Unravelling tropical estuary health through a multivariate analysis of spatiotemporal phytoplankton diversity and community structure in relation to environmental interactions

The study explores phytoplankton diversity and community structure as indicators of Karanja Estuary health. Surface water samples were collected between January 2022 and March 2023, revealing 139 phytoplankton species across three stations and seasons. Cluster and multidimensional scaling analysis delineated three distinct spatiotemporal clusters within the estuary. Peak phytoplankton abundance and diversity during the post-monsoon period contributed to increased salinity, pH and optimal nutrients. The monsoon season witnessed the lowest phytoplankton abundance due to decreased water transparency and light penetration. Biota-environment (BIO-ENV) analysis highlighted water temperature, biochemical oxygen demand, ammonia, silicate and chlorophyll-a (ρ = 0.9084) as significant factors influencing phytoplankton distribution. Canonical correspondence analysis unveiled significant correlations between Skeletonema costatum, Skeletonema sp., Thalassionema frauenfeldii and Thalassionema nitzschioides with nitrate, ammonia, biochemical oxygen demand (BOD) and dissolved oxygen. Diatom species Asterionellopsis glacialis, Chaetoceros sp., Ditylum brightwellii, Ditylum sol and Pseudo-nitzschia pungens were associated with total phosphorus, chlorophyll-a and silicate. High BOD values suggest a significant presence of organic matter, potentially depleting oxygen through decomposition. A high Shannon–Weiner diversity index implies a diverse phytoplankton community with multiple species, indicating ecosystem resilience and stability. This situation may arise when a stressed ecosystem maintains diversity with pollution-tolerant species, which may be temporary and lead to detrimental long-term consequences. The ecological quality status (EcoQS) using six phytoplankton community indices concluded that all the stations ranged from ‘high’ to ‘bad’ condition, with disagreement among the six indices. This study employs a holistic approach, integrating physical, chemical and biological parameters with multivariate analysis techniques to acknowledge the complexity of ecosystem health.