Post-Monsoon Phytoplankton Productivity and Community Structure in the Lower Reaches of the Kallada River, South India

Phytoplankton productivity is a key driver of ecological functioning in riverine and estuarine ecosystems. The present study assesses post-monsoon phytoplankton productivity and community structure in the lower reaches of the Kallada River, Kerala. Phytoplankton samples were collected from four stations - West Kallada, Munroe Island, Perugalam, and Koduvila - during October-February 2023. A total of about 35 phytoplankton taxa were recorded across the study area, with Bacillariophyceae forming the dominant group at all stations. Perugalam and West Kallada exhibited higher phytoplankton density and diversity, with Shannon-Wiener diversity values ranging from 2.8 to 3.3, indicating favourable conditions for sustained primary productivity. Diatom dominance ratios varied between 0.48 and 0.74, reflecting moderate to high productivity across stations, with the highest value observed at Koduvila due to dominance of Melosira varians. Munroe Island showed moderate productivity characterized by a mixed assemblage influenced by river-estuarine interactions. Cluster analysis and NMDS ordination revealed clear spatial segregation of stations based on phytoplankton composition and indicator taxa. Findings of the present study highlight pronounced spatial heterogeneity in phytoplankton-based productivity and provide baseline information for ecological assessment and management of tropical river-estuary systems.