Analyses of Macrobenthos of Hatiya and Nijhum Dweep Islands at Higher Taxonomic Resolution
M. Belal Hossain,
This study was carried out to describe the distribution and community pattern
of benthic macrofauna collected from two nationally and internationally important
islands, Hatiya and Nijhum Dweep in the period of January to June, 2010 using
hand-held mud corer (10x10x10 cm) from seven stations. The coarser (order) level
of taxonomic resolution was used to investigate the community attributes among
the sampling stations. The study yielded a total of 10688.89 ind. m-2
macrofauna from all stations. Ten major taxa were identified from two islands
and polychaete being the dominant. The average population density was 1526.98±1453.375
ind. m-2. There was a significant difference (p<0.05) in faunal
density between Hatiya and Nijhum Dweep with later having higher mean density.
The maximum (1.97) Shannon diversity index was found in St1 and the lowest (1.14)
in St5. The highest similarity (76.6%) was found between St2 and St5. Multivariate
analysis was conducted at order level of benthos using PRIMER. Bray-Curtis similarity
measures among macrozoobenthic communities separated the stations into several
cluster groups which was supported by nMDS ordination map.
Received: September 19, 2012;
Accepted: October 12, 2012;
Published: June 04, 2013
Benthic macrofauna has been the subject of many studies throughout the world
by realizing their importance in ecological and environmental studies (Gray
et al., 1990; Smith and Simpson 1993; James
et al., 1995; Aweng et al., 2012;
Asadujjaman et al., 2012). They are of considerable
importance in aquatic food chain and especially the main food sources of commercially
important demersal fishes (Hossain et al., 2009).
They provide a linkage between substratum and sea bed and water column predators
(Gray and Elloitt, 2009). They have long been used as
indicator of pollution. There is an increasing awareness of biodiversity and
the protection of all members of the biological community. All organisms, microscopic
to macroscopic, play active role in balancing the ecosystem. Loss or extinction
of any group of organisms hampers the functioning of the ecosystem. So, there
is need to identify and protect the benthic population of conservation interest
(Allen, 2000). In decision VI/26 of Convention on Biological
Diversity (CBD) targeted to fulfill the three objectives, to achieve a significant
reduction of the current rate of biodiversity loss at the global, regional and
national level as a contribution to poverty alleviation and to the benefit of
all life on earth by 2010 (CBD, 2004). But unfortunately
the benthic habitats of Bangladesh coastal area are still comparatively untouched
or understudied. Only a few unpublished (Alam, 1993;
Belaluzzaman, 1995) and published (Mahmood
et al., 1993; Hossain et al., 2009;
Hossain, 2011; Asadujjaman et al.,
2012) works described the benthic macrofauna of Bangladesh coastal area.
The reason may be due to lack of taxonomic expertise, time consuming, cost and
labor intensive work. Majority of benthos monitoring studies, have often been
criticized by the scientific communities due to the high costs and long time
involved in sampling and taxonomic work (Warwick, 1993).
So, Warwick (1988) and other investigators Gray
et al. (1990), Smith and Simpson (1993) and
James et al. (1995) concluded from their investigations
that studies of benthic macrofaunal communities, where strong gradient exist,
can be conducted even if specimens are identified to the level of phylum. Of
course, decreasing the level of taxonomic resolution will cause a certain loss
of information. But it is better to use the coarser level taxonomic information
rather than doing wrong identification (Warwick, 1988).
Therefore, in the present study coarser taxonomic resolution as suggested by
Warwick (1988) was followed.
Hatiya and Nijhum Dweep are two small islands, formed of sediment carried by
the mighty Meghna River, situated in the northern part of Bay of Bengal, Bangladesh.
These islands are nationally and internationally important (Khan,
2012). The eulittoral zone is the area of the foreshore and seabed that
is exposed to the air at low tide and submerged at high tide. Both biotic and
abiotic factors are strongly active in this zone and create a challenging environment
for the organisms living therein. So, the aim of this study is to describe and
characterize the macrofaunal community pattern at coarser taxonomic level in
the eulittoral zone of the selected area.
MATERIALS AND METHODS
Study area: Hatiya and Nijhum Dweep are two small islands located (GPS
reading: 22°22'N 91°7.5'E and 22°04'N 91°00'E, respectively)
in the mouth of Meghna river, central coast of Bangladesh (Fig.
1). These two islands have an area of about 1508 km2. They are
frequently subject to cyclones and destructive ocean waves (Encyclopedia
Britannica, 2012). The clusters of islets of Hatiya and its surroundings
comprise various types of habitats e.g., network of intertidal creeks inside
mangroves, massive mudflats, grassland, reed land, sand flats, sand beaches,
sand dunes and dipper channels. These varieties of habitats harbor a high plant
and animal biodiversity (Khan, 2012). At high tide a
significant portion of Nijhum Dweep becomes covered in water, apart from the
inhabited areas. In 2001, it was designated as the Nijhum Dweep National Park
by Bangladesh government. The forestry department of Bangladesh created lush
mangrove forests in Nijhum Dweep as part of conservation efforts for the area.
The inter-tidal mudflats and sand-flats serves as the southern most stop-over
for nearly 100 species of migratory birds, around a dozen of which are considered
to be globally critically endangered (Khan, 2012). Recently
a group of scientists from home and abroad surveyed the area and proposed to
declare it as RAMSAR site (Khan, 2012).
|| Location of sampling stations in Hatiya and Nijhum Dweep
Sampling locations: Seven sampling stations were selected around the two islands for macrofauna collection (Table 1).
Sample collection and analysis: Three replicate sediment samples were collected from each station with hand-held mud corer (10x10x10 cm) having a mouth opening of 0.01 m2 during pre-monsoon (January-June, 2010). The samples were washed through a 0.5 mm metallic sieve. Sieved organism samples were preserved with other residues in the plastic container with 10% buffered formalin and few drops of Rose Bengal. Organisms were sorted and enumerated under major taxa using Binocular microscope with digital camera (model No: XSZ21-05DN). Benthic invertebrates were identified upto order level. An attempt has been made to identify the macrobenthos up to lowest taxonomic level but due to time limitation, taxonomic expertise, lack of fund and appropriate literature it was not possible.
|| Sampling stations of Hatiya and Nijhum Dweep Islands with
The coarser level (order) of taxonomic data as suggested by Warwik (1988) were analyzed using the statistical package SPSS v.17 and multivariate data analyzing software, Plymouth Routines In Marine Ecological Research (PRIMER v.6). Classification and Ordination (Nonparametric multidimensional scaling, nMDS) were performed by using Bray-Curtis similarity measure for community analysis.
RESULTS AND DISCUSSION
Density, diversity and similarity: The macrofauna comprised ten major
taxa (order) and yielded a total of 10688.89 ind. m-2 with a mean
density of 1526.98±1453.375 ind. m-2 from all stations and
the highest being 4511.11 ind. m-2 in St 6 and the lowest 433.33
ind. m-2 in St 1 (Fig. 2). The mean density of
benthos was 1137.78 ind. m-2 in Hatiya and 2500 ind. m-2
in Nijhum Dweep. The dominant taxon was polychaete contributing 45.03% of total
fauna. Polychaete, oligochaete, crab were common in all stations. Bubble plot
show polychaete and oligochaete were abundant in St 6 (Fig. 3a,
b) whereas shrimp larvae and crab were dominant in St3 (Fig.
3c and d). Evenness and Shannon diversity indices are
two important components of the diversity indices. Evenness index expresses
how evenly the individuals are distributed among the different species. Pielous
evenness index and Shannon diversity are widely used indices for comparing diversity
between various habitats. The value of Shannon diversity is usually found to
fall between 1.5 and 3.5 and only rarely it surpasses 4.5. The evenness has
very good discriminating ability between the habitats and the value varies from
0 to 1. In this study the highest (0.95) evenness value was found in St1 and
the lowest (0.56) in St 6 (Table 2). The maximum (1.97) Shannon
diversity index was found in St1 and the lowest (1.14) in St5 (Table
2). Similarity matrix data (Table 3) shows the highest
similarity (76.6%) was found between St 2 and St 5. The highest density and
diversity observed in the undisturbed and less eroded stations. Polychaete and
oligochaete were found to be comparatively abundant in the disturbed and eroded
stations (St 6) whereas crustaceans were dominant in undisturbed and less or
non-erosion prone stations (St 3).
Community analysis: The most useful approaches for the preliminary analyzing
of benthic data are classification and ordination. Generally, classification
starts with a matrix of similarity coefficients between all possible pairs of
|| Density (ind. m-2) of macrobenthos in different
stations, at 95% confidence interval
||Bubble plot of (a) Polychaete, (b) Oligochaete, (c) Shrimp
larvae and (d) Crab distribution in the sampling stations (St)
The most similar pair of individuals is joined to form a new super individual
and the process is repeated until all individuals have been joined into a single
group and successive grouping is plotted against some measure of the homogeneity
of the new group and then a dendogram is produced (Fig. 4).
||(a) Dendogram plot and (b) nMDS plot of sampling stations
(St) using Bray-Curtis similarity matrix
|| Diversity indices in the study area
|| Similarity matrix among the stations
The structure of the dendogram summarizes trends in the species abundance data.
In this study, the computer package, PRMER has been used for classification
and ordination of abundance data. Cluster analyses were carried out, based on
the square root transformed abundance of all taxa found in the study area, using
the Bray-Curtis measure of similarity and group-average sorting. The analysis
split the stations mainly two groups (indicated as A and B) at about 30% similarity
level (Fig. 4). Group A consists of two stations (St 4 and
St 6) and group B of four stations (St 3, St 7, St 2 and St 5). St1 is not grouped
into any cluster remaining totally separate from other stations. In group B,
St 7, St 2 and St 5 are in the same cluster whereas, St 3 remained separate.
The nMDS mapping of abundance data supports the cluster groups.
||Species composition of (a) Cluster group A and (b) Cluster
It shows at the both 60 and 40% similarity level St 2, St 5 and St 7 are tightly
closed with each other but St 4 and St 6 remained close at only 40% similarity
level (Fig. 4) and St 3 and St 1 are scattered in the nMDS.
The species composition of each cluster groups is presented in Fig. 5. Both Groups are dominated by polychaetes followed by oligochaete and shrimp larvae. In group A, polychaete and oligochaete together constituted 77% of total abundance whereas in group B, polychaetes and shrimp larvae covered 62%.
Though, a certain amount of information about community identities and their
temporal drifts will be lost, analyses of higher taxonomic levels are more likely
to reflect a contamination gradient than are analyses based on species abundances
(Gray et al., 1990). While community identity
cannot be recognized at order level, the inter-annual variability may still
be recognized. Although, environmental impact assessment studies traditionally
require identification of the individuals collected to the species level, several
studies have found analyses at the level of family to be acceptable, since little
information appears to be lost (Warwick, 1988). This
suggests that one way of reducing the cost of benthic surveys could be to shorten
the time Surveys of soft-bottom macrofauna, integral to a majority of marine
pollution monitoring studies. One possible reason is that sampling strategies
used are very labour-intensive. There are instances, however, where a correct
identification to a higher taxonomic level would be better than an incorrect
identification to the species level which by requiring considerable expertise
is often more error prone. Using higher taxa would be useful, especially when
carrying on a pilot study about the general features of a new environment (Gray
et al., 1990; Smith and Simpson, 1993; James
et al., 1995). Although, gross, results will help in developing a
subsequent, more appropriate and detailed sampling design. Warwick
(1988) firstly pointed out that there were theoretical advantages to conducting
both multivariate and univariate analyses at various hierarchical levels of
taxonomic aggregation in programmes aimed at detecting the biological effects
of marine pollution. Anthropogenic effects modify community at a higher taxonomic
level than do natural environmental variables, due to the inability of species
to adapt through evolution in response to a recent pollutant disturbance (Smith
and Simpson, 1993; James et al., 1995). So,
the developing countries e.g., Bangladesh can use benthic community attributes
for environmental impact assessment analyzing abundance data at higher taxonomic
level which will be cost effective.
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