Drought disrupts atmospheric carbon uptake in a Mediterranean saline lake

Saline inland lakes play a key role in the global carbon cycle, acting as dynamic zones for atmospheric carbon exchange and storage. Given the global decline of saline lakes and the expected increase of periods of drought in a climate change scenario, changes in their potential capacity to uptake or emit atmospheric carbon are expected. Here, we conducted continuous measurements of CO2 and CH4 fluxes at the ecosystem scale in 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 storage and analyzed meteorological variables like air temperature and radiation, known to influence carbon fluxes in 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.

Methods
We carried out continuous and interannual measurements of CO2 and CH4 fluxes at the ecosystem level in the saline lake Fuente de Piedra using the Eddy Covariance (EC) method.

-Study site: Fuente de Piedra is a shallow, saline lake in an endorheic basin in Málaga, Andalusia, Spain, covering approximately 17 km² with a maximum depth of 1.5 meters. Designated as a Ramsar site in 1983, it benefits from extensive historical data on water storage and meteorological conditions. Salinity levels in the lake, influenced by the hydrological cycle, vary from oligosaline to hypersaline, with groundwater inflow, streams, and surface runoff contributing to its water, while sediment samples show homogeneity in organic carbon, nitrogen, and the C ratio.

-Field measurements of greenhouse gas fluxes and meteorological drivers: We used the eddy covariance method to measure CO2, CH4, and energy exchanges every 30 minutes from August 2021 to May 2023, including two dry summer periods. In addition to gas measurements, we recorded various environmental and soil state variables every 10 seconds, averaging them every 30 minutes using a data logger. These measurements included photosynthetic photon flux density, air temperature, relative humidity, net radiation, and soil heat flow, with sensors placed at specific heights and depths to capture representative data. Furthermore, groundwater level, daily precipitation, air temperature, and incident solar radiation were monitored using a piezometer and meteorological station near the lake, with equipment maintained bi-weekly.

-Greenhouse gas flux data processing, quality control and partitioning: Greenhouse gas flux data were processed and quality controlled using EddyPro® software, following international standards, including tilt correction, time lag compensation, and spectral corrections. High-quality flux data were selected based on quality check flags and sensor cleanliness, with footprint modeling applied to exclude data influenced by the adjacent terrestrial environment, resulting in 18% and 8% good quality daytime data for CO2 and CH4, respectively, and energy balance closure of 76%.

-Predicting greenhouse gas fluxes as responses to meteorological drivers: We examined the relationship between CO2 and CH4 fluxes and meteorological drivers. Using half-hourly flux data, we calculated 24-hour integrated values and selected dates with over 50% of the anticipated data points to represent daily patterns accurately. A linear regression with forward model selection, evaluated by the Akaike Information Criterion (AIC), identified the most effective model, and the influence of predictors was analyzed using the slope coefficients with a significance level of alpha = 0.05.