Spatiotemporal distribution and composition of phytoplankton assemblages in Bhitarkanika: A Mangrove Estuary

Abstract

Phytoplankton form the foundation of aquatic food webs and are sensitive indicators of environmental conditions in estuarine ecosystems. This study assessed the spatiotemporal distribution of phytoplankton in the Bhitarkanika mangrove-estuarine ecosystem, Odisha, India, and examined their relationship with key physicochemical variables. Sampling was conducted at 28 stations across Bhitarkanika National Park, Dhamra River and Gahirmatha during pre-monsoon and post-monsoon seasons. Water quality parameters, including temperature, salinity, pH, dissolved oxygen, biological oxygen demand, total suspended solids, turbidity, and nutrients, were analysed using standard methods. Phytoplankton abundance, chlorophyll-a, and total chlorophyll were determined through laboratory and algal analyser-based assessments.

Results showed significant seasonal variations in environmental conditions and phytoplankton dynamics. Post-monsoon conditions were associated with lower salinity and temperature, higher nutrient availability, and greater phytoplankton abundance. Mean phytoplankton density increased from 251.5 ± 102.0 no. mL⁻¹ in the pre-monsoon period to 589.0 ± 766.2 no. mL⁻¹ in the post-monsoon period. Cyanobacteria (blue-green algae) increased markedly during the post-monsoon season, while diatoms and cryptophytes were comparatively less represented. Correlation analysis revealed positive relationships between phytoplankton abundance, chlorophyll concentrations, and nutrient levels, particularly nitrate, ammonia, and orthophosphate. In contrast, dissolved oxygen showed strong negative relationships with phytoplankton biomass and chlorophyll-a. Multivariate analyses identified salinity, nutrient availability, suspended solids, and turbidity as major factors influencing phytoplankton distribution and community structure.

These findings highlight the role of monsoon-driven hydrological and biogeochemical processes in regulating phytoplankton assemblages within the Bhitarkanika mangrove-estuarine ecosystem and provide baseline information for ecological monitoring, biodiversity assessment, and sustainable management of coastal and estuarine environments.