ABSTRACT
The present study was carried out to investigate the plankton diversity of Manas River, Assam from January 2015 to November 2017. A total of 57 genera of plankton were recorded from the river. Population of phytoplankton was represented by 42 genera belongs to the family Bacillariophyceae (14 genera), Chlorophyceae (21 genera), Cyanophyceae (5 genera) and Xanthophyceae (1 genera) and Phaeophyceae (1 genera). Zooplankton population was represented by 15 genera belong to Copepoda (4 genera), Cladocera (6 genera) and Rotifera (5 genera). The population density of plankton varied from season to season and found maximum (88800 u/l) in retreating monsoon and minimum in pre monsoon (54800 u/l). The Shannon-Weiner diversity index of plankton population of the river was recorded 2.957 which indicates moderate to high diversity. Present study revealed that pH, water temperature, water velocity, total hardness and chloride were the most driving force for annual variation in both phytoplankton and zooplankton structure