The influence of river discharge on gravel bar hyporheic microbial community structure and putative metabolic functions

1. Microbial communities in the hyporheic zone (HZ) are important in self-purification as the riverbed is metabolically active and responsible for the retention, storage and mineralisation of organic matter transported to the surface water. However, studies exploring HZ microbial community responses to disturbances (e.g. floods) remain scarce. 2. Here, we characterised the microbial community structure among the three (downwelling, upwelling and intermediate) HZ points within and among gravel bars at high and low discharge levels in a dam-regulated river using 16S rRNA metabarcoding. 3. We observed significant dissimilarity in the microbial community at low discharge exhibiting local adaptation due to gravel bar spatial environmental heterogeneity. Moreover, the homogenisation effect resulted in similar microbial community structures among the three points within the gravel bars at high discharge. Microbial communities across adjacent gravel bars were dissimilar, potentially attributing to different bar morphologies. 4. Our study highlights the role of spatial environmental heterogeneity in the biological processes that govern microbial community structure at three hyporheic points in gravel bars at two discharge levels. 5. Our results are essential to understand the HZ microbial communities’ response to the river discharge levels. ### Competing Interest Statement The authors have declared no competing interest.