The Indian Sundarbans, a UNESCO World Heritage Site, represent one of the world’s largest and most dynamic ecosystems. Phytoplankton, as primary producers, form the foundation of the aquatic food web and play a crucial role in carbon sequestration within this estuarine deltaic landscape. However, the ecosystem is increasingly being influenced by fluctuations in salinity, nutrient influx, and climate-related stressors. The present study investigates the ecological relationship between physicochemical parameters and phytoplankton population in selected regions of the Indian Sundarbans. The study was conducted over one year, from November, 2023 to October, 2024 across four sampling stations along the Ganga River - Godkhali, Gosaba, Canning, and Jharkhali. Water samples were analysed for key physicochemical parameters, including temperature, pH, biological oxygen demand (BOD), and chemical oxygen demand (COD).

Seasonally, the highest water temperature was recorded at Canning, while the lowest was observed at Jharkhali. Maximum pH was recorded at Canning during the pre-monsoon period, whereas the minimum was noted at Godkhali and Gosaba in the post-monsoon season. BOD peaked during the monsoon at Canning and reached its lowest at Godkhali in post-monsoon. Similarly, COD was highest at Canning in pre-monsoon and lowest at Godkhali in post-monsoon. Phytoplankton density was highest during the post-monsoon period, followed by pre-monsoon and monsoon. Canning and Godkhali consistently showing the highest and lowest density of phytoplankton in each season respectively. Bacillariophyceae dominated at most sites. One-way ANOVA, correlation study and regression analysis revealed that phytoplankton density was primarily influenced by BOD, followed by temperature. Two-way ANOVA indicated that seasonal and combined physicochemical interactions significantly affect phytoplankton dynamics.