Diversity, Community Assemblage, and Environmental Determinants of Phytoplankton in a Subtropical Transboundary Coastal River

The health of subtropical transboundary coastal rivers is closely linked to phytoplankton diversity, seasonal fluctuations, and community structure. This study presents the first comprehensive multivariate assessment of phytoplankton diversity and environmental drivers in the Dakatia River. Canonical Correspondence Analysis (CCA), Analysis of Similarity (ANOSIM), and Similarity Percentage (SIMPER) were applied across eight geo-referenced stations under varying anthropogenic pressures to establish a baseline data for this cage-culture-intensive river system. The analysis revealed distinct spatial and seasonal variations in phytoplankton diversity and taxonomic composition, highlighting shifts in community assemblages and the influence of physicochemical factors on these dynamics. A total of 37 genera were identified across six major classes: Bacillariophyceae (36.50%), Chlorophyceae (25.91%), Cyanophyceae (17.00%), Euglenophyceae (13.20%), Ulvophyceae (5.06%), and Zygnematophyceae (2.31%). Phytoplankton abundance was highest in winter (16,958.33 ± 6418.75 cells L^-1), followed by the monsoon (14,572.92 ± 2982.90 cells L^-1) and summer (13,739.60 ± 1857.76 cells L^-1). The Shannon-Wiener diversity index, species evenness, and species richness exhibited seasonal fluctuations, ranging from 2.14 ± 0.22 to 2.47 ± 0.15, 0.65 ± 0.07 to 0.75 ± 0.07, and 2.67 ± 0.55 to 4.29 ± 0.38, respectively, reflecting a moderate level of phytoplankton diversity in a dynamic and complex river ecosystem. Cluster analysis delineated five distinct phytoplankton communities at a 50% similarity threshold. ANOSIM results indicated significant seasonal dissimilarity (<i>p</i> < 0.001), while no significant spatial dissimilarity (<i>p</i> > 0.05) was observed. SIMPER analysis identified <i>Melosira</i> sp. as the primary contributor to interseasonal dissimilarities, followed by <i>Oscillatoria</i> sp. and <i>Gomphosphaeria</i> sp. Pearson's correlation and CCA highlighted ammonia, temperature, salinity, nitrate, and phosphate as key environmental drivers shaping phytoplankton abundance, diversity, and community composition. These findings have provided critical insights for ecosystem management, biodiversity conservation, and sustainable water resource governance.