Seasonal and Tidal Variations of Major Nutrients in Passur River System of the Sundarbans, Bangladesh

Abstract

The Rupsha–Passur River system is one of the largest and most important river systems in the Sundarbans region. It is the largest freshwater supplier to the Sundarbans. The unique habitat of the Sundarbans mangrove ecosystem is dependent upon its hydrological and chemical regimes. Therefore, a comprehensive study to understand hydrological behavior and the changes that have taken place due to anthropogenic activities in and around the area is fundamental to the management of natural resources and the environment. This study aimed to evaluate tidal and seasonal variations in the concentrations of nitrogen (NH₄⁺, NO₃⁻), phosphate, sulphate, and ammonia in a riverine mangrove forest during the post-monsoon (October–January 2010), dry (February–May 2011), and monsoon (June–September 2011) seasons in the Passur River of the Sundarbans. Spatial and temporal measurements of nutrients were made in the Passur River of the Sundarbans mangrove forest to understand dissolved nutrient dynamics. Seasonal measurements of physical parameters and dissolved nutrients were made at several locations during daytime at both high and low tides. The five selected stations, extending from the upstream to downstream regions, exhibited uniformity with respect to water temperature. Significant spatial variations were observed in parameters such as water salinity, pH, and dissolved oxygen (DO). Along with tidal influences, anthropogenic factors associated with adjacent cities and towns exerted a regulatory influence on parameters such as nitrogen, phosphate, sulphate, and ammonia. Tidal water temperatures during the sampling period averaged 30.6 ± 0.4°C in the monsoon season, 30.06 ± 0.4°C in the post-monsoon season, and 20.8 ± 0.4°C in the dry/winter season. Salinity values measured during the monsoon averaged 5.42 ppt, whereas salinity during the post-monsoon and dry seasons averaged 8.2 ppt and 11 ppt, respectively. Nitrate, NH₄⁺, phosphate, and sulphate concentrations ranged from 0.08 to 1.23 ppm, 0.013 to 0.080 ppm, 0.045 to 1.35 ppm, and 7.30 to 126.92 ppm, respectively, throughout the study period.

The study results showed that the nutrient concentration ranges were higher than the Environmental Quality Standard (EQS) of Bangladesh. The data and information presented in this study will serve as a baseline for future studies on the dynamics of water chemistry and its effects on the environment.