Investigation of tidal impact on nutrient exchange in some coastal rivers in south-west Bangladesh

Abstract

The objective of this research was to investigate the influence of tides on nutrient concentrations in the Pasur and Baleshwar rivers of southwest Bangladesh. The sampling of river water was carried out to study the variation of salinity and nutrient concentrations at different tidal phases during both spring and neap durations. Water samples were collected from the Mongla station of the Pasur river and Sarankhola station of the Baleshwar River for salinity and dissolved nutrients (NO3–, PO 3–, NO -and NH –N). The results revealed that the nutrients are significantly influenced by variations in tidal water levels. Expected patterns for salinity variation with tidal water level fluctuation were observed both at the Pasur River and at the Baleshwar River during the tidal cycles of spring tide and neap tide. At both the sites, salinity was relatively higher during spring periods compared to neap periods: 573 μS/cm during spring tide and 559 μS/cm during neap tide at Pasur River; and an average of 433 μS/cm during spring tide and 430 μS/cm during neap tide at Baleshwar River. The fluctuations of salinity during the tidal phases were found to be very small even during the winter season: 57 μS/cm during the spring period with a 2.91 m tidal range and 26 μS/cm during the neap period with a 2.44 m tidal range in the Pasur River; while it was only 37 μS/cm during the spring period with a 2.18 m tidal range and only 38 μS/cm during the neap period with a 1.94 m tidal range in the Baleshwar River. From the measured data, it was found that during neap tide in the Pasur River pH fluctuated from 7.77 to 8.1; nitrate fluctuated between 2.8 to 4.3 mg/l, ammonia 0.14 to 0.29 mg/l, nitrite 0.02 to 0.032 mg/l, phosphate 0.2 to 0.5 mg/l and alkalinity 110 to 125 mg/l. In the same river, during spring tide, pH fluctuated from 7.7 to 8.11, nitrate fluctuated between 2.8 to 4.8 mg/l, ammonia 0.14 to 0.30 mg/l, nitrite 0.02 to 0.03 mg/l, phosphate 0.3 to 0.5 mg/l and alkalinity 110 to 125 mg/l. In the Baleswar River, during the neap tide, pH varied from 7.98 to 8.17, nitrate fluctuated between 2.6 to 4.6 mg/l, ammonia 0.13 to 0.31 mg/l, nitrite 0.01 to 0.022 mg/l, phosphate 0.2 to 0.6 mg/l and alkalinity 105 to 130 mg/l. In the same river, during spring tide, pH ranged from 7.99 to 8.19, nitrate fluctuated between 2.1 to 5.4 mg/l, ammonia 0.15 to 0.28 mg/l, nitrite 0.011 to 0.021 mg/l, phosphate 0.2 to 0.7 mg/l and alkalinity 105 to 120 mg/l. During both the spring and neap periods, nitrate, nitrite, and phosphate concentrations decrease as the tidal water level rises and vice versa. So, it appears that constituents of nitrate, nitrite, and phosphate are supplied from the upstream sides of the Pasur and Baleshwar Rivers to the sampling sites, which can be primarily attributed to the agricultural and municipal runoffs. In the case of ammonia, during both the spring and neap periods, it was found that concentration increased with the rise of tidal water levels and vice versa for both the Pasur and Baleshwar rivers. So, it appears that ammonia was supplied from the seaside towards upstream through both the river systems. Comparison of peak values of ammonia concentration between Pasur and Baleshwar Rivers did not show significant difference which could be related to the impact of Sundarbans Mangrove Forest at the downstream side of Pasur river. So, it was concluded that the sampling locations were too far from the mangrove forest and were not appropriate to make any concrete conclusions on the impact of mangrove forests on the nutrient flux through the selected coastal rivers. The data generated from this study will guide continuing efforts to support the sound management of coastal ecosystems.