Seasonal Variation and Diversity of Tripos (Dinoflagellata) in Relation to Environmental Drivers in the Hooghly River Estuary, Bay of Bengal

Planktonic dinoflagellate Tripos spp. were sampled and documented from three selected sites within the Hooghly Estuary, Bay of Bengal. The study focused on their diversity, abundance patterns, and the environmental factors regulating them. 14 species of Tripos were identified, with Tripos furca and Tripos fusus being the dominant. Seasonal patterns indicated that post-monsoon (POM) and pre-monsoon (PRM) exhibited the highest diversity, characterized by low dominance (D = 0.18 to 0.23), high Simpson diversity index (1–D = 0.78 to 0.82), and elevated Shannon diversity index values (H' = 1.75 to 1.89). In contrast, monsoon (MON) displayed the highest evenness (E = 0.90 to 0.94) but reduced diversity. Tripos fusus showed maximum abundance (highest cell density 1.92 ´ 103 Cells/L) during the post-monsoon (POM) season, when nutrient levels and salinity were elevated and the water temperature was relatively lower.  Multivariate Non-metric Multidimensional Scaling (NMDS) ordination with environmental vectors displayed a clear separation of seasonal groups, primarily influenced by temperature, salinity, and nutrient gradients. Similarity Percentage (SIMPER) analysis identified Tripos fusus, Tripos furca, and Tripos lineatus are the major contributors to seasonal dissimilarity in the assemblages. Individual rarefaction analyses applied to the abundance further revealed higher species accumulation during late POM and early PRM months. Overall monsoon-influenced hydrological changes are the dominant force regulating Tripos diversity and community composition in the Hooghly Estuary.