Spatial Patterns of Diffusive Methane Emissions Across Sediment Deposited Riparian Zones in Hydropower Reservoirs

In dammed rivers, sediment accumulation creates potential methane emission hotspots, which have been extensively studied at sediments deposited on the riverbed. However, methane emissions from sediment deposited riparian zones at the land-water interface remain poorly understood. We investigated the spatial pattern of diffusive methane emissions from a riparian zone on a sediment-deposited island in the heavily dammed Upper Mekong River. High diffusive methane emissions (maximum 10.4 mg h^-1m^-2) were observed at the island center, while low-to-negative diffusive methane emission was found around the island periphery, whose flux varied between -0.2–1.6 mg h^-1m^-2. The low-to-negative diffusive methane emission zone accounted for 89.1% of the island area, of which 9.1% was a methane sink zone. Microbial processes in the riparian zone, affected by hydrological variations, were responsible for the low methane flux in this area. Under reservoir operation, the water level in the reservoir frequently fluctuated with a maximum oscillation of 2.54 m, which enhanced hyporheic exchange and created redox gradients along the hyporheic flow path. From the island periphery to the island center, there was a decrease in dissolved oxygen (from 4.8 to 0.43 mg L^-1) in hyporheic water, an increase in sediment methanogen abundance for methane production and a decrease in sediment methanotroph abundance for methane oxidation. This study adds to our understanding of methane emissions from dammed rivers and helps to evaluate their contributions to global methane emissions from inland waters.