Water-Level Fluctuations Rather than Water Content Changes Induced by Reservoir Operation Impact Bacterial Functioning for Nitrogen Transformation in Riparian Zones

Riparian zones act as critical “sinks” for nitrogen transformation and “buffers” for nitrogen removal in reservoirs. The operation of hydropower reservoirs significantly alters water levels in these zones and impacts the biogeochemical transformation of nitrogen. This is primarily driven by bacteria residing in the sediments of riparian zones. However, the impact of water level alteration on the bacterial community and its nitrogen-transformation function remains unclear. This study investigates whether the changes in water contents or the water-level fluctuations resulting from reservoir operation more profoundly affect bacterial functioning for nitrogen transformation and whether the observed differential functioning arises from community dynamics. Through a long-term field investigation in a reservoir on the upper Mekong River, we discovered that fluctuation frequency, rather than changes in water contents, significantly increased the abundance of genes encoding nitrogen-transformation enzymes, especially those related to anaerobic ammonia oxidation. These fluctuations created conditions that maximized the bacterial potential for nitrogen removal without significantly altering the community structure. Our findings suggest that the differential functioning observed is instead driven by ecological strategies within the bacterial community rather than by community dynamics. This offers new insights into optimizing reservoir management for improved nitrogen removal.