The significant role of reservoirs in the carbon cycling and the recent growth of dam building have increased
the interest to evaluate carbon fluxes in these systems. In this work, we study the spatial and temporal
variability in CO₂ (diffusive) and CH₄ (diffusive and ebullitive) fluxes from the water surface in a Mediterranean
reservoir (El Gergal, Spain). During a whole year, CO₂ and CH₄ fluxes were measured monthly from the
reservoir lacustrine and riverine zone. Additionally, we measured CO₂ flux from dry sediments of the reservoir
drawdown area. CO₂ and CH₄ diffusive fluxes from the water surface were higher during mixing period, and
the CO₂ flux was related to pH and Chl-a, suggesting primary production as a relevant driver. Conversely, CH₄
ebullitive flux was higher during thermal stratification. Dry exposed sediments constituted a net CO₂ source
to the atmosphere showing higher CO₂ emissions in areas influenced by river or intermittent stream
discharges. During the mixing period, the reservoir water surface was a net source of CO₂ to atmosphere
whereas it was a CO₂ sink during the stratification period. However during the stratification period the
reservoir drawdown zone increased due to lower water input and larger water demand, and consequently
larger areas of previously submerged sediments became dry and exposed to the atmosphere. Therefore,
ecosystem net CO₂ uptake during the thermally stratified period could be significantly diminished in
Mediterranean reservoirs in future conditions, due to the predicted increase in the frequency and intensity
of droughts expected for this region.