Study area: The Bakkhali river estuary is located at the south-eastern coast of the Bay of Bengal. This river is relatively wide (about 67 km) compared to other rivers of the Cox’s 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. 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. 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.
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 different references.
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.
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. Soil organic matter was detected following standard procedure. Soil organic carbon was calculated dividing the organic matter by a factor of 1.9 following the procedure described by Nelson and Sommers.
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. The Shannon-Wiener diversity, 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:
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) using the following formula:
where, S is total species and N is total individuals.
Pearson’s 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.