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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 km(2)). 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. This study was funded by the projects Alteration of carbon sinks and sources in shrinking inland waters (Alter-C), the Spanish Ministry of Science, Innovation and Universities (refs: PID2020-114024GB-C31 funded by MCIN/AEI/10.13039/501100011033/) and Effects of the drawdown of Enobieta Reservoir (Navarre) on the biodiversity and functioning of river ecosystems (DESEMBALSE), Foundation BBVA (ref: PI064-17). AM was supported by an FI grant from the Agencia de Gestio d'Ajuts Universitaris i de Recerca (AGAUR) of the Generalitat de Catalunya. DvS and BO acknowledge support through the Consolidated Research Group 2017SGR0976. RM acknowledges support by the Generalitat de Catalunya through the Consolidated Research Group 2017SGR1124, and by the CERCA program. AE and MA support of the Basque Government through the Consolidated Research Group IT951-16. AM got a predoctoral grant by the University of the Basque Country (UPV/EHU). DvS is a Serra Hunter Fellow.

Aging dams and the rising efforts to restore stream ecosystems are increasing the number of dam decommissioning programs. Although dam decommissioning aims at improving in-stream habitat, biodiversity, and ecosystem functioning in the long term, it might also cause ecological impacts in the short term due to the mobilization of the sediment accumulated in the reservoir. Benthic biofilm in particular can be impaired by episodes of high turbidity and scouring. We conducted a multiple before-after/control-impact experiment to assess the effects of the drawdown of a large dam (42 m tall), a first step to its decommissioning, on biofilm structure (biomass and chlorophyll-a) and functioning (metabolism, nutrient uptake, and organic matter breakdown). Our results show that the reservoir drawdown reduced the autotrophic biofilm biomass (chlorophyll-a) downstream from the dam, which in turn lowered metabolism. However, nitrogen and phosphorus uptake by the biofilm was not affected. Organic matter breakdown was slower below the dam than in nearby undammed reaches before and during drawdown. All drawdown effects quickly disappeared and reaches downstream from the dam approached values found in nearby undammed reaches. Thus, our results indicate that the effects of reservoir drawdown on stream biofilms exist but may be small and disappear rapidly. This research was supported by the Fundación BBVA (064-17). The authors also acknowledge the financial support from the Basque Government (Consolidated Research Group: Stream Ecology 7-CA-18/10) predoctoral fellowships from the University of the Basque Country UPV/EHU (M.A.). The authors are especially grateful to the municipality of Donostia-San Sebastian and the staff of Artikutza for their continuous support during the experiment.