Effect of hydroperiod on CO2 fluxes at the air-water interface in the Mediterranean coastal wetlands of Donana

Wetlands are productive ecosystems that play an important role in the Earth's carbon cycle and thus global carbon budgets. Climate variability affects amount of material entering and the metabolic balance of wetlands, thereby modifying carbon dynamics. This study presents spatiotemporal changes in air-water CO2 exchange in the vast wetlands of Donana (Spain) in relation to different hydrological cycles. Water sources feeding Donana, including groundwater and streams, ultimately depend on the fluctuating balance between annual precipitation and evapotranspiration. Hence, in order to examine the contribution of the rainfall pattern to the emission/capture of CO2 by a range of aquatic habitats in Donana, we took monthly measurements during severely wet, dry, and normal hydrological years (2010-2013). During wet hydrological cycles, CO2 outgassing from flooded marshes markedly decreased in comparison to that observed during subsequent dry-normal cycles, with mean values of 25.8419 and 5.28mmolm(-2)d(-1), respectively. Under drier meteorological conditions, air-water CO2 fluxes also diminished in permanent floodplains and ponds, which even behaved as mild sinks for atmospheric CO2 during certain periods. Increased inputs of dissolved CO2 from the underground aquifer and the stream following periods of high rainfall are believed to be behind this pattern. Large lagoons with a managed water supply from an adjacent estuary took up atmospheric CO2 nearly permanently. Regional air-water carbon transport was 15.2GgCyr(-1) under wet and 1.24GgCyr(-1) under dry meteorological conditions, well below the estimated net primary production for Donana wetlands, indicating that the ecosystem acts as a large CO2 sink. Plain Language Summary Wetlands are productive ecosystems that play a significant role in the Earth's carbon cycle, representing important contributors to the global carbon budget. Climate variability affects productivity in wetlands, thereby modifying their capacity to act as sink or source for atmospheric CO2. This study addresses how the hydrological cycle influences the capacity for carbon sequestration in the Mediterranean wetlands of Donana. Covering a surface of 3560km(2), Donana region includes a rich variety of landforms and vegetation types representative of Mediterranean lowlands. Its wetlands are one of the most emblematic protected areas in Europe with a high biotic diversity and unique importance for aquatic wildfowl. The region experiences annual cycles of flooding (October to March) and desiccation (May to June), with the inundation cycles being dependent on rainfall. Our measurements suggest that drier meteorological conditions result in lower CO2 emissions to the atmosphere from flooded marshes and permanent ponds whereas large lagoons with man-managed water supply from an adjacent estuary behave as carbon sequestrators. Predictions indicate an increasing frequency of droughts over the Mediterranean region due to climate change. These results highlight the importance of water management in Donana for wetlands conservation and their role in the atmospheric CO2 exchange.