The influence of drought and salinity on Greenhouse Gas emissions in “Fuente de Piedra” endorheic lagoon

Wetlands represent 15% of global organic carbon storage and act as natural “blue carbon”, playing a significant role in the global carbon sink. However, due to climate change and anthropogenic activities (such as desiccation), they can become an important atmospheric CO₂ source.  In many arid areas, lagoons may constitute the greatest part of the natural waters in temperate latitudes. They are usually very shallow or even temporary since evaporation exceeds precipitation. The most common lagoons are saline lakes in endorheic basins, which are strongly dependent on the hydrological budget. “Fuente de Piedra” lake (hereafter FdP) is a shallow and saline endorheic lagoon located in the province of Málaga, in the south of Spain. It is an important nature reserve because of its population of nesting flamingos in the Western Mediterranean, and is also the largest lagoon in Andalusia and is part of Ramsar since 1999. Based on previous studies, we hypothesize that FdP will be a net sink of CO₂ but probably a yearly source of CH₄ and N₂O. However, its magnitude is still to be determined. Regarding the effect of drought, due to the contradictory results found in the literature, it is difficult to predict how GHG balances will behave during periods of drought and flooding. Therefore, the main objective of this study is to quantify CO₂, CH₄ and N₂O fluxes in FdP and their seasonal variability. In this regard, the Picarro G2508 spectrometer, is being used every 15 days in four locations over a transect (dry sediments, wet sediments, shore and lagoon) to measure CO₂/CH₄/N₂O fluxes since March 2021. At the same time, the eddy covariance technique is being used since August 2021 to quantify CO₂, CH₄ and H₂O exchanges at the ecosystem level. Positive values of fluxes denote a net release to the atmosphere, while negative values indicate a net uptake. Preliminary results of Picarro measurements show that, during the drought period there is a significant effect of salinity for CO₂ emissions with maximum value 0.3 µmol m^-2 s^-1 when sediments are covered by salt and 2.8 µmol m^-2 s^-1 when salt was removed. Regarding measurements of the transect, during the flooding period CO₂ and CH₄ fluxes ranged respectively from -3 µmol (CO₂) m^-2 s^-1 and 0.008 µmol (CH₄) m^-2 s^-1 in the lake to 1.7 µmol (CO₂) m^-2 s^-1 and zero (CH₄) µmol m^-2 s^-1 in the dry sediments. On the other hand, no N₂O emissions where detected. Regarding the eddy covariance measurements at the ecosystem level, CO₂ and CH₄ flux values ranged from 10 µmol (CO₂) m^-2 s^-1 and 0.01µmol (CH₄) m^-2 s^-1 to zero µmol (CO₂) m^-2 s^-1 to -0.05 (µmol CH₄ m^-2 s^-1) during drought period (no measurements for the flooding period were taken yet). As a preliminary conclusion FdP seems to act as a source of CO₂ during the drought period, while for CH₄, FdP seems to act as a slight sink. However, more measurements are needed in order to provide stronger conclusions about the drought and the flooding period.