, zooplankton and the fish stock in the food
chain. Biological characteristics are related to density and diversity of organisms.
Diversity is an important indicator or parameter of human interference with a natural
ecosystem which often leads to reduce diversity. This parameter is presented in several
ways by different researchers. One of the common diversity indices is Shannon-Wiener
index (H′). Dredging has manifolds impacts on aquatic ecosystem including bottom
disruption, increase turbidity, reduce oxygen availability in the water column, alter water
chemistry, cause long-term community disruption and alteration. Several studies have
indicated that species diversity and abundance of fish, shellfish and benthic invertebrates
1*
Assistant Professor, Dept. of Zoology, Govt. B.L. College, Khulna, Email: nazrulkabir09@gmail.com
2
Professor, Department of Zoology, University of Dhaka
60
A.K.M. Nazrul Kabir and M. Niamul Naser
are reduced after dredging [1,2,3]. In these cases, all water bodies, full recovery does not
occur or require several years. Recently Kabir and Naser [4] studied the dredging effects
on benthos fauna in the same baor of Meherpur district. Thus, the present study was
undertaken to know the recovery rate of zooplankton abundance and benthic composition
and their diversity in recently dradged Harda baor in Meherpur district.
Materials and Methods
Description of the study area: The present study was carried out in Harda baor of
Meherpur district (2304424.20 N and 8804617.77 E) between September 2006 and
August 2008. The Harda baor is located in Pirojpur Union under Sadar Upazilla of
Meherpur district. The area of this baor is 21.29 ha and is rectangular in shape. This baor
has been re-excavated in 2005 financed by IFAD for the purpose of aquaculture. As a
result, soft muddy bottom and bottom fauna were removed from the bottom of the baor
where the soil type is clay. The water depth varies from 81 cm to 231 cm during the study
period. Baor is affected by pesticides and fertilizers washed away from the surrounding
crop lands. Fish landing centre is at the northern side.
Zooplankton Sampling and analysis: For investigation, plankton samples were
collected monthly between August 2006 and September 2008. Specimens were collected
with a plankton net with a mesh size of 55 µm, and were fixed in 4% buffer solution for
further study. The collection, estimation and identification of zooplankton were done
following literature [5,6,7, 8, 9, 10, 11, 12, 13, 14,15].
B
C
A
Plate 1: Showing the sampling station (A, B and C) of study baor
The Impacts of Dredging on the Recovery Rate of Zooplankton Abundance
61
Benthos Sampling and analysis: The benthos samples were collected monthly at three
stations within the depth of 25-50 cm at station- A, 51-75 cm depth at station-B and
beyond the 76 cm depth at station-C using Ekman dredge with an area of 225cm2 between
7.30 a.m. and 11.30 a.m during the study period. After collection, soft muddy bottom
sediments were taken into plastic bucket and slurry was prepared and then transfer for
washing through a series of standard brass sieves of 2.0, 0.92 and 0.2 mm mesh size.
Organisms remaining on the sieves were collected with care by a pair of pointed forceps
with cotton or dropper and poured into a separate jar containing 6% formaldehyde with
few drops of glycerol for preservation. The jars were labeled properly and stored in pack.
The samples were taken to the Advanced Fisheries Research Laboratory, Department of
Zoology, University of Dhaka, for further analysis. The collected organisms were sorted
manually into major taxonomic groups by using magnifying glass and further sorted
organisms were preserved group wise into 4% buffered formaldehyde with label. Sorted
organisms were identified to the level of lowest possible taxa with the help of light and
compound microscope, except for molluscs shell. Identification of the specimen was done
following literature [8, 13, 16, 17]. The benthic organisms were counted separately and
averaged by number. The abundance was expressed as the number per meter square
(indiv/m2).
Diversity indices: Shannon-Weiner diversity index (H`) and evenness (J`) were also
estimated. Diversity index of plankton during each month was calculated by using the
formula:
s
H or H `  (pi) (log2pi)
i 1
Where, S= number of species
pi = the proportion of individuals belonging to ith species.
Species evenness (J`) was calculated from the observed species diversity and from the
equation of Hmax of Lloyd and Ghelardi [18]. Index of species evenness was measured
by using the following formula:
H`
J`= Hmax
where, H` = Observed species diversity, Hmax= Maximum species diversity= Log2 S
where, S= the number of species in the community.
Results and Discusssion
Zooplankton species composition: The present investigation was carried out during the
study period from September 2006 to August 2008. A total of 63 taxa (7 taxa Protozoans,
36 taxa Rotifers, 14 Copepods and 6 Cladocerans) and 74 taxa (11 protozoans, 44
Rotifers, 13 Copepods and 6 Cladocerans) were recorded in the first and second sampling
year, respectively. Das and Bhuiyan [19] recorded 55 species of planktonic organisms
62
A.K.M. Nazrul Kabir and M. Niamul Naser
including 25 rotifers, 14 Cladocerans, 10 copepods, and 8 Ostracods from two ponds and
two lakes of Dhaka city. Monthly fluctuations of total zooplankton abundance and major
groups are discussed in the following chronology.
Total zooplankton abundance: The zooplankton population varied both qualitatively
and quantitatively with months (Fig.1). The abundance of zooplankton ranged from 55
indiv/l (7.93) in September`06 to 1588 indiv/l (557.36) in June`07 with mean value of
(Mean± SE :488 79.33 indiv/l) and from 315 indiv/l in December`07 to 5331 indiv/l in
May`08 with mean value of (Mean ± SE :1231241.69) [Table1] in the first and second
sampling year, respectively. In the present investigation zooplankton abundance showed
less population during the first sampling year (Mean ± SE: 488 79.33 indiv/l) than
second sampling year (Mean ± SE: 1231241.69). Very less population of zooplanktion
in the first sampling year may be due to the effect of dredging in the lake water. Ohimain
et al. [20] assessed the impact of dredging on zooplankton community in a tropical
mangrove ecosystem and they found reduction in the population (by 91%) and taxa
(72%). Monthly fluctuations of major zooplanktonic groups are discussed in the
following chronology.
Protozoans: Protozoans was the 3rd dominant group and it constituted 6% of the total
zooplankton abundance during the first and second sampling year, respectively (Figure
2). Rahman et al.[21] recorded protozoa as the 4th dominant group among zooplankton
from Hamil beel, Bangladesh. In the first sampling year Protozoans abundance ranged
from 0 indiv/l in June-July`07 to 183 indiv./l in October`06 with mean value of (Mean ±
SE :28±8.67) indiv/l while in the second sampling year it fluctuated from 4 indiv/l in
June`08 to 183 indiv./l in October`07 with mean value of (Mean ± SE :66±10.15) indiv/l
(Table 1).
Rotifers: Rotifers placed 1st in position among zooplankton group and it formed 71%
and 78% of the total zooplankton abundance in the first and second sampling year,
respectively (Fig. 2). Rahman et al.[21] obtained almost similar observations from Hamil
beel, Bangladesh. Its abundance ranged from 42 indiv/l in September`06 to 956 indiv/l in
May`07 with mean value of (Mean ± SE: 338±51.60) indiv/l and from 148 indiv/l in
July`08 to 3369 indiv./l in May`08 with mean value of (Mean ± SE: 901±155.31) indiv/l
during first and second sampling yaer, respectively (Table 1). Average value of rotifers
was three times higher in the second sampling year than first sampling year. This may be
due to the dredging effects on baor water during the first sampling year and it also
revealed that the zooplankton population can recover very quickly.
Copepods: Copepods was the second dominant group among zooplankton which formed
17 and 15% of the total zooplankton abundance during the first and second sampling
year, respectively (Fig. 2). During first sampling year the abundance fluctuated from 2
indiv/l in September`06 to 532 indiv/l in June`07 with mean value of (Mean ± SE:
80±26.37) indiv/l while in the second sampling year it fluctuated from 3 indiv/l in
April`08 to 1761 indiv/l in May`08 with mean value of (Mean ± SE :173±91.28) indiv/l
(Table 1). In the present study the copepods abundance was found more than two times
The Impacts of Dredging on the Recovery Rate of Zooplankton Abundance
63
higher (Mean ± SE: 173±91.28) in the second sampling year than first sampling year
(Mean ± SE: 80±26.37). This may be due to the good quality of water in the baor during
second sampling year than first sampling year which deteriorated after dredging.
Cladocerans: Cladocerans was the leas abundant group among zooplankton and it
accounted for 6 and 1% of the total zooplankton abundance in the first and second
sampling year respectively (Fig. 2). During first sampling year its abundance ranged from
0 indiv./l to 233 indiv./l with mean value of (Mean ± SE: 26±14.09) indiv/l. The highest
peak was observed in June`07. Whereas in the second sampling year its abundance
fluctuated from 0 to 125 indiv./l with mean value of (Mean ± SE: 13±8.44) indiv/l and
the highest value was found in May, 2008 (Table 1). Ehsan et al.[22] stated that
Cladocerans were less in abundance in winter months in Chanda Beel, Bangladesh.
Zooplankton species diversity: Monthly variations of species diversity indices (species
richness, Shannon-Weiner index and evenness) are shown in Table 2. Species richness
during first sampling year ranged from 12 to 21 taxa with mean value of (Mean ± SE:
17±0.66) taxa while in the second sampling year it ranged from 13 to 24 taxa with the
mean value of (Mean ± SE: 17±0.85). This less number of taxa may be due to the effect
of dredging in the lake water. Ohimain et al. [23] also found reduction in taxa richness by
72% due to dredging effect from tropical ecosystem and also concluded that dredging
resulted in heavy impacts on zooplankton population. During first sampling year species
diversity (H`) ranged from 1.78 to 3.74 with mean value of (Mean ± SE: 2.98±0.09)
while in the second sampling year it varied from 1.54 to 3.80 with mean value of (Mean
± SE: 2.82±0.12) (Table 2). Diversity index value greater than 3.00 indicates clean water.
Values in the range of 1.00 to 3.00 are characteristics of moderately healthy conditions
and values less than 1.00 characterize heavily deterioration condition [24]. In the present
investigation, diversity index was found always less than 3 indicating the condition of the
studied baor is not fully suitable for aquatic life which may be due to the effect of
dredging in the baor water. Ohimain et al. [20] mentioned that dredging resulted in heavy
impacts on zooplankton diversity from tropical ecosystem. In their study the result of
diversity analysis before dredging shows that diversity was equally high from all
sampling stations and after dredging there was a significant reduction in diversity at
dredged canal. During the first sampling year evenness (J`) varied from 0.42 to 0.86
(Mean± SE: 0.74±0.02). In case of second sampling year it ranged from 0.43 to 0.85
(Mean ± SE: 0.59±0.02).
Benthic species composition: A total of 16 taxa (2 chironomids, 10 oligochaetes and 4
moluscs) from first sampling year and 18 taxa (3 chironomids, 13 oligochaetes and 2
molluscs) were identified from second sampling year (Table 3). The dominance of these
groups of macro-benthos has been reported earlier by Rahman and Das [25] from
Rajdhala and Padmai beel of Netrokona district, Bangladesh. The probable cause of the
occurrence of these dominant groups of bottom fauna may be due to the favorable
condition for their growth. Monthly fluctuations of total benthos abundance and major
benthic groups are discussed in the following headings:
64
A.K.M. Nazrul Kabir and M. Niamul Naser
Total benthos: In the first sampling year, total benthos abundance ranged from 44
indiv/m2 (25.40) in September`06 and November`06 to 1154 indiv/m2 in January`07 with
mean value of (Mean ±SE:44781.82) while in the second sampling year it ranged from
134 indiv/m2 in April`08 and June`08 to 1023 indiv/m2 in March`08 with mean value of
(Mean ± SE: 38960.81) (Table-3). Rahman and Das [25] found the highest density of
benthos in December and the lowest in September from Rajdhalla beel, Netrokona
district, Bangladesh. The frequency of availability of macro benthos varied in different
points, months and seasons. These differences might be due to the differences of bottom
habitat and various physico-chemical as well as other environmental conditions.
Chironomids: Chironomid larvae were the second dominant group among the total
benthic fauna and it formed 36 and 39% of the total benthos abundance in the first and
second sampling year, respectively (Fig. 4). During first sampling year the highest
abundance of total Chironomids was recorded 489 indiv/ m2 in February`07 and the
lowest abundance was 88 indiv/m2 in October`07 with average value of (Mean ± SE
:15936.90) and it was absent in September`06 and November`06. In the second
sampling year it ranged from 0 indiv/m2 in February`08 to 400 indiv/m2 in March`08 with
mean abundance of (Mean ± SE: 153 31.69)[Table 1]. Rahman and Das [25] also
observed the highest number in December and the lowest in July from Rajdhalla beel of
Netrokona district, Bangladesh.
Oligochaetes: Oligochaetes was the first dominant group among the total benthic fauna and
it accounted for 60 and 59% of the total benths abundance in the first and second sampling
period, respectively (Fig. 4). Ahmed et al. [26] also found similar result from Kaptai lake. In
the first sampling year the oligochaetes abundance ranged from 44 indiv/ m2 (25.40) in
September`06, November`06, July`07 and August`07 to 967 indiv/ m2 (580.68) in
January`07 but absent in October`06 and June`07 (Mean± SE :273  61.04) while in the
second sampling period the highest abundance of oligochaetes was recorded as 608
(209.08) indiv/m2 in March`08 and the lowest as 15 () indiv/m2 in August`08 with mean
value of (Mean ± SE :229 44.19) indiv/m2 [Table 3]. Rahman and Das [25] observed the
maximum abundance of benthos in December and minimum in November from Rajdhala
and Padmai beel of Netrokona district, Bangladesh. High occurrence of Oligochaets in
winter season may be due to low temperature and low feeding tendency of fishes.
Molluscs: Molluscs was the third and least dominant group among benthic fauna and its
constituted of 3 and 2% of the total benthos abundance in the first and second sampling
year, respectively. Rahman and Das (2001) also recorded the molluscs as third dominant
group in Rajdhala and Padmai beel. During first sampling period the Molluscs species
were found in December`06, February`07, May`07 and August`07 as 44 indiv/m2 while in
the second sampling year total Molluscs abundance ranged from 0 to 15 indiv/m2 with
mean value of (Mean ± SE :8 2.26 indiv/m2) (Table 3). Rahman and Das [25] found that
the maximum number of molluscs in July from beel waters. Molluscs were totally absent
in different months from the baor water, possibly due to lack of aquatic vegetation as
shelter after dredging. Haynes and Makarewicz [27] also found significantly greater
densitites at undredged stations.
The Impacts of Dredging on the Recovery Rate of Zooplankton Abundance
65
Benthos species diversity: Monthly variations of species diversity indices (species
richness, Shannon-Weiner index and evenness) are shown in Table 4. The monthly
variations of species richness during first sampling year ranged from 1 to 4 (Mean ± SE:
3±0.25) taxa. During second sampling year it ranged from 1 to 5 taxa (Mean ± SE:
3±0.22) (Table 4). The lowest species richness may be due to dredging effects in the baor
water. Alves et al. [28] also found to cause changes in number of taxa due to dredging
activity. In first sampling year species diversity (H`) ranged from 0 to 1.62 (Mean ± SE:
0.88±0.13) while in the second sampling year it fluctuated from 0.24 to 1.93 with mean
value of (Mean ± SE: 1.26 ±0.10). It should be recalled here that diversity (H`) values
less than 1, 1 to 3, and greater than 3 have been regarded as features of eutrophic,
mesotrophic and oligotrophic water, respectively [29]. The diversity index (H`), 1 or less,
indicating the poor quality of water [30]. This low diversity index in Harda baor may be
due to the effects of dredging. During first sampling year evenness (J`) varied from 0 to
0.94 with mean of (Mean ± SE: 0.53±0.07) while in the second sampling year it ranged
from 0.24 to 0.95 with mean value of (Mean ± SE: 0.76±0.05).
Conclusion
In the present investigation zooplankton abundance showed less population during the
first sampling year, (Mean ± SE: 488 79.33 indiv/l) than second sampling year (Mean
±SE:1231241.69) while average value of total benthos abundance were recorded as
(Mean ±SE:44781.82) and (Mean ± SE:38960.81) indiv/m2 in the first and second
sampling year respectively. From this study it was found that the recovery rate of
zooplankton population occurred very quickly than benthos population. The recovery of
benthos population may be required several years. During dredging, vegetation, soil and
sediments are usually removed by dredger which causes the loss of habitat, destruction of
flora and fauna and interrupt breeding cycles. Several studies have indicated that species
diversity and abundance of benthic invertebrates are reduced after dredging [1, 2, 3]. In
these cases, all water bodies; full recovery does not occur or require several years. In
conclusion, it can be observed from this study that dredging acts on the abundance of
zooplankton and benthos in large or in groups. The complete dredging of Harda Baor
showed negative impacts on productivity in the study.
References
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A.K.M. Nazrul Kabir and M. Niamul Naser
The Impacts of Dredging on the Recovery Rate of Zooplankton Abundance
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A.K.M. Nazrul Kabir and M. Niamul Naser
Table 2: Monthly variations of zooplankton diversity indices [Shannon-Weiner index
(H`), Species Richness (SR), Maximum Species diversity (H max) and
Equitabilility or Evenness (J`)] of recently dredged Harda baor.
Months
H`
SR
Hmax
J`
Sep`06
3.070.14
120.33
3.620.04
0.850.04
Oct`06
2.710.19
121.20
3.600.14
0.750.03
Nov`06
3.120.10
151.52
3.890.14
0.800.00
Dec`06
3.600.04
190.88
4.260.06
0.840.02
Jan`07
3.740.15
201.76
4.330.13
0.860.02
Feb`07
3.050.19
212.33
3.870.33
0.790.07
Mar`07
2.780.19
192.40
4.250.17
0.650.03
Apl`07
1.780.09
192.02
4.190.16
0.420.01
May`07
2.610.01
181.76
4.120.14
0.630.01
Jun`07
3.140.26
141.85
3.740.21
0.840.02
Jul`07
2.680.11
140.33
3.840.03
0.700.02
Aug`07
3.510.11
202.08
4.300.14
0.820.01
1st year mean
2.980.09
170.66
4.000.05
0.740.02
Sep`07
3.800.31
246.35
4.510.34
0.840.06
Oct`07
3.170.13
182.40
4.170.18
0.760.01
Nov`07
2.160.14
172.33
4.030.19
0.530.07
Dec`07
3.000.43
132.51
3.630.31
0.810.05
Jan`08
3.060.15
152.33
3.900.20
0.780.02
Feb`08
2.610.23
202.33
4.270.16
0.610.03
Mar`08
2.330.36
190.00
4.250.00
0.550.08
Apl`08
3.250.19
172.72
4.010.22
0.800.02
May`08
2.420.18
172.02
4.090.17
0.560.08
Jun`08
1.540.14
133.17
3.570.33
0.430.02
Jul`08
3.590.12
191.33
4.210.10
0.850.05
Aug`08
2.870.32
131.45
3.710.15
0.760.06
2nd year mean
2.820.12
170.85
4.030.07
0.690.02
The Impacts of Dredging on the Recovery Rate of Zooplankton Abundance
71
72
A.K.M. Nazrul Kabir and M. Niamul Naser
Table 4: Monthly variations of benthic diversity indices (H`, SR, Hmax and J`) in recently
dredged Harda baor of Meherpur district.
Months
H`
SR
Hmax
J`
Sep`06
00
10.33
00
00
Oct`06
00
10.0
00
00
Nov`06
00
10.33
00
00
Dec`06
1.340.34
30.57
1.520.28
0.850.08
Jan`07
1.230.61
41.45
1.520.78
0.540.27
Feb`07
1.040.52
31.00
1.440.44
0.640.14
Mar`07
1.560.46
41.15
1.860.46
0.810.05
Apl`07
1.300.23
30.57
1.520.28
0.860.07
May`07
1.620.08
30.33
1.720.14
0.940.02
Jun`07
0.380.38
20.66
0.520.52
0.240.24
Jul`07
1.200.13
30.33
1.380.19
0.880.08
Aug`07
0.960.56
2 0.88
1.000.57
0.630.31
1st year mean
0.880.13
30.25
1.040.14
0.530.07
Sep`07
1.650.17
30.33
1.720.14
0.950.02
Oct`07
1.600.16
30.33
1.720.14
0.930.03
Nov`07
1.680.29
40.57
1.960.21
0.840.05
Dec`07
1.930.30
50.88
2.160.29
0.880.04
Jan`08
1.270.35
31.00
1.440.44
0.900.04
Feb`08
1.070.53
31.20
1.440.72
0.540.27
Mar`08
1.430.07
40.33
1.860.14
0.770.02
Apl`08
0.830.44
20.57
0.860.46
0.640.32
May`08
0.870.07
20.0
1.000.00
0.870.07
Jun`08
0.240.24
10.33
0.330.33
0.240.24
Jul`08
1.610.15
30.33
1.720.14
0.930.01
Aug`08
0.930.51
20.88
10.57
0.660.33
2nd year mean
1.260.10
30.22
1.430.12
0.760.05
The Impacts of Dredging on the Recovery Rate of Zooplankton Abundance
73
6000
5500
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
0
Sep
Oct
Nov
Dec
Jan
Feb
First sampling year
Mar
Apl
May
Jun
Jul
Aug
Second sampling year
Fig. 1: Monthly variations of zooplankton abundance in recently dredged Harda baor of
Meherpur district during September 2006 to August 2008 (first sampling period: September`06August`07 and second sampling period: September`07-August`08)
(A)
(B)
Fig. 2: Percentage composition of zooplankton abundance in recently dredged Harda baor of
Meherpur district (A= First sampling period: September 2006-August 2007; B=Second sampling
period: September 2007-August 2008)
1400
1200
1000
800
600
400
200
0
Sep
Oct
Nov
Dec
Jan
Feb
First sampling year
Mar
Apl
May
Jun
Jul
Aug
Second sampling year
Fig. 3: Monthly variations of benthos abundance in recently dredged Harda baor of Meherpur
district during September 2006 to August 2008 (First sampling period: September 2006-August
2007 and second sampling period: September 2007-August 2008)
74
A.K.M. Nazrul Kabir and M. Niamul Naser
(A)
(B)
Fig. 4: Percentage composition of benthos abundance in recently dredged Harda baor of
Meherpur district (A= First sampling period: September 2006 – August 2007; B=Second
sampling period: September 2007 –August 2008)