Zooplankton diversity patterns, assemblage, and environmental determinants in a seasonally dynamic tropical river

Objective This study examined the composition, diversity, and assemblage of zooplankton in relation to environmental factors in the coastal Dakatia River, Bangladesh, across wet and dry seasons. Methods Zooplankton samples were collected seasonally (monsoon and winter) from each station, alongside measurements of environmental parameters. Population density, diversity indices, assemblage patterns, and environmental drivers of the zooplankton community were analyzed. Results A total of 25 zooplankton genera were recorded, dominated by Rotifera (36%), followed by Copepoda (26.49%), Cladocera (21.25%), and crustacean nauplii (16.26%). Zooplankton density varied significantly ( p < 0.05) between the seasons, with the highest mean (144.88 ± 22.94 ind./L) during the winter (dry) and the lowest (120.25 ± 14.04 ind./L) in the monsoon (wet) season. Diversity indices indicated moderate diversity, ecological stability, habitat suitability, and generally low dominance of taxa. Cluster analysis delineated five and four major assemblage clusters during monsoon and winter, respectively, at 70% similarity. Analysis of similarity confirmed significant ( p < 0.05) spatial and temporal variation among the stations and between the seasons, while similarity percentage analysis identified Keratella sp., Brachionus sp., Bosminopsis sp., crustacean nauplii, Cyclops sp., and Lecane sp. as the most influential contributors to community dissimilarity. Environmental parameters varied significantly across seasons ( p < 0.05). Among them, pH exerted the greatest effect during the monsoon, while canonical correspondence analysis further revealed that pH in the monsoon and nitrate in the winter were the primary drivers structuring zooplankton assemblages. Conclusion These findings reveal moderate zooplankton diversity and community stability, providing key indicators of ecological integrity and productivity in tropical river ecosystems.