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

Abstract

Dam decommissioning (DD) is a viable management option for thousands of ageing dams. Reservoirs are great carbon sinks and reservoir drawdown results in important carbon dioxide (CO2) and methane (CH4) emissions, but we ignore the effects of DD on the carbon dynamics in reservoirs. We studied the effects of DD on CO2 and CH4 fluxes from impounded water, exposed sediment, and lotic water before, during, and three to ten months after drawdown of the Enobieta Reservoir, North Iberian Peninsula. During the studied 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 day-1) and lotic water (188.11 ± 86.09) decreased over time but remained higher than in impounded water (-36.65 ± 83.40). Areal CH4 fluxes did not change over time and were noteworthy only in impounded water (1.82 ± 1.11 mmol m-2 day-1). Total ecosystem carbon (CO2 + CH4) fluxes (kg CO2-eq day-1) were higher during and after than before reservoir drawdown due to higher CO2 fluxes from exposed sediment. The reservoir was a net sink of carbon before reservoir drawdown and became an important emitter of carbon along the first ten months after reservoir drawdown. Future studies should examine mid- and long-term effects of DD on carbon fluxes, identifying the drivers of areal CO2 fluxes from exposed sediment and incorporating DD in the carbon footprint of reservoirs.