Study area: The study area was situated in the estuarine environment of Bakkhali, Cox’s Bazar (20º85'40"-20º46'92" N and 91º96'60"– 92º34'37" E;) which has a maritime climate, and the temperatures buffered by the nearby ocean (Khan and Karim, 1982). The maximum air temperature, water salinity and rainfall observed in summer ranges from 31.1º-33.3ºC, 33.47- 34.71‰, 172.0-214.0 mm and minimum of them in dry season ranges from 24.8º-29.8ºC, 5.81- 12.81‰, 1.0-6.0 mm, respectively in the estuarine environment of Bangladesh (Mahmood et al., 1978; Mahmood, 1986). Drought conditions prevail during the winter months i.e., November-February and rainfall is confined to the monsoon period. Normally, 80-90% of the annual rainfall occurs during the monsoon months (June-September). The area of P. coarctata bed is about 10 ha. The Bakkhali estuary is an important river estuary with a harbor and local fishery. The estuary is approximately 0.5 km wide and >10 m deep at its mid point. The tide in the area is semi-diurnal and maximum tidal amplitude is 3 m at spring tide.
Study area: The study area was situated in the estuarine environment of Bakkhali, Cox’s Bazar (20º85/40"-20º46/92" N and 91º96'60"– 92º34'37" E) which has a maritime climate, and the temperatures buffered by the nearby ocean (Khan and Karim, 1982). The maximum air temperature, water salinity and rainfall observed in summer ranges from 31.1º-33.3ºC, 33.47- 34.71‰, 172.0-214.0 mm and minimum of them in dry season ranges from 24.8º-29.8ºC, 5.81- 12.81‰, 1.0-6.0 mm, respectively in the estuarine environment of Bangladesh (Mahmood et al., 1978; Mahmood, 1986). Drought conditions prevail during the winter months i.e., November-February and rainfall is confined to the monsoon period. Normally, 80-90% of the annual rainfall occurs during the monsoon months (June-September). The area of P. coarctata bed is about 10 ha. The Bakkhali estuary is an important river estuary with a harbor and local fishery. The estuary is approximately 0.5 km wide and >10 m deep at its mid point. The tide in the area is semi-diurnal and maximum tidal amplitude is 3 m at spring tide (Mahmood, 1986).
Morphological study: P. coarctata was randomly collected using trowel from the two stations. The collected samples were placed in the pre-labeled plastic bags and brought back to the Laboratory of Estuarine, Coastal and Aquaculture Research (LECAR), Institute of Marine Sciences and Fisheries, University of Chittagong. In the laboratory, the samples were washed carefully in running water. The leaf length, leaf width, rhizome length, internode length and shoot height were recorded. Specimens of P. coarctata were preserved in the museum of the Institute of Marine Sciences and Fisheries for future reference.
Estimation of shoot density and standing crop: A permanent plot of 50 m x 50 m at each station was established following the method described by Brower and Zar (1984). Five quadrates each of 20 cm x 20 cm, were randomly selected for measuring the shoot density in the permanent plots. Shoot density was investigated at monthly interval for six months following the procedure described by English et al. (1994). All plants including roots and rhizomes inside the quadrates were collected with a trowel and taken in plastic bags, and brought to the LECAR. The samples were washed and counted manually in the laboratory. The shoot number recorded from the individual quadrate was expressed as shoots m-2. For standing crop, the above ground parts (leaves and sheaths) were separated from roots and rhizomes. Any epiphytes or algal growth were removed carefully from the plants by scraping with a knife. Above and below ground parts of P. coarctata were then dried in an oven at 100°C for 4 hours to obtain dry weight (DW) following the procedure described by Schubauer and Hopkinson (1984) and Norhadi (1993). The standing crop values obtained from this study were expressed in gm per unit area (g m-2).
Soil sampling: The surface sediment (<10 cm deep) samples were collected from the permanent plots. Soil pH (wet) was recorded in situ using a pH meter (soil pH tester, TAKAMURA electric works ltd., Japan). Soil samples were collected using a hand made mud corer (5.0 cm diameter), placed in the plastic bags and labeled. After collection, all samples were brought to the LECAR within 2-4 hrs for analysis. In the laboratory, the samples were dried at 100°C for 24 h and powdered, sieved through a 0.5 mm stainless steel sieve and stored in a desiccator. All determination was in triplicate and the mean value was used to obtain a representation of each parameter. Dry soil pH was determined by soil tester following the procedure described by Boyd (1995). Soil texture was analyzed by hydrometer method following the procedure described by Bouyoucos (1962), soil organic matter following combustion method by Boyd (1995) and soil carbon by Nelson and Sommers (1982).
Pore water sampling: Pore water samples were collected from the permanent plots. After digging a soil sample, accumulation of pore water could usually be observed at the bottom of the hole. Once the small hole was filled with water, it was rapidly transferred into air tight plastic bottles. Pore water pH and salinity were measured in situ using a pen pH meter (S237734, HANA Instruments) and refractrometer (News-100, TANAKA, Japan), respectively. The collected water samples were preserved by HCl for further analysis and brought to the laboratory placing inside an ice box. Pore water ammonium concentration was measured by phenol hypochlorite procedure described by APHA (Anon, 1976).
Statistical package for social science (SPSS) verson-10 was used to analyze the mean values of morphometric parameters, shoot density, above and below-ground biomass, and soil and pore water parameters. A simple t-test was used to compare the means of two sets of observations at two stations.