The atmospheric water cycle of a coastal lagoon: An isotope study of the interactions between water vapor, precipitation and surface waters

The interaction between the different components of the water cycle at ecosystem scale is of great interest in hydroecology. Stable isotopes of hydrogen and oxygen in the water molecule can be used to study these processes quantitatively due to their different physical properties. This study aims at quantifying the contribution of local moisture sources to the isotopic signature of water vapor in the specific estuary system of a coastal lagoon. Here we present a framework of the isotopic composition of the atmospheric water cycle components (water vapor and precipitation) for the coastal lagoon of Venice (Italy) and we discuss the interaction between the atmospheric component and the surface component of the local water cycle, i.e. the atmospheric moisture and the lagoon water. The Venetian Lagoon is an enclosed basin with low freshwater influx, resulting in the presence of large horizontal gradients of the isotopic composition of surface water. This feature allows to determine the sensitivity of the isotopic signal in near surface atmospheric moisture under a range of humidity conditions. We observed that when the lower atmosphere is close to water vapor saturation pressure condition, the isotopic composition of water vapor at the ground appears to be in isotopic equilibrium with lagoon surface water in a range of approximately 6–26 km from the sampling point. On the other hand, under unsaturated conditions, we estimated that lagoon evaporation flux accounts on average for ∼27% to the daily (06–18 Local Sidereal Time) total evapotranspiration surface flux. This is the first study that discusses the spatio-temporal variability of water vapor, precipitation and surface water isotopic composition for a coastal lagoon, thus including the major components of the local water cycle in an estuarine system with the isotope approach.