The drawdown phase of dam decommissioning is a hot moment of gaseous carbon emissions from a temperate reservoir

Dam decommissioning (DD) is a viable management option for thousands of ageing dams. Reservoirs are large carbon sinks, and reservoir drawdown results in important carbon dioxide (CO₂) and methane (CH₄) emissions. We studied the effects of DD on CO₂ and CH₄ fluxes from impounded water, exposed sediment, and lotic water before, during, and 3–10 months after drawdown of the Enobieta Reservoir, north Iberian Peninsula. During the study period, impounded water covered 0–100%, exposed sediment 0–96%, and lotic water 0–4% of the total reservoir area (0.14 km²). Areal CO₂ fluxes in exposed sediment (mean [SE]: 295.65 [74.90] mmol m⁻² d⁻¹) and lotic water (188.11 [86.09] mmol m⁻² d⁻¹) decreased over time but remained higher than in impounded water (−36.65 [83.40] mmol m⁻² d⁻¹). Areal CH₄ fluxes did not change over time and were noteworthy only in impounded water (1.82 [1.11] mmol m⁻² d⁻¹). Total ecosystem carbon (CO₂ + CH₄) fluxes (kg CO₂-eq d⁻¹) were higher during and after than before reservoir drawdown because of higher CO₂ fluxes from exposed sediment. The reservoir was a net sink of carbon before reservoir drawdown and became an important emitter of carbon during the first 10 months after reservoir drawdown. Future studies should examine mid- and long-term effects of DD on carbon fluxes, identify the drivers of areal CO₂ fluxes from exposed sediment, and incorporate DD in the carbon footprint of reservoirs.