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. Plain Language Summary: Inland waters, including reservoirs used for many different purposes, contribute greenhouse gases like carbon dioxide and methane to the atmosphere. With 34 times more global warming potential than carbon dioxide, methane is of particular concern since reservoirs emit disproportionally large amounts of it. However, our ability to quantify reservoir emissions is limited by high variability in these emissions over space and time. In this study, we measured emissions from several stations across six reservoirs and compared these emissions to a previous study from 2012 to understand potential longer‐term variation in emissions and assess driver variables. Overall, we found that algal productivity was an important driver of fluxes among the reservoirs and could in fact lead to carbon dioxide sequestration. However, the seasonal phenology in algal productivity and related variables were also important to understand when considering emissions variability within a reservoir and over time. As we continue to collect emissions measurements from reservoirs, studies focused on multiple spatial and temporal scales (i.e., day and night) can improve upscaled estimates of greenhouse gas emissions to better quantify the role of reservoirs in the global carbon cycle. Key Points: Reservoirs are a major source of carbon emissions, but spatial and temporal variability leads to uncertain modeled or upscaled estimatesAcross six reservoirs, summer CO2 diffusion was the dominant flux pathway and showed sequestration, linked to increased algal productivityChanges in algal productivity influence the direction of CO2 diffusion with key implications for assessing net reservoir emissions [ABSTRACT FROM AUTHOR]