Bangladesh has a large coast line of about 711 km offering a good potential for exploration of coastal and estuarine water for fishery resources, production of salt and navigation etc. Phytoplankton, Zooplankton and Benthos are very important components of aquatic biotic communities, playing a great ecological roles in wetland ecosystem functions. They play significant roles in the energy pathway and nutrient significant roles in the energy pathway and nutrient cycling and they also constitute an important link in the aquatic food chain as food resource for fishes and other animals. The distribution of aquatic organisms is the result of interactions among their ecological role, the physical conditions that characterize the habitat and food availability [1]. Thus, the community structure of plankton and benthos of Karnawphuli depends on a number of factors namely water quality, type of substrate, particle size of sediment, water flow, sediment, air- water temperature and salinity as well as environmental conditions surrounding the watercourse. The Karnawphuli is the principal river of Chittagong region of Bangladesh [2]. The biggest sea port of the country is situated in the bank of this river for its advantageous geographical position. The country earns a lot of foreign currencies through port activities. Though it has great economic value, but the water body of river, estuary and sea is impacted much due to accumulation of waste from agricultural land, urban run-off, industries, power plant sewage and  sedimentation. Unplanned urbanization, industrialization, discharging untreated sewage on the bank of the Karnawphuli River has exerted unbearable pressure and stress on the river.

Water, sediment and benthic faunal samples were collected from the intertidal zone of the Karnawphuli estuary (22°23.387'-22°23.411'N and 91°80.981'-91°80.987'E) Chittagong, Bangladesh during the period of December 2015 (fig-1). All the samples were collected from two sites namely Jetti no. 10 and Jetti no. 15. Among the two sites, Site 2 (Jetti no. 15) was set in front of a power station named Barakah Power Plant Ltd and lighter age vessel standing place through which sewage of power plant and the discharge of vessels wastage falls into the Karnawphuli estuary and thus it was considered as impacted site (figure-2). The other site like Site 1 (Jetti no. 10) considered as moderately impacted site (figure-2). Sampling site is highly crowded by ships and economically one of the most important place in Bangladesh. At the time of acquiring sample about 53 small transport boat, about 13 troller and about 33 lighterage vessel has been located around the sampling area.

Figure 1:  Karnawphuli Estuary.

Figure 2: The map of indication about the area of sampling.

Subsurface water samples were collected from two sites during the high tide condition for measuring water temperature, salinity, pH, plankton and all the process determined followed by Standard Method. Water temperature was measured by using standard mercury filled centigrade thermometer having a range of from 0°to 100°C [7]. In the site 1 at the time of 12.50 pm, water temperature was recorded 20°C and the weather temperature was 23°C and in the site 2 at the time of 1.11pm, water temperature was recorded 22°C and the weather temperature was 23°C. Water pH was determined by using a digital pen pH meter (HANNA Instruments), Water pH meter was calibrated before every measurement. As recorded in the site 1, at the time of 12.50 pm water pH was 6.5 and in the site 2, at the time of 1.11 pm water pH was 7. In the site 1, at the time of 12.50pm salinity measured 20 ppt and in the site 2, at the time of 01.11pm salinity was measured 21 ppt. In the site 1 at the time of 12.50pm water transparency measured up to 9cm and in the site 2 at the time of 1.11pm water transparency measured up to 10cm through sacchi desk. Sediment and benthic faunal samples were collected from the study sites by using Grab Sampler [3]. One grab sampler was used in the Site 1 (Jetti no. 10) and Site 2 (Jetti no. 15) by turns. One sediment sample was taken from each station for analysis of soil temperature, soil water salinity, soil texture, sediment texture, particle density. Grab sampler used to collecting sediment sample from the High tide mark station at High tide period of various sampling time. The diameter of core sampler is 10 cm and the length 20 cm [4]. The collected sediment samples were kept into a polythene bag (500gram to 1 kg). In the laboratory collected sediment samples were spread on the sheet of paper or trays and dried in air (exposure to sun light hastens the drying). The larger aggregates were broken down preferably in a wooden mortar and pestle and pass the sediment sample through a 0.30mm sieve [5]. The sieved sediment sample in then weighted and the mouth of the plastic container are well capped (each container are marked as necessary). The containers were stored in a cold place in laboratory [6]. Though immediate analysis is ideal. Storing in low temperature (4°C) is perhaps the best way to preserve most samples until the next day use. Sediment sample preserve in low temperature (less than 25°C) room in the plastic pack for further analysis. Soil texture was determined by the method described by Bouyoucos method (1962) [7]. 25g of oven dry soil (150°C) was taken in a breaker. After that 50ml distilled water was added. The contents were stirred thoroughly with a glass for half an hour. Then 10ml 1N NaOH solution was added, stirred thoroughly. The contents were transferred to a 1000ml sedimentation cylinder. The volume of contents was making up to 1000ml by adding distilled water. The suspension was allowed settle and reading was taken by a soil hydrometer and thermometer exactly after 1min and after 2 hrs [8]. A similar blank was run without soil but following all above criteria. Benthic faunal communities were sampled by taking sediments with grab sampler at each of the station. The collected samples were sieved through a 0.5mm mesh. Mud sample has been collected from Jetti no. 10 and 15 by using grab sampler from 5 meter and 7 meter depth from water level. The sample has been preserved by using 5% of formalin solution. After that it has been colorized by Euosine and kept for 24 hours. Then Benthos organisms has been sorted from the raw sample and assembled according to species. Phytoplankton and Zooplankton samples were collected from surface water. Plankton net is conical in shape and consists of ring (rigid/flexible and round/square) [9], the filtering cone and the collecting bucket for collection of organisms (phytoplankton and zooplankton). The collected sample was immediately preserved in 5% neutralized and transferred to laboratory for analysis. For efficient sorting the samples were stained with rose bangle and let for overnight. All the plankton attained pink (eosin) color rendering easy identification. The stained plankton was sorted out from debris with fine brush, needle. Forceps and a complex microscope was used during sorting. The sorted organisms were preserved in 5% formalin [10]. The sorted organisms were brought under microscope and identified following Islam (1982 & 2003). The total numbers of major taxa of zooplankton and phytoplankton were determined and their concentrations were compute as ind/m³ of water and their other respective percentage [11].

The mean value of different hydrological factors like water temperature, water salinity and water pH were 21±1°C, 20±1%, and 6.8±0.2.5 respectively. Mean values of different pedological factors like percentage (Diagram-1) sand, silt, clay assemblages consist of 10 benthic species/taxons of the habitat were 73.28%, 21.6%, 5.12% and mean air temperature of the study area during the sampling period.

Among 10 species/taxon, 4 species of molluscs, 2 species of annelids, 3 species of arthropods 1 species of Chaetognatha, a vast majority of bacteria are found in the sediments of benthic zone; Diatoms, ciliates are also found in the present study. The abundance of the benthic species are Neptunea lyrata, Pecten albicans Sagitta elegans, Gammarus roeseli, Pectin aria koreni, Nereis pelagisa, Littorina littoriea, Carcinus meanus, Caprella mutica, Buccinum undatu. After analysing mud sample collected from grab sampler there are 44 benthic organisms of 20 species recorded (given in table-1).

Table 1: A chart of the species recorded from the collected mud sample.

After analysing the sample of phytoplankton, there were 22 types of species recorded from two sites of sampling (name of the species given in table-2).

Table 2: Phytoplankton recorded from both of the sampling sites.

From the collected sample the total number of zooplankton specimens/individuals of all groups is equal to total counts of the specimens (say x) is divided to Volume of water filtered (V). The volume of the water filtered is normally expressed in cubic meters. It is calculated as follows.

V = (A x R)/k (m³); V₁=4.450 m3 and V₂= 75.438m³, Where

K = Calibration constant= 0.63

A = Mouth area of the net= (3.14) × (.12)²  =.04522 m² R = Flow meter reading and = R₁= 62, R₂= 1051

V = Volume of water filtered.

After analyzing the sample of zooplankton, there were 9 types of species recorded from two sites of sampling (name of the species and the total faunal composition are given in table-3).

Table 3: Faunal composition

After analyzing the samples it is observed that the species richness of phytoplankton, zooplankton and benthos community was positively influenced by salinity and percentage of silt while it was negatively influenced by percentage of sand and the density of clay. Salinity, percentage of sand, slit, clay and water transparency showed higher contribution to explain the variance in species diversity in the present observations.

The hydropedological factor like total suspended solids was identified as the best variables highly correlated with the phytoplankton, zooplankton and benthic faunal assemblages of the study area in the present study. Islam found total suspended solids (TSS), sand content of soil were identified as the best variables highly correlated with the benthic faunal assemblages of the study area. Yuan reported that salinity and sediment were the main factors affecting benthic macro fauna. Some other studies conducted by different authors in different locations also proved that the sedimentation and the percentage of sand, slit and clay have a great influence on the existence and amount of phytoplankton, zooplankton and benthic organism.. found that the grain size defined distinct species assemblages. indicated that distribution of benthos could be explained by the classification of physical characteristics of sediment in tidal flats. postulated that the species diversity is mainly controlled by the fluctuations in the environment that lead to less diversity. Environmental variability is believed to be the key to the changes in biotic structure expressed by variation in taxa richness and abundance that was similar with the present findings. A number of environmental variables including sediment structure; temperature and salinity have been correlated with the abundance, density and diversity of estuarine organisms. The main environmental factors affecting the distribution and structure of estuarine organisms reported by most authors were salinity and sediment characteristics, especially mud or clay content. Mud content and water content of the sediments of the sediments were found to influence the abundance intertidal biotic communities that were reported by Growenewald. Hirst reported that variations in salinity can promote marked shifts in species representation and community structure because marine organisms have different salinity tolerances. In this study water and sedimentation, salinity, water temperature, water pH and water transparency also influenced various phytoplankton species that lived in the estuarine region at different significant levels either positively or negatively which confirm the findings of the previous studies.

It can be concluded that the abundance and diversity of phytoplankton, zooplankton and benthos community of the intertidal zone of the Karnawphuli estuary is significantly influenced by some physio- chemical parameters of the existing environment. Indiscriminate discharge of untreated sewage and industrial wastes should stop now to save the environment as well as the biotic community of this estuary.

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