Using an “isotopic spike” from a tropical storm to understand water exchange on a large scale: case study of Hurricane Rafael in the Lesser Antilles archipelago, October 2012

RATIONALE: Studies of wetland eco-hydrology in tropical coastal area are scarce, and the use of water stable isotopes can be of great help. Key constraints for their analysis are (i) the small difference in δ18 O values between seawater and old evaporated freshwater, and (ii) the fact that the presence of old brackish water limits the determination of the water origin and dynamic. METHODS: The water of tropical storms displays distinctively depleted heavy stable isotopes, in comparison with usual tropical rainfall without strong convective thunderstorms. During tropical storms, such as Hurricane Rafael in mid-October 2012, the rainfall δ18 O signal can be decreased by many units. This effect is called an "isotopic spike", and it could be used as a temporal marker of the water fluxes. RESULTS: Water samples, with δ18 O values as low as = -8.9 ‰, were collected in the islands of Guadeloupe and Saint-Martin during Hurricane Rafael, whereas the usual range of groundwater or mean rainfall δ18 O values is around -2.8 ± 0.5 ‰, as measured from 2009 to 2012. These water "isotopic spikes" allow us to show a surface fresh water uptake by mangrove trees in Guadeloupe, and in Saint-Martin, to calculate the water renewal of the salt ponds and pools. CONCLUSIONS: The "isotopic spikes" generated by tropical storms, are generally used to track back past storm events, as recorded in trees and stalagmites. Here, the propagation of isotopic spike is followed to improve the understanding of the freshwater circulation and the water dynamic within coastal ecosystems influenced by seawater.