Wetland is also known as beels in Assam, India enriched with various types of Zooplanktons, Phytoplanktons and Ichthyofauna. These wetlands are the transitional zones between the terrestrial and aquatic environments. The north-Eastern part of India is blessed with biological reservoirs, especially the Oxbow lakes. An Oxbow lake (named Dhir beel in Assam, India) was investigated from June 2016 to May 2019. Results revealed the presence of the group Chlorophyceae (48%) among the Phytoplanktons and Cladocera (33%), showing the highest among the Zooplanktons and 83 species of fishes belonging to 56 genera, under 29 families. Of these, Gudusia chapra was the most abundant (8.71%) and Anguilla bengalensis was the least abundant (0.15%) in this wetland. The diversity index of Site-IV and Site-V were higher in comparison among the five sites selected in terms of fish diversity. The present study showed a correlation between the increase in species richness and the decline in species abundance. This might be due to the decrease in the planktonic population, possibly resulting from anthropogenic interference and natural siltation.