Monsoon-driven dynamics of chlorophyii-a: evaluating physical and physicochemical controls in the meghna estuary, Bhola district

The Meghna estuary, vital for Bangladesh’s fisheries, transportation, and coastal livelihoods, faces threats from nutrient loading, sedimentation, and upstream effluents. This necessitates systematic water quality monitoring to ensure ecological and economic sustainability. Hence, this study assessed physical, physicochemical, and biogeochemical parameters during the monsoon, using in situ CTD measurements and laboratory double-extraction spectrophotometric analysis for chlorophyll and nutrients. Temperatures averaged 30.6°C, exceeding national standards with minimal spatial variation, while turbidity increased from 104.8 NTU at the surface to 149.1 NTU at 5 m depth. In addition, pH (6.9-7.2) and dissolved oxygen (DO) (4.0-5.2 mg/L) stayed within acceptable ranges with a vertical decreasing pattern, lowest at the most turbid station. Chl-a averaged 0.87 at the surface and 0.39 at depth, peaking at Station 3 (1.55 surface, 0.54 depth) with high phosphate and low turbidity. Chl-a strongly correlated with DO surface, depth) and phosphate surface, depth), reflecting nutrient-driven productivity. Ammonia and silicate concentrations rose upstream and mid-estuary, and correlated negatively with Chl-a concentration. While phosphate levels peaked downstream, exceeding national standards and positively correlating with chl-a, indicating a potential threat of eutrophication due to nearby anthropogenic activities. Eutrophication might initiate dense algal blooms that, upon decomposition, severely deplete dissolved oxygen and create hypoxic or anoxic “dead zones” where most aquatic life cannot survive. These findings emphasise the need for continuous monitoring and coordinated management to maintain the estuary’s ecological health and economic role.