Introduction: Seasonal and tidal variations in nutrient concentration and water quality were investigated in the western Sundarbans of Bangladesh during the post-monsoon, winter and monsoon seasons during 2010-2011. Methods: Water collected from the surface, middle and bottom layers at six locations of the Kholpetua-Arpangashia river system during high and low tides was analyzed for temperature, salinity, pH, dissolved oxygen (DO), total dissolved solids (TDS), electrical conductivity (EC) and dissolved nutrients (NO 3-N, PO 4-P, SO 4 and NH 3-N). Results: The study revealed that nutrient concentrations were influenced by seasonal changes. Mean nutrient levels during post-monsoon, winter and monsoon seasons, respectively, were in the following ranges

Introduction: The Rupsha-Passur River System (RS) is one of the biggest and important river systems in the Sundarbans estuarine ecosystem. It is the largest fresh water supplier into this mangrove forest. A comprehensive study was undertaken to evaluate the tidal and seasonal variations in nutrient concentrations (nitrate, phosphate, sulphate and ammonia) of the RS during October, 2010 (Post monsoon), February, 2011 (Dry winter) and August 2011 (Monsoon). In-situ measurement and laboratory analysis were conducted to measure nutrients and other physico-chemical parameters at discrete water layers of five study points under different tidal cycles. Results: Following post monsoon, NH 3-N concentration showed seasonal variation with increasing trend (0.0703 to 0.0803 mg/L) in dry winter and then significant dropping (0.013 to 0.019 mg/L) in monsoon period. During field observation, no significant tidal variation in ammonia concentration was identified among the study sites. Post monsoon and winter time observation recorded lower SO 4 content (7.301 to 37.508 mg/L) at all the sampling stations while in monsoon period, most of the stations showed higher concentration up to 126.92 mg/L. Contrary to post monsoon season with comparatively higher PO 4-P value (0.314 to 1.347 mg/L), winter and monsoon period sampling indicated low phosphate concentrations (0.045 to 0.5 mg/L) in the Rupsha-Passur RS. Most of the study sites showed considerable tidal changes in phosphate value during post monsoon and monsoon season while only two stations went through tidal fluctuation in sulphate content during monsoon period. Concentrations of NO 3-N were found to fluctuate between 0.083 and 1.233 mg/L with no distinct seasonal distribution pattern. Tidal variation of NO 3-N in the experimental sites was not so prominent during post monsoon period as of winter data. Conclusion: Present study will serve as a basis for future hydrological and environmental studies in the world's largest Sundarbans intertidal mangrove forest. Study results indicate how nutrient dynamics of such diversified estuarine system are influenced by varying weather conditions. Daily fluctuations in nutrient concentrations and other physicochemical properties due to semidiurnal tidal activity were also figured out through the study. Information generated from the research works will guide all concerned for any future conservation and management initiatives for the world heritage site.

Mangrove provides a unique ecological environment for diverse communities and Sundarbans is a rapidly changing ecosystem due to various anthropogenic activities. In order to assess the spatial and temporal variation of total Nitrogen and total Phosphorus concentration in Major River Systems of Sundarbans, a study was carried out from September, 2010 to February, 2011. Fourteen sampling location from major River systems were chosen. During post monsoon and winter seasons the range of total Phosphorus (0.326~0.409 mg/L and 0.091~0.371 mg/L respectively) and total nitrogen (2.52~3.50 mg/L and 3.43~5.25 mg/L respectively) were observed in Rupsha-Passur River system. On the other hand the range of total Phosphorus (0.475~0.144 mg/L and 0.060~0.113 mg/L respectively) and total nitrogen (2.31~3.61 mg/L and 3.22~5.95 mg/L respectively) were found in Arpangashia-Malancha River system during post monsoon and winter seasons. The nutrients of water of Baleswar-Bhola River system during rainy and dry seasons were found in the range of total Phosphorus (0.106~0.364 mg/L and 0.053~0.075 mg/L respectively) and total nitrogen (2.59~3.57 mg/L and 2.87~5.60 mg/L respectively). Total Nitrogen and total phosphorus levels were relatively higher than the EPA standards for surface water during the two seasons. The Dynamic nutrients level observed in the study area may have severe consequences on the in-dwelling aquatic flora and fauna.

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Rahaman, S.M.B.; Rahaman, M.S.; Ghosh, A.K.; Gain, D.; Biswas, S.K.; Sarder, L.; Islam, S.S., and Sayeed, A.B., 2015. A spatial and seasonal pattern of water quality in the Sundarbans river systems of Bangladesh. Journal of Coastal Research, 31(2), 390-397. Coconut Creek (Florida), ISSN 0749-0208.Comprehensive field measurements were made in three major river systems (RSs) of the Sundarbans during October 2010 to August 2011 under post-monsoon, dry winter, and monsoon seasons to assess the pollution status. Fifteen experimental sites were set for collection of water samples from discrete layers under different tidal conditions. Salinity, temperature, pH, and dissolved oxygen (DO) were determined in situ and chemical oxygen demand (COD), biological oxygen demand (BOD), total nitrogen (TN), total phosphorus (TP), nitrate, phosphate, and ammonium were measured through laboratory analysis. Mean DO was recorded between 3.93 and 7.37 mg/L with an average 5.44 mg/L. Highest average DO was noticed at Rupsha-Passur RS during post-monsoon, whereas the lowest was in Kholpetua-Arpangashia RS during monsoon. Mean BOD ranges between 0.3 and 4.1 mg O 2 /L, with an average of 2.04 mg O 2 /L, where maximum and minimum concentrations were measured at Kholpetua-Arpangashia RS in monsoon and dry winter respectively. Mean COD was found to vary from 20 to 240 mg O 2 /L, with an average 121.33 mg O 2 /L, exceeding the permissible range of 4-8 mg O 2 /L. Rupsha-Passur RS shows lowest average COD during post-monsoon, whereas Kholpetua-Arpangashia RS indicated highest COD in winter. Measured COD was higher in the dry season than in the wet season because of precipitation that causes dilution in the RSs. The study indicates the existence of organic pollution, though the DO concentration was within the permissible limit. Analyzed water samples showed nitrate, phosphate, and ammonium values in the range of 0.083 6 0.03 to 0.627 6 0.42 mg/L, 0.011 6 0.005 to 0.449 6 0.26 mg/L, and 0.038 6 0.018 to 0.176 6 0.16 mg/L respectively. TN and TP concentrations were not high enough in most cases, as there is a chance of eutrophication due to algal pollution. Study results will help identify factors responsible for affecting the biodiversity of the Sundarbans intertidal mangrove forest.ADDITIONAL INDEX WORDS: Sundarbans, BOD, COD, nutrients, mangrove forest, aquatic ecosystem, organic pollution.