Enhanced greenhouse gas emission from exposed sediments along a hydroelectric reservoir during an extreme drought event

An active debate has been underway on the magnitude and duration of carbon (C) emissions from hydroelectric reservoirs, yet little attention has been paid to stochastic C emissions from reservoir sediments during extreme climatic events. A rare opportunity for field measurements of CO₂ efflux from a hydroelectric reservoir in Korea during an extreme drought event was used to examine how prolonged droughts can affect microbial organic matter processing and the release of CO₂, CH₄ and N₂O from exposed sediments. Chamber measurements of CO₂ efflux along an exposed sediment transect, combined with high-frequency continuous sensor measurements of the partial pressure of CO₂ (pCO₂) in the reservoir surface water, exhibited extraordinary pulses of CO₂ from exposed sediments and the turbulent inflowing water in contrast to a small CO₂ sink in the main water body of the reservoir and a low efflux of CO₂ from the flooded sediment. Significant increases in the production of CO₂, CH₄ and N₂O observed in a laboratory incubation of sediments, together with enhanced activities of phenol oxidase and three hydrolases, indicate a temporary activation of microbial organic matter processing in the drying sediment. The results suggest that drought-triggered pulses of greenhouse gas emission from exposed sediments can offset the C accumulation in reservoir sediments over time scales of years to decades, reversing the trend of declining C emissions from aging reservoirs.