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−1 m−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 and 1.6 mg h−1 m−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 dissolved oxygen (DO) gradients along the hyporheic flow path. From the island periphery to the island center, there was a decrease in DO (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. Greenhouse gas emissions from reservoirs can undermine the green credentials of hydropower. In dammed rivers, sediment accumulation can create potential methane emission hotspots to the atmosphere. Extensive research has been conducted on methane emissions from sediments deposited on the riverbed, but little is known about methane emissions from sediment deposited riparian zones, at the land‐water interface. We investigated methane emissions from sediments deposited in the riparian zone of a reservoir island and found low‐to‐negative diffusive methane emissions around the island periphery, likely due to the effects of hyporheic exchange between the reservoir and island on microbial processes. Frequent water level fluctuations in the reservoir enhanced hyporheic exchange, introducing oxygen‐rich water from the reservoir to the island, and thus regulating microbes responsible for methane production and consumption. Understanding of methane emissions in dammed rivers is important to evaluate their contributions to globally important greenhouse gas emissions from inland waters. Low‐to‐negative diffusive methane emissions occurred at the edge of the riparian zoneDissolved oxygen gradients were measured between hyporheic and river water at the edge of the riparian zoneDissolved oxygen gradients appeared to regulate microbial processing of methane emissions Low‐to‐negative diffusive methane emissions occurred at the edge of the riparian zone Dissolved oxygen gradients were measured between hyporheic and river water at the edge of the riparian zone Dissolved oxygen gradients appeared to regulate microbial processing of methane emissions