Enhanced riparian denitrification in reservoirs following hydropower production

It is commonly assumed that dams retain nutrients and cause eutrophication in reservoirs after impoundment. In this study, we explored denitrification in a riparian zone at the land-water interface in a hydropower reservoir of the upper Mekong River. Results demonstrated that hydropower production intensified water level fluctuations in the reservoir and significantly accelerated riparian denitrification process at the land-water interface; the denitrification rate exhibited a strong positive relationship with the period of water level rising-falling cycle (r(2) = 0.98, P < 0.05). Intensified water level fluctuations created frequently alternate wetting-drying cycles and enhanced hyporheic exchanges in the riparian zone, which accelerated organic carbon mineralization and thereby increased carbon source supply to fuel denitrification. In addition, river impoundment after dam construction appreciably enlarged the hot spots for riparian denitrification. The accelerated denitrification as well as the enlarged hot spots area jointly enhanced nitrogen removal from impounded rivers. These findings could advance fundamental knowledge on the environmental impacts of hydropower development on rivers.