## 1 Introduction  
Bacterial communities play a pivotal role in the welfare of water environments [1]. However, they vary spatially, depending on different aspects like environmental factors, community adaptation and human activities [2; 3; 4]. The dynamics of bacterial communities in developing countries are poorly understood [5; 6] including the present study area: selected regions of the Bay of Bengal, Bangladesh.  
Banglades, located in the north of the Bay of Bengal, is known as a deltaic country and 70% of the area is covered by water. Bangladesh is an agricultural country that is located between the 20\({}^{\circ}\)34\({}^{\prime}\) and 26\({}^{\circ}\)38\({}^{\prime}\) N latitudes and 88\({}^{\circ}\)01\({}^{\prime}\) and 92\({}^{\circ}\)41\({}^{\prime}\) E longitudes in South Asia. Like Bangladesh, the Bay of Bengal rim region is currently under pressure due to invasion of human pollutants [7]. Wastes in this zone affect the physical, biological and chemical qualities of the water environment, and as such have negative impacts on human health for (e.g., pathogenic bacteria have caused typhoid fever and pediatric bloodstream infections in this area, which have been documented in hospitals [8]).  
Microbiota are the base of biogeochemical cycles [9] and act as a pollutant biodegradation, so it is essential to assess a microbial community structure's changing nature in aquatic systems. The structures of bacterial communities are affected by the hydrodynamics of storm events [10], physiochemical parameters, longitudinal distance, upstream inputs affects and fecal indicator bacteria concentrations. Further, bacterivory, substrate availability, variation in hydrological and nutrient conditions [11], land use changes, distance from the pristine headwaters, rainfall and pH [12] influence the structure of microbial communities in riverine environments.  
Bacterial community diversity and dynamics, including pathogens [13] and their impacts on water [4], have been characterized through different molecular methods [14; 15]. Among different methods, the intensive application of next-generation sequencing (NGS) helps to explore the diversity, dynamics and sensitivity of microorganisms [12; 16; 17; 18; 12]. [7] studied the different bacterial diversities in physical ecosystems using 16S rRNA-based and independent culture studies approach where bacterial lineages were found without cultivated representatives.  
NGS provides a powerful approach by which to explore the structure of bacterial communities [19]. Without prior cultivation, it is possible to get information about bacteria (phyla and family) from environmental samples, whereas NGS is a diagnostic method for direct detection [20]. In addition, NGS can be used for metagenomics studies [21] and assessing the presence of microbiota in large rivers [12]. NGS methods could be a valuable tool in various water quality monitoring applications. They are useful for the development and assessment of molecular diagnostic tools to find the abundant bacterial indicators in water resources [22].  
Season-wise (e.g., winter, pre-monsoon, monsoon and post-monsoon) the water cycle in the Bangladesh region of the Bay of Bengal varies and shows different characteristics [5]. There are many people living around this area and, as such, this zone is controlled by both natural and human pressures. Most importantly, the diversity and dynamics of bacterial communities in this area are still not known [8].  
We directed this research to examine the distribution of bacterial communities and the impact of water temperature. To determine the bacterial distribution through the region, Illumina next-generation sequencing analyses [23; 24; 25] have been used to identify bacteria from water samples in the study region.