1.2 Sampling

The voyages were conducted on board passenger ships of Bangladesh Inland Water Transport Authority (BIWTA) during monsoon and post-monsoon seasons from the study sites at Sandwip, Hatiya, Bhola, Barisal and Chandpur. Water samples were collected by a one liter Kemmerer water sampler from the surface (Table 1) and sub-surface (at 3 m depth) (Table 2) from each site. A Petersen Grab Sampler (APHA, 1974; Suess, 1982) having an opening of 0.024 m² was used to collect sediment samples. At each site, 4 grabs were collected for benthos identification and 2 grabs for analysis of sediment physico-chemical parameters.

1.3 Surface and sub-surface water analysis

The in situ air and water temperature were recorded using a Graduated Centigrade Thermometer; water pH was determined using pH paper (color pH ast ®, pH, indicator, strips, Cat.9582. Made in Germany); turbidity was measured using a white secchi disc of 30 cm diameter (Pipkin et al., 1977); water salinity was determined using a hand held refractometer (ATAGO, S/Mill, salinity. 0-100‰, Japan). Dissolved Oxygen concentration was determined by the Winkler titration Method (H. O. PUB. No. 607. 1955); Total Suspended Solids (TSS) and Total Organic Matter (TOM) were determined following Jin-Eong et al. (1985). BOD was determined by light and dark bottle method (APHA, 1974). Alkalinity, H^-CO₃ and CO₂ were determined following APHA (1975).

1.4 Bottom sediment analysis

The collected sediment samples for analysis were kept in labeled poly-bags. The in situ sediment pH was determined by a Soil pH tester (Takemura Electric Works Ltd. Japan), soil in situ temperature was recorded (by using a graduated Centigrade thermometer) immediately. Then samples weight (using spring balance) were recorded on board the vessel and were transferred to the laboratory for analysis. In the laboratory the samples were mixed thoroughly to prepare a composite sample and were dried at room temperature. The dry soil samples were sieved at 2 mm mesh. The sieved samples were dried again in an oven at 105°C for 12 hours. The soil extract water pH was determined by a pH meter. The soil composition (percentage of sand, silt and clay) were determined (Boyd and Tucker, 1992). Finally the soil texture were calculated.

1.5 Benthos samples analysis

The bottom sediment were collected for macrobenthos study using same Petersen Grab sampler and were sieved using 0.5 mm mesh. The sieved samples were preserved in a labeled plastic container with 70% alcohol (APHA, 1974) and transferred to laboratory for study. In the laboratory, Bengal Rose was added to samples (APHA, 1975) and left overnight for proper staining to facilitate benthos sorting. The sorted organisms were stored in vials with 70% ethanol for further study and identified as well as for photographs. The benthos were identified following Alam (1993); Ahmed (1990); Harkanta and Parulekar (1994); Jegadeesan and Ayyakkkannu (1991); Chandran et al. (1982); John and Yonge (1977); Belaluzzaman (1995); Hossain (1983) and Barua (1983).

1.6 Statistical analysis

One Way Analysis of Variance (ANOVA) with Post-hoc LSD test was done to test the influence of physic-chemical variables in the distribution of benthos (SPSS v.22). Canonical Correspondence Analysis (CCA) was done to find the ecological relationship between physico-chemical parameters and benthos community for both seasons (using PAST v.3.1). Cluster analysis (Dendrogram) was performed to show the similarity among the sites based on abundance taxa (using PRIMER v.6). Shannon-Wiener’s diversity of benthos was also determined (using PRIMER v.6).

2 Results

2.1 Physico-chemical parameters of surface and subsurface water

The depth of the sampling sites ranged from 4 to 13 m (Table 3). The air temperature varied 25.5 to 33°C in monsoon and 23 to 29°C in post-monsoon; The surface water temperature recorded 28 to 31°C in monsoon and 21 to 23°C in post-monsoon; and subsurface water temperature remained 28 to 29.5°C in monsoon and 21 to 22°C in post-monsoon.

During monsoon, 1‰ salinity was recorded only at Sandwip, while at all other sites salinity was almost zero. Whereas in post-monsoon water salinity was 15‰ at Sandwip and 8‰ at Hatiya (Table 1) and other sites it was recorded zero.

The secchi depth of surface water was lowest at Sandwip and gradually increased towards upstream sites and was recorded maximum at Chandpur during the both seasons. The lowest secchi depth was recorded as Sandwip (4 cm) and highest was at Chandpur (55 cm) during monsoon. On the other hand it was lowest at Sandwip (35 cm) and highest at Chandpur (60 cm) during post-monsoon season. Between the seasons the Total Suspended Solids (TSS) and Total Organic Matter (TOM) were more in both surface and subsurface water during monsoon than that of post-monsoon season. During monsoon maximum and minimum TSS were recorded 10.83 mg/L and 0.31 mg/L at Sandwip and Chandpur respectively. Whereas in post-monsoon TSS was maximum 1.34 mg/L at Hatiya and minimum 0.30 mg/L at Sandwip. The TOM was recorded most was 0.86 mg/L at Sandwip and lowest 0.48 mg/L at Chandpur during monsoon; on the other hand it recorded highest 0.64 mg/L at Sandwip and lowest 0.54 mg/L at Bhola during post-monsoon.

In the present study the surface water pH was slight acidic during monsoon whereas it was almost neutral in post-monsoon. It was also observed that the subsurface water pH was slightly more than that of surface water. The Dissolved Oxygen (DO) concentration was comparatively more during monsoon than post-monsoon at all sites. Again DO was more in sub-surface than surface water in most sites. The Biological Oxygen Demand (BOD) was recorded more during monsoon than post monsoon at all sites. Again subsurface BOD was relatively more than that of surface water. During the study CO₂, Alkalinity, HˉCO₃ concentration of surface and subsurface water were determined. It was observed that their concentration was relatively less in subsurface water than surface water.

2.2 Physico-chemical parameters of bottom soil

The in situ soil pH recorded 6.5 to 7.2 (at Sandwip and Barisal respectively) during monsoon and 6.8 to 7.1 (at Barisal and Sandwip respectively) during post-monsoon. The bottom soil extracted water pH ranged 6.5 to 7.1 (minimum at Hatiya and Chandpur and maximum at Barisal) during monsoon and 6.2 to 6.8 (at Barisal and Sandwip) during post-monsoon season. It was observed that the in situ bottom sediment poured water pH was more than the soil extracted water pH for both season at all sites (except Hatiya in monsoon). In all sites and seasons the subsurface water OM concentration were recorded a little more than that of OC. In the present study the water OC ranged from 0.06 ml/L to 0.83 ml/L (at Sandwip in monsoon and at Chandpur in monsoon respectively) whereas the OM varied between 0.11 ml/L and 1.43 ml/L (at Sandwip in monsoon and at Chandpur in monsoon respectively). But in case of bottom soil OM and OC content were more in post monsoon than that of monsoon (except at Hatiya). During the study OM content ranged from 0.06% to 0.8% (at Chandpur during monsoon and at Sandwip during monsoon respectively).

Sand-silt–clay percentage of sampling sites were used to calculate soil texture (Table 3). The soil texture of Hatiya, Bhola and Barisal remain similar during both seasons whereas at Sandwip and Chandpur it showed little change (Figure 2).

2.3 Benthos

A total of 17 major taxa of macrobenthos were identified of which 10 and 15 were recorded during monsoon and post-monsoon season respectively (Table 4). During monsoon, the highest density of macrobenthos was recorded at Chandpur (7658.7 Indivs/m²) and the lowest (208.1 Indivs/m²) at Hatiya. Whereas during post-monsoon the maximum was 27180.0 Indivs/m² at Barisal and the minimum was 1248.7 Indivs/m² at Bhola. Polychaetes were recorded at all sites and seasons (except Hatiya in monsoon). Amphipoda and Nemertea were abundant at Sandwip and Hatiya in both seasons. Nemertean, Oligochaeta, Polychaeta were the dominating at Chandpur during the post-monsoon.

2.4 Multivariate analysis

2.4.1 Canonical corresponding analysis (CCA)

The Redundancy Analysis (RDA) triplot is used to explaining the favorite abiotic ecological factors for distinguishing dominant benthos during monsoon and post-monsoon seasons and also to show the influence of environmental parameters (Figure 3; Figure 4).

During the monsoon season, when a low-salinity environment prevailed in majority portion of the estuary, most of the benthos showed low positive relation with salinity. Moreover, Amphipod, Isopoda, Diptera, Megalopa, Nemertean, Polychaetes larvae showed close affinity to water temperature and organic carbon. Again Branchiopod and Coleopteran showed close affinity to silt soil. Archiogestropoda, Cumacea, Doliolida and Oligochaeta exhibited no affinity to clay and soil pH and organic matter (Figure 3) Moreover, Polychaetes and Mesogastropoda showed that they were not influenced by physico-chemical factors during post-monsoon (Figure 3).

During the post-monsoon season, most of the benthos showed low affinity to salinity (Figure 4). The analysis showed Archiogestropoda, Cumacea, Megalopa, Pollychaetes, Pollychaetes larvae have close positive relation with clay, water temperature, soil pH, organic matter and organic carbon. There is negative effect of silt with Branchiopod, Doliolida, Egg, and Nemertean. Again some of the benthos group like Bivalves, Coleopterans, Diptera, Mesogastropoda, Oligochaeta, exhibited no influenced by environmental factors during post-monsoon (Figure 4).

Therefore, a complete study on the inter-relation among the macrobenthos and environmental factors was performed for each season, and the triplot in the RDA was helpful in both picturing all the data points plotted in the coordinate system and diagnosing the inter-relationship among benthos and environmental factors.

2.4.2 Cluster analysis

Cluster analyses (CA) were executed using square root and Bray Curtis Similarity to show the similarity among the sites in terms of benthos occurrence. The dendogram of the benthos, based on their occurrence and abundance pattern along with different seasons, helped extensively in understanding the similarity in their distribution in the estuary. During the monsoon season Hatiya and Sandwip formed a cluster that means they are almost similar in case of benthos distribution (Figure 5a). During post-monsoon Hatiya and Sandwip as well as Chandpur and Barisal formed cluster indicating that they are similar in terms of benthic community occurrence (Figure 5b).

2.5 Diversity index

In the present study benthos diversity (H') and evenness (J') spatially and temporally (Figure 6a; Figure 6b) did not show much variation (0.92±0.52 and 0.85±0.40 for the monsoon and post-monsoon seasons respectively). The highest diversity (H') was recorded at Bhola (1.53) during monsoon season and the lowest was at Chandpur (0.29) during the monsoon season (Figure 6a).

Again, benthos evenness (J') also showed less variation (0.64±0.32 and 0.51±0.26 during monsoon and post-monsoon seasons respectively) (Figure 6b). The maximum evenness value was found at Sandwip (0.95) during the monsoon season and the minimum was at Barisal (0.20) during the post-monsoon season (Figure 6b).

3 Discussion

The abundance and distribution of benthos are closely associated with bottom sediment physico-chemical variables. Benthic organisms are considered as indicator of past and present conditions (Bruse et al., 1975; Gamlath and Wijeyaratne, 1997). The presence of unusually large benthic species as well as some others absence can be used as bioindicator of water quality (Hart and Fuller, 1978; Stanford et al., 1994). Spatio-temporal variability in benthic fauna has been studied in coastal aquatic systems in many parts of the world (Morrisey et al., 1992; Ieno and Bastida, 1998; Biles et al., 2003; Giberto et al., 2004). Differences in environmental variables play critical role in distribution pattern of benthic organisms. Soil texture and salinity control distribution of benthic organisms in coastal and estuarine region (Self and Jumars, 1978). Perkin (1976) also reported that benthic community structure depends on environmental factors such as salinity, temperature, dissolve oxygen, depth, organic matter, soil texture and size of sediment particles.

The Estuarine environment is exposed to diverse change in physic-chemical variables due to incessant mixing of fresh water with marine water (George et al., 2012). The water quality estimation is very important to determine ecosystem that have great impact on the occurrence of aquatic lives (Chang, 2008). In the present study, water quality was recorded slightly acidic with low salinity in major part of the estuary. The other physico-chemical parameters viz; H^-CO₃, water temperature, secchi depth, CO₂, DO, alkalinity, salinity and pH showed significant response for the variations in benthos community structure (p<0.05).

Water temperature variation is significant function in macrobenthic assemblages (Ndome et al., 2012). Unanam and Akpan (2006) reported that the distribution of macrobenthos reduced with increase in temperature. The present study area (the lower Meghna River) is under tropical monsoon region and the atmospheric temperature is maintained by seasonality. It was observed that the surface water temperature was 2 to 5.5°C less than air temperature whereas subsurface water temperature remained similar or up to 2°C less than that of surface water temperature (Table 1; Table 2). Aken (2008) mentioned that water temperature is an important parameter which influences the dissolution-precipitation, adsorption–desorption, oxidation–reduction and physiology of biotic community in an aquatic environment.

Salinity plays a great role in the occurrence and distribution of macrobenthos (Pearson, 1970; Pearson and Rosenberg, 1978; Rosenberg, 2001; Ysebaert et al., 2003). In this study it was observed that the surface and subsurface water salinity were almost similar. During monsoon, only (1‰) water salinity was recorded at Sandwip, while at all other sites salinity were almost zero. During post-monsoon water salinity was 15‰ at Sandwip and 8‰ at Hatiya (Table 1; Table 2) and other sites it was zero. The abundance pattern of Amphipods (F=1.575E4 & p=0.000) showed strong relation to salinity during post-monsoon. Kumar and Khan (2013) reported that there was a significant positive correlation between salinity and benthic faunal diversity (r=0.381; p<0.05). Jones (1987) described that Polychaetes, Crustaceans and mollusks mostly interact with salinity. Hodda and Nicholas (1985) investigated the meiofauna around the Hunter river-estuary, New South Wales, Australia and described salinity as the key factor influencing the relative abundance of species distribution.

In the present study in situ subsurface water pH and soil extracted water pH changes were almost similar. It was observed that the subsurface water pH and soil extracted water pH remained slightly acidic at all sites and seasons except for Barisal in monsoon. Hossain and Marshall (2014) reported that pore-water of Sungai Brunei Estuary, Borneo was slightly acidic. The distribution and abundance of Polychaetes (F=193.78 & p=0.49) was greatly affected by soil pH in both monsoon. Gaskill (2014) reported that benthic macro-invertebrate richness is significantly correlated with pH (p = 0.034) and species richness decreased because of lower pH.

In the study it was observed that both the OC and OM percentage were more during post-monsoon than monsoon season at all sites. The maximum amount of soil Organic Matter (OM) 1.43% was found at Sandwip and the lowest 0.11% was recorded at Chandpur in monsoon. During post-monsoon the highest OM (1.04%) was at Bhola and the minimum (0.36%) was at Barisal (Table 3).

The total organic carbon and grain size affect habitat selection, feeding behavior and survival of benthic organisms (Jegadeesan and Ayyakkannu, 1992; US Army Corps of Engineers, 1996). Murugan and Ayyakkannu (1991) found a high significant correlation between benthic fauna and organic carbon in the South East coast of India. The Organic matter and sediment characteristics have massive influence in the occurrence of benthic fauna (Pearson, 1970; Pearson and Rosenberg, 1978; Rosenberg, 2001; Ysebaert et al., 2003). According to Levin and Talley (2000) soil organic matter and soil grain size have an effect on benthic communities.

Investigating soil composition it was observed that sand percentage was highest and at five sites in both season. The soil texture remained unchanged at Hatiya, Barisal and Chandpur sites; whereas at Sandwip and Bhola it changes slightly Figure 2 (A and B). Greater benthos species occurred more at Chandpur and Barisal and the soil texture of these two sites were sandy loam and loam respectively. Harakanta and Parulekar (1985) reported benthic fauna are relatively rich in finer soft bottom which have similarity with the present investigation. In case of Sandwip and Hatiya soil texture were clay loam and sandy clay loam which are comparatively compact for benthos to penetrate at Sandwip, Hatiya and Bhola. During post-monsoon percentage of sand was highest among all sites and the abundance was lowest. Changes in soil texture composition have a significant effect on the abundance and distributions of some invertebrates; particularly deposit feeders (Whitlatch, 1980). Jones (1987) reported that fine sediment generally contained more organic carbon and carbonate than coarse sediment.

4 Conclusion

In the present study, a total of 17 major taxa were identified of which 10 and 15 major taxa were recorded during monsoon and post-monsoon season respectively. Macrobenthos abundance was greater during post-monsoon than monsoon in the study sites. In the lower Meghna water was slightly acidic. Salinity, soil pH and water temperature were found responsible for the variations in benthos community assemblage (p<0.05). Canonical Corresponding Analysis (CCA) also indicated that Amphipoda, Isopoda, Diptera, Megalopa, Nemertea, Polychaeta larvae showed close affinity to water temperature and organic carbon. Cluster analyses of sites in terms of benthos occurrence showed that during the monsoon season Hatiya and Sandwip formed pair cluster in case of benthos distribution. In the present study benthos diversity (H') and evenness (J') spatially and temporally did not show much variation.

5 Limitation

In the present study the sampling sights covered an area of about 172 km and samples were collected from passenger ship. So it was difficult to take sample at a time from all sites as well as to maintain tidal cycle or tide log. Again as samples were collected from passenger ship on board samples have to collect where the ship anchored for disembarkation of passengers and goods.

Acknowledgments

First, author is gratified to his mentor and supervisor (late) Dr. Nuruddin Mahmood, Professor Institute of Marine Sciences, University of Chittagong, Bangladesh. He is also thankful to Mr Simul Bhuyan, Mr. and Md. Zakaria for their continuous inspiration. The author is pleased to Mr. Musa for his cooperation in the field. I am also grateful to my wife Asma Bint Shafiq for her beloved support. Last but not least sincerely acknowledge my parents late Dr. AQM Shamsul Alam and Mrs. Sajjadun Neas.

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