pCO₂ variability in the surface waters of the ultra-oligotrophic Levantine Sea: Exploring the air–sea CO₂ fluxes in a fast warming region

Seasonal pCO₂ variability was studied in the Southeast Levantine (SE-Levantine) during 2009–2015 with the aim of quantifying air–sea CO₂ fluxes in this ultra-oligotrophic, warm and highly evaporative marginal sea. Mixed layer pCO₂ varied significantly between 560 ± 9.0 μatm in August (summer) and 350 ± 8.7 μatm in March (winter). Comparison of pCO₂ to Sea Surface Temperature (SST) yielded a strong positive correlation (n = 135, r² = 0.94), suggesting that the seasonal variations are the result of a thermodynamic effect on the carbonate system in seawater. Using the coupling between pCO₂ and SST, we calculated the mean monthly values and the air-sea fluxes in this region. These calculations indicated that this region is a net source of CO₂ to the atmosphere over an annual cycle, with an average flux of 845 ± 270 mmol C m² y^{− 1} (~ 0.98 Tg C y^{− 1}). Flux estimation for three other sub-basins of the East Mediterranean (South of Crete) with similar oceanographic conditions, were calculated from mean seasonal SST and wind data (MEDATLAS). The results suggest that these regions may either be a weak source or sink for atmospheric CO₂. However, considering the relatively rapid warming trends (~+1 °C/decade) observed in the Levantine basin and the anticipated thermodynamic effect on pCO₂ levels in seawater, these sub basins could become more pronounced sources for atmospheric CO₂. Thus, with the fast warming and increased stratification of its surface water the CO₂ source of the Eastern Mediterranean is expected to expend spatially.