Drought disrupts atmospheric carbon uptake in a Mediterranean saline lake

Abstract. Saline and intermittent inland lakes play a key role in the global carbon cycle, acting as dynamic zones for atmospheric carbon exchange. Given the global decline of saline lakes and the expected increase of periods of drought in a climate change scenario, changes are expected in their potential capacity to uptake or emit atmospheric carbon. In this study, we conducted continuous measurements of CO₂ and CH₄ fluxes at the ecosystem scale within a saline endorheic lake of the Mediterranean region over nearly 2 years. Our focus was on determining net CO₂ and CH₄ exchanges with the atmosphere under both dry and flooded conditions, using the eddy covariance (EC) method. We coupled greenhouse gas flux measurements with water availability and analyzed meteorological variables like air temperature and radiation, known to influence carbon fluxes in saline lakes. This extensive data integration enabled the projection of the net carbon flux over time, accounting for both dry and wet periods on an interannual scale. We found that the system acts as a significant carbon sink by atmospheric CO₂ uptake in wet conditions, with uptake ceasing in periods of drought. Moreover, increased air temperatures during wet phases slightly decrease the CO₂ uptake efficiency. Regarding CH₄, we measured uptake rates that exceeded those of well-aerated soils such as forest soils or grasslands. Additionally, we observed that CH₄ uptake during dry periods was nearly double that of wet periods. However, the absence of continuous data prevented us from correlating CH₄ uptake processes with potential environmental predictors. Our study challenges the widespread notion that wetlands are universally greenhouse gas emitters, highlighting the significant role that endorheic saline lakes can play as natural sink of atmospheric carbon. However, our work also underscores the vulnerability of these ecosystem services in the current climate change scenario, where drought episodes are expected to become more frequent and intense in the coming years.