Composition of Macrobenthos in the Bakkhali Channel System, Coxs Bazar with Notes on Soil Parameter
M.K. Abu Hena,
Macrobenthos in coastal environment that play a significant role in the food web. It could also use as a good indicator of aquatic ecosystem health. The abundance and composition of macrobenthos in Bakkhali channel system, Coxs Bazar were conducted in relation to the soil parameters. Samples were collected using Ekman Berge bottom grab from five different stations of Bakkhali channel. Macrobenthos were comprised of five major groups namely Polychaeta (9.96-30.31%), Oligochaeta (3.68-59.707%), Crustacea (0.02-58.40%), Bivalvia (1.40-82.09%) and Gastropoda (0.08-4.25%). Total number of macrobenthos was higher at station I (9000 individuals m-2) and station II (8517 individuals m-2) compared to other stations. Shannon diversity index among the stations ranged from 0.65-1.04. Soil pH and soil moisture ranged from 6.1-6.4 and 23.44-31.29%, respectively. The highest organic carbon concentration was observed at station I (2.11%) and lowest at station III (1.40%). Maximum fraction of sand by weight was found at stations II (81.88%) and III (87.88) while the highest fraction of clay (21.52%) and silt (8.0%) were recorded in station I. It was observed that benthic bivalves were positively correlated (r = 0.891, p>0.05) with silt fraction of the sediments.
Received: September 19, 2012;
Accepted: October 06, 2012;
Published: January 23, 2013
Macrobenthos in estuarine ecosystem provides significant support to the aquatic
food web. They contribute to ecosystem stability through the sustenance of fishery
resources including birds. The composition, abundance and distribution pattern
of macrobenthos can act as an ecosystem index by indicating trophic structure,
quality of water and the eutrophication level of the aquatic ecosystem (Mehdi
et al., 2005). Physicochemical parameters of the environment may
influence macro-benthic organisms either positively or negatively depending
on their sources. Excessive input of nutrients and changes of soil parameters
can cause long or short-term shifts in benthic species composition, abundance
and richness (Aura et al., 2011).
Benthic faunas developed naturally in aquatic ecosystems. Besides the trophic
relationship with microbes, they have important roles in estuarine ecosystems,
especially with regards to food supply, productivity, fish growth and nutrient
cycling. The composition of benthic faunal compositions in the estuarine ecosystems
depends on several factors like siltation, water quality, sediment condition
and temperature. Previous studies revealed that the macrobenthos were essential
for many estuarine species i.e., fishes, shellfish and avifauna through their
entire life stages. A number of studies argued that the richest fisheries of
the world are closely related to the benthic community, particularly demersal
fishes and shrimps are closely related to the benthic communities as their major
source of food (Longhurst, 1957; Chong
and Sasekumar, 1974). Longhurst (1957) investigated
the relationship between demersal fishes and soft bottom benthos in the West
African estuary and found that macro invertebrates are the main diet for the
Benthic fauna can influence water chemistry, regulate sediment properties and
control nutrient cycle by mobilizing and rearticulating sediment and organic
matter (Coull, 1970). Many literatures are available
on the descriptive and correlative studies of benthic faunal taxa with environmental
factors (Coull, 1970). Benthic organisms may be dependent
on textural composition of estuarine sediments that might limit the distributions
of certain organisms (Davis, 1971). However, the study
on macrobenthos composition and abundance in the estuarine channel system is
scanty in Bangladesh. Therefore, the main objective of this study was to investigate
the macrobenthos composition and their abundance in the channel system of Bakkhali
river estuary of Coxs Bazar. Several physicochemical parameters of soil
were also investigated to understand the relationship between the soil parameters
and the macrobenthos composition in this river system.
MATERIALS AND METHODS
Study area: The Bakkhali river estuary is located at the south-eastern
coast of the Bay of Bengal (Fig. 1). This river is relatively
wide (about 67 km) compared to other rivers of the Coxs Bazar district.
Bakkhali river estuary has a semidiurnal tidal regime and heavily influenced
by monsoonal wind. The tidal range of Bakkhali river varied between 0.07 m and
4.42 m during neap and spring tide respectively (Mahmood,
1986). The estuarine zone is also characterized by long intertidal mudflats
where mangrove vegetation (Avicennia alba, Avicennia marina and
Acanthus ilicifolius), macro algae (Ulva intestinalis), salt tolerant
grass Imperata cylindrica, cord grass Porteresia sp. and seagrass
Halophila beccarii are present (Abu Hena et al.,
2007). The lower part of this estuary is heavily influenced by anthropogenic
and industrial activities including fish harbours, fish processing plants and
a large number of fish and shrimp farms. The large amount of organic and inorganic
waste changes the chemical characteristics of the water body by producing toxic
substances, which ultimately affect the biodiversity of this estuarine system.
Five sampling stations were selected for the present study namely Station I
(Bakkhali Mouth), Station II (North Nuniya Chora), Station III (Kustura Ghat),
Station IV (Mazir Ghat) and Station V (Gudar Para). The distance from one station
to another station was 1 km (Fig. 1).
Collection of macrobenthos: For macro benthic fauna, samples were collected
using a small boat during March-April, 2007. Sampling was done using an Ekman
Berge bottom grab having a mouth opening of 0.04 m2. Three samples
were collected from each station with three replicates. Samples were sieved
through 500 μm mesh screen to retain macrobenthos. All samples were preserved
immediately with 10% buffered formalin. The organisms were counted and calculated
for total amount in m2 for macrobenthos. The major taxonomic group
of benthos was identified following the references described by Arnold
and Birtles (1989), Chuang (1961), Berry
(1972), Lim (1963), Huys et
al. (1996), Day (1967) and Fauchald
Soil collection and analysis: Soil samples were collected from each
station in the study area with 3 replicates. Soil samples were collected using
grab sampler from a depth of 0-10 cm at each sampling location and the samples
were kept in self sealed plastic bags.
|| Location of the study area showing the study stations in
the Bakkhali channel system, Coxs Bazar
In the field the soil pH (wet) and soil temperature were detected in situ
using a pH meter (Soil pH tester, Takamura Electric Works Ltd) and thermometer
(Centigrade thermometer, Japan), respectively.
For determination of soil moisture, initially the soil sample were taken in
petri-dish and heated in incubator for 1 hour and cooled it in the desiccators.
Then, the final weight was measured to detect the moisture content. Soil texture
was measured following the procedure described by Bouyoucos
(1962). Soil organic matter was detected following procedure described by
Boyd (1995). Soil organic carbon was calculated dividing
the organic matter by a factor of 1.9 following the procedure described by Nelson
and Sommers (1982).
Data analysis: The Shannon diversity index is commonly used to describe
the diversity of the particular community and as an indicator for the assessment
of an ecosystem with regards to abundance and diversity (Bahls
et al., 1992). The Shannon-Wiener diversity (Shannon
and Weaver, 1949), the Pielou evenness, richness and Fisher-alpha were calculated
by PAST Version 2.13. Shannon diversity index (H´) and evenness (E) were calculated
for each of the sample based on the following formula (Magurran,
where, pi is the relative cover of the ith species = (ni/N), ni is the number of individual species counted, N is the total number of species:
where, log S is the natural log of the total number species.
Richness was measured by Margalef index (d) (Margalef, 1968)
using the following formula:
where, S is total species and N is total individuals.
Pearsons correlation coefficient (r) was used to identify relationships between the abundance of macrobenthos and soil parameters. Stepwise multiple regression analysis was used to examine the effect of macrobenthos abundance with soil parameters using statistical software SPSS version 19.0. All the statistical significance were tested at a 95% confidence level.
Abundance and composition of macrobenthos: The total number of macrobenthos at station I (9000 individuals m-2) and station V (8517 individuals m-2) were higher compared to the other sampling stations. The major groups of macrobenthos were comprised of the Bivalvia, Polychaeta, Oligochaeta, Crustacea, and Gastropoda. The density range of macrobenthos were; Polychaeta (401-2126 individuals m-2), Oligochaeta (148-1822 individuals m-2), Crustacea (1-4974 individuals m-2), Bivalvia (52-6276 individuals m-2) and Gastropoda (3-171 individuals m-2) (Table 1). In term of percent composition of macrobenthos, the figures recorded were 9.96-30.31%, 3.68-59.707%, 0.02-58.40%, 1.40-82.09% and 0.08-4.25% for Polychaeta, Oligochaeta, Crustacea, Bivalvia, and Gastropoda, respectively (Table 1).
The Shannon diversity index was found 0.88 for station I, 1.03 for station II, 0.97 for Station III, 0.65 for station IV and 1.03 for station V. The diversity indices were not uniform among the stations. The evenness indices among the stations were ranged between 0.38 and 0.56. The diversity index of station II and station V were higher than the other sampling stations (Fig. 2).
Physicochemical parameters of estuarine sediments: Soil pH among the
sampling stations ranged between 6.1 and 6.4. Soil moisture ranged from 23.44-31.29%.
Organic carbon displayed considerable spatial variability where the highest
concentrations were observed at station I (2.11%) and lowest at station III
|| Abundance of macrobenthos (individuals m-2) recorded
from the Bakkhali channel system, Coxs Bazar
|| Physicochemical parameters of soil collected from the Bakkhali
channel system, Coxs Bazar
|| Pearsons correlation matrix of macrobenthos abundance
and physicochemical parameters of soil from Bakkhali channel system, Coxs
|*Correlation is significant at the 0.05 level (2-tailed)
||Diversity profile of different sampling stations from Bakkhali
channel system, Coxs Bazar
Maximum fraction of sand by weight was found at stations III and II (87.88
and 81.88%, respectively), while the highest fraction of clay (21.52%) and silt
(8.0%) were found at station I (Table 2). In the Pearsons
correlation matrix, it is observed that the benthic bivalve group was positively
correlated with silt fraction (r = 0.891, p>0.05). No significant correlations
were found between other benthos groups and soil parameters (Table
The five major groups of macrobenthos recorded from the channel system of Bakkhali
estuary were mostly common and dominant taxa of the coastal and estuarine environment
of Bangladesh. These species are also common in the coastal seagrass and mangroves
habitats. Usually the diversity of macrobenthos is higher in the tropical and
sub tropical climate. The number of macrobenthos group observed in the present
study is similar to the number of groups encountered in other tropical tidal
areas (Aura et al., 2011).
The abundance of macrobenthos was higher in the Stations I and V, assuming
that the habitat and environmental condition of those stations probably be suitable
for benthos compared to other sampling stations. The variation among the stations
could also probably be due to higher input of nutrients from the sediments.
Generally, relatively soft surface and high detritus or organic matter (>3%)
in these stations may cause for high food diversity hence supports greater benthic
organisms (Arshad et al., 2011; Aura
et al., 2011). Siddique et al. (2012)
observed that salt marsh sediment of Bakkhali estuary especially at Stations
III and IV features metal pollution from urban runoff and other coastal activities
which eventually may affects on benthic community. Studies by Hossain
(2003) recorded 20 major taxa of macrobenthos in the Meghna river estuary
that is higher than the present study. Compared to the studies done by Belaluzzaman
(1995) the abundance of macrobenthos is lower in the channel system of the
Bakkhali estuary, which probably due to the unsustainability of the estuarine
bottom due to continuous runoff that leads to increase sediment loads in the
estuarine basin (Aura et al., 2011). The variation
of diversity index among the stations in the Bakkhali estuary could probably
be due to other unknown factors which might regulate the index values. The diversity
index (1.17-4.64) recorded in the present study is similar with Belaluzzaman
(1995) and Islam (2003).
The physicochemical property of the sediment plays a regulatory role in determining
the abundance and seasonal variation of benthic organisms in the estuarine environments.
Estuaries are cordial to benthos which is able to cope with the harsh environment.
In the present study, highest amount of organic matter (4.0%) was found at station
I. The mean concentration of organic matter of Bakkhali channel system (3.29%)
is almost similar to results (3.19±1.00%) recorded from Karnafully river
(Siddique and Aktar, 2012). However, the mean concentration
of organic matter is lower than the results obtained by Weis
et al. (2001) for wetland sediments in Tijuana Estuary, California
(range of 8.7-13.8%).
Among different sampling locations, both stations I and IV observed to have
higher fraction of clay and silt than the other stations. The mean percentage
of clay particles in Bakkhali river estuary was found to be 16.07%, which is
lower that the mean value obtained from Karnafully river (33.52±3.81%;
Siddique and Aktar, 2012) and Tijuana estuary (59.42±8.93%;
Weis et al., 2001). The mean percentage of silt
was 5.05%, which is lower than the results (24.2±2.34) obtained by Weis
et al. (2001). Usually, soil condition or sediment particles are
important parameter to colonize the benthic fauna in the estuarine environment
(Chou et al., 2004). According to Chou
et al. (2004), sedimentation not only affects faunal communities
in the estuarine ecosystem but also changes sediment composition, organic matter
and nutrient input. The present study showed that the abundance and benthic
fauna diversity was higher in the areas where organic matter is rich (Bhat
and Neelanktans, 1988).
This study revealed that the abundance of macrobenthos depends on soft sediment surface and concentration of high detritus and organic matter. The maximum abundance of macrobenthos was related to nutrients input in this channel systems. However, the lower abundance of macrobenthos probably due to continuous environmental disturbance likes sedimentation, urban runoff and pollution in this river, which might badly affect in the river ecosystems.
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