2.1 Study area The study was carried out from Bangladesh Fisheries Research Institute, Riverine Station, Chandpur (N-23014.6160 and E90037.8180 ) during the period of January, 2014 to December, 2014 covering the area of Chandpur sadar (Chandpur), Meghna ghat (Narayangonj) and Bhairab (Kishorgonj). The samples of water were collected from selected sites. The characteristics of sampling sites are described in Table 1.
2.2 Assessment of physico-chemical parameters Common physical tests of water include colour, odour, temperature, transparency, solids concentrations e.g., Total Dissolved Solid (TDS) and turbidity. Colour and odour were examined through external sensory organ whereas other physical properties were evaluated through latest models of digital temperature and turbidity meter. Transparency was measured through secchi disk reading. Chemical parameters such as Conductivity (μs/cm), Transparency, TDS (mg/L), Dissolve Oxygen (mg/L), pH, Total alkalinity, Total hardness, ammonia (NH3), BOD (mg/L) and COD (mg/L) were analyzed on the same day of sampling. Free CO2 content was determined by Phenolphthalein indicator method. In this method, at first we take 100 ml water sample then add 6 drops Phenolphthalein indicator and finally titrated with NaoH until pink color. Total alkalinity was estimated by using phenolphthalein and methyl orange indicator method. In this method, at first we take 100 ml water sample then add 6 drops Phenolphthalein indicator and 6 drops methyl orange. Finally this solution titrated with 0.2 N H2SO4 until orange color. Total hardness was determined by EDTA titrimetric method. HACH test kit (Model-FF-2, USA) was used to measure pH, dissolved oxygen (DO), ammonia. Total Dissolved Solid (TDS), conductivity was measured by EC meter (HANNA instruments: H19143). The Biochemical Oxygen Demand (BOD) of polluted water is the amount of oxygen required for the biological decomposition of dissolved organic matter. Usually, the time is taken as 5 days and the temperature 20 °C as per the global standard. The BOD test is among the most important method in sanitary analysis to determine the polluting power, or strength polluted water. It serves as a measure of the amount of clean diluting water required for the successful disposal of sewage by dilution. The test has its widest application in measuring waste loading to treatment plants and in evaluating the efficiency of such treatment systems. The test consists in taking the given sample in suitable concentrations in dilute water in BOD bottles. Two bottles are taken for each concentration and three concentrations are used for each sample. One set of bottles is incubated in a BOD incubator for 5 days at 20 °C; the dissolved oxygen (initial) content (D1) in the other set of bottles will be determined immediately. At the end of 5 days, the dissolved oxygen content (D2) in the incubated set of bottles is determined. In this method, at first we take 100 ml original water sample then add 2 ml MnSo4 and 2 ml alkyl iodide. After that add 2 ml concentrated H2So4 and mixed well. Then add 6 drops starch solution and finally titrated with Na2S2O3 until colorless. COD results are reported in terms of mg of oxygen. At first 50 ml sample was taken and allow with 1g H2SO4 and glass beads. Then 5 ml H2SO4 and Ag2SO4 was added very slowly. Cool in room temperature. After cooling 25 ml 0.0417 M K2Cr2O7 was added. Then solution containing flask was added in condenser. Then 70ml of H2SO4 and Ag2SO4 was added through open end of condenser and continuous shaking was done for well mixing. Cool the solution containing flask in room temperature. Then double volume of distilled water was added. An excess of oxidizing agent is added, the excess is determined by another reducing agent such as Ferrous Ammonium Sulphate (FAS). An indicator ferroin (0.0417 N) is used in titrating the excess dichromate against ferrous ammonium sulphate.
2.3 Assessment of biological parameters Sampling of plankton assemblage from sub-surface water (four times in a day at 6, 12, 18 and 24 hrs to know the diurnal variation) were done by filtering 50 liters of river water through a 25 µ and 50 µ plankton net for qualitative (preferably up to species level) and quantitative analyses. For plankton counting, the S-R (Sedgwick Rafter) cell was used which is 50 mm long, 20 wide and 1 mm deep. Before filling the S-R cell with sample, the cover glass was diagonally placed across the cell and then samples were transferred with a large bore so that no air bubbles in the cell covers were formed. The S-R cell was let standard for at least 15 minute to settle planktons. Then planktons on the bottom of the S-R cell were counted by microscope. By moving the mechanical stage, the entire bottom of the slide area was examined carefully. Organisms lying between two parallel cross hairs were counted as they passed a vertical line. Identification of plankton (phytoplankton and zooplankton) up to generic level was made according to Presscot (1964), Needham and Needham (1963) and Belcher and Swale (1978) [6]. Number of plankton in the S-R cell was derived from the following formula [5]: Number of species/Liter = C × 1000 mm3 / L × D × W × S
Where, C = Number of organisms counted L = Length of each stripe (S-R cell length) in mm D = Depth of each stripe in mm W = Width of each stripe in mm S = Number of the stripe.
Sampling of macro-invertebrates was done using an Ekman's dredge and no. 40/60mm sieve for qualitative and quantitative analysis.