Effect of hydroperiod on CO₂fluxes at the air-water interface in the Mediterranean coastal wetlands of Doñana

Abstract

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 Doñana (Spain) in relation to different hydrological cycles. Water sources feeding Doñana, 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 Doñana, 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 25.84 ± 19 and 5.2 ± 8 mmol m−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 15.2 GgC yr−1 under wet and 1.24 GgC yr−1 under dry meteorological conditions, well below the estimated net primary production for Doñana wetlands, indicating that the ecosystem acts as a large CO2 sink.

Methane Emissions From the Salt Marshes of Doñana Wetlands: Spatio-Temporal Variability and Controlling Factors

“…Rainfall was quite variable during the study period ( Figure 2B), typical of the Mediterranean climatic region that is characterized by a rainy season (usually extending from October to March) and a dry season (from June to September) when rainfall is almost absent. The inter-annual precipitation pattern in this geographical area also fluctuates often, which affects the flooding regime in the wetlands (Huertas et al, 2017). Particularly, between March 2016 and March 2018 which corresponds to 3 hydrological years (starting 1 September 2015), total annual rainfall was 493 mm from September 2015 to August 2016, 539 mm from September 2016 to August 2017 and 500 mm from September 2017 to March 2018.…”

“…The floodplain is highly variable depending on annual rainfall (Huertas et al, 2017), and is also presently affected by the estuarine intrusion, that in turn is controlled by the tide amplitude. In fact, water levels were too low for sampling during the summer months and early fall.…”

“…Therefore, over almost two decades, hydrology in the Doñana wetlands mostly depended on the annual rainfall regime, small streams situated in the northern side of the Park (e.g., La Rocina, Figure 1) and to a lesser extent, groundwater from the Almonte-Marismas aquifer (Olías et al, 2008). As a result, inundation cycles in the marshland became highly variable (Díaz-Delgado et al, 2016;Huertas et al, 2017). The scientific community and policy-makers designed an ambitious restoration program, the so called Doñana 2005, aimed at restoring partial connectivity between the estuary and the marshes and since 2014, estuarine waters have inundated the Doñana salt marshes again.…”

“…Prior to connection, biogeochemistry in aquatic systems of the DNP was examined in relation to the patterns of the variable hydrological cycle occurring in the region (Morris et al, 2013;Huertas et al, 2017). Under wet cycles, prolonged flooding resulted in a marked heterotrophy and a significant CO 2 outgassing (Morris et al, 2013) whereas during drynormal hydrological cycles, air-water CO 2 fluxes decreased, with permanent floodplains and ponds of the marshland even behaving as mild sinks for atmospheric CO 2 for short periods (Huertas et al, 2017). Alternatively, aquaculture lagoons with man-managed water supply from the estuary continuously behaved as carbon sinks, which highlighted the importance of water management in the Doñana, not only for wetland conservation, but also for its role in the atmospheric CO 2 exchange.…”

“…Alternatively, aquaculture lagoons with man-managed water supply from the estuary continuously behaved as carbon sinks, which highlighted the importance of water management in the Doñana, not only for wetland conservation, but also for its role in the atmospheric CO 2 exchange. Nevertheless, regardless of the hydroperiod, net primary production of the surrounding terrestrial vegetation largely offset aquatic CO 2 effluxes and hence, considering the net ecosystem exchange, the Doñana wetlands were categorized as a large carbon sequestrator (Huertas et al, 2017).…”