Seasonal and spatial variation of flagellate communities in a tropical river

Heterotrophic flagellates (HF) play an essential function in the microbial food loops as it is an agent of biochemical cycling of limiting nutrients and being a useful bio-indicator of environmental changes. However, in spite of its profound role in aquatic ecosystem studies regarding the evaluation of the spatial and seasonal patterns of the flagellate communities are lacking in Bangladeshi water, and therefore, study on community pattern of HF in a tropical river like Padma River could be a best example for other tropical rivers of the world. The present investigation was performed with the primary objectives to evaluate flagellate community structure, their spatial and seasonal distribution patterns, and the role of hydro-biological and physicochemical variables in determining their variability. The study was conducted at selected four study sites in Padma river, Bangladesh for a period of two distinct season (dry and wet season) in 2018. Standard procedures were followed in collecting, analyzing and interpreting of sampled data. The study revealed a total of 18 genera with different degrees of specificity to hydro-biological and physicochemical variables. It further establishes significant seasonal patterns, rather than spatial differences in the distribution of the flagellate communities. We found that the higher precipitation intensity during monsoon was the critical determinant in hydrological, biological, and physicochemical fluctuations which caused a significant decline in the value of the above variables. Likewise, total phytoplankton, zooplankton, and total heterotrophic bacterial abundance was also declined five-folds during the wet season. Similarly, total abundance of HF showed a significant decline in the community during the wet season. Genus wise relative abundance (RA%) of Euglena (23.81 %) was the highest during the dry season. In conclusion, this research and data analysis techniques propose unique insights on the flagellate links to environmental and hydro-biological regime in a tropical river with global implications.