Seasonal nutrient distribution in the Rupsha-Passur tidal river system of the Sundarbans mangrove forest, Bangladesh

Abstract

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. Following post monsoon, NH3–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 SO4 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 PO4–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 NO3–N were found to fluctuate between 0.083 and 1.233 mg/L with no distinct seasonal distribution pattern. Tidal variation of NO3–N in the experimental sites was not so prominent during post monsoon period as of winter data. 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.