Ecological Health Assessment in a Sub-Tropical Lake: Integrating Water Quality Index with Multivariate Statistical Methods

Abstract This study provides a comprehensive ecological health assessment of Bangladesh's Kaptai Lake through an integrated analysis of physicochemical and biological parameters across pre-monsoon, monsoon, and post-monsoon seasons. Data from five monitoring stations revealed significant seasonal variations, with the pre-monsoon period characterized by peaks in NH₃ (0.16 ± 0.04 mg L − 1 ), phytoplankton density (14.28 ± 1.52×10⁵cells L − 1 ), and gross primary production (3.38 ± 0.30 g C m − ² d − 1 ), while post-monsoon exhibited maximum DO (6.56 ± 0.27 mg L − 1 ) and conductivity (112.6 ± 12.57 µS cm − 1 ). Spatial analysis identified distinct gradients, including significantly higher DO at Langadu (7.05 ± 0.46 mg L − 1 , p < 0.01) and WQI values indicated comparatively poorer water quality conditions at Naniarchar, although spatial differences were not statistically significant. Multivariate analysis identified mineralization and productivity gradients as major sources of environmental variability, with the first two principal components explaining 46.8–60.1% of the total variance. Ecosystem health assessment using the Direct Measurement Method (DMM) ranked post-monsoon conditions as the most favorable, with Kaptai Dam station maintaining the healthiest balance (Bz/Bp = 0.48, structural Exergy = 28.32 MJ mg − 1 ). Overall, WQI indicated generally favorable water quality conditions for aquatic life. The successful application of WQI coupled with DMM indicators demonstrates that the lake is seasonally influenced by anthropogenic activities, highlighting the critical need for targeted management strategies to mitigate seasonal water quality deterioration and maintain critical ecosystem services in this economically vital freshwater resource.