The rotations of the French icebreaker Astrolabe which supplies the French Antarctic base Dumont d’Urville four times a year, offer a unique scientific opportunity for studying the long-term changes of the Southern Ocean. As part of the SURVOSTRAL program led by Rosemary Morrow (LEGOS, Toulouse), salinity measurements made on board this ship over the past twenty five years have already highlighted significant salinity variability near the continent. But today it is impossible to quantitavely interpret these variations in salinity in terms of changes in precipitation, the sea ice cycle, variations in ocean circulation, or even the melting of the polar ice cap adding fresh water on the ocean surface. In this project we propose to measure continuously, in conjunction with SURVOSTRAL measurements, stable water isotopes of the ocean surface, along the path of the Astrolabe. Last year in February 2020, we carried out a very convincing feasibility study on board the R3 rotation. We wish, as part of this project, to initiate a first rotation with a system now stable and operational. As with the February 2020 feasibility study, it will involve installing a laser spectrometer coupled to a Continuous Water Sampler module. LOCEAN will provide the instrument, and will provide technical expertise for the installation, the collection of measurements, and the scientific analysis of measurements. The major goal of the project is to document the freshwater cycle of the Southern Ocean and its changes, leveraging on the observational effort of temperature and salinity implemented in recent decades.