Shifts in Carbon Emissions Versus Sequestration From Hydropower Reservoirs in the Southeastern United States

Reservoirs are a significant source of carbon (C) to the atmosphere, but their emission rates vary in space and time. We compared C emissions via diffusive and ebullitive pathways at several stations in six large hydropower reservoirs in the southeastern US that were previously sampled in summer 2012. We found that carbon dioxide (CO₂) diffusion was the dominant flux pathway during 2012 and 2022, with only three exceptions where methane (CH₄) diffusion or CH₄ ebullition dominated. CH₄ diffusion rates were positively associated with water temperature. However, we found no clear predictors of CH₄ ebullition, which had extremely high variability, with rates ranging from 0 to 739 mg C m⁻² day⁻¹. For CO₂ diffusion, the direction of the flux shifted between 2012 and 2022, where all but three stations across all reservoirs emitted CO₂ in summer 2012, but every station sequestered CO₂ in summer 2022. Here, indicators of greater algal production were associated with CO₂ sequestration, including surface chlorophyll-a concentration, surface dissolved oxygen saturation, and pH. Additional sampling campaigns outside the summer season highlighted the importance of seasonal phenology in primary production on the direction of CO₂ diffusive fluxes, which shifted to positive CO₂ fluxes by the end of August as productivity decreased. Our results demonstrate the importance of capturing CO₂ sequestration in field and modeling measurements and understanding the seasonal drivers of these estimates. Measuring C emissions from multiple pathways in reservoirs and understanding their spatiotemporal responses and variability are vital to reducing uncertainties in global upscaling efforts.