Key physico-chemical parameters (salinity, temperature, turbidity and dissolved oxygen) were measured in surface water during longitudinal transects in the Loire and Gironde estuaries in summers 2017 and 2018. This objective of this work was to determine the distribution of the dissolved oxygen and to detect potential severe desoxygenation. The transects were scheduled in order to begin the measurements at high tide from a site located upstream of an area where severe deoxygenation have been already been reported. Then, the transect was realised by sailing at low speed downstream with a multiparameter probe SAMBAT, maintained at 0.5 m below the surface, that collected a measurement every 2 minutes.

There are at least a dozen small hyper-turbid estuaries facing the Bay of Biscay, geographically situated between the two major estuaries of the Gironde and the Loire. MAGEST and SYVEL high-frequency multi-site monitoring revealed that the Loire, and to a lesser extent the Gironde, are subject to summer hypoxia. These observations raised the question of the potential occurrence of hypoxia in the small estuaries in between, motivating an investigation of dissolved oxygen in one of them, the Charente estuary. Oxygen and salinity sensors were placed at L'Houmée (2019), Tonnay-Charente (2018; 2019), Rochefort (2020; 2021; 2022), Martrou (2020) during summer, the most critical period for dissolved oxygen; a multiparameter probe was placed at Tonnay-Charente from April to November 2020. Longitudinal investigations along the estuary axis were also carried out during the summers of 2018 and 2019. All the measurements were acquired at 0.5 ± 0.2 meters below the surface. The dataset enabled us to identify the occurrence of summer hypoxia and an oxygen depletion zone in the Charente estuary. These results resulted in the implementation of high-frequency monitoring at Tonnay-Charente, operational since November 2020.

The French Atlantic coast hosts numerous macrotidal and turbid estuaries that flow into the Bay of Biscay that are natural corridors for migratory fishes. The two best known are those of the Gironde and the Loire. However, there are also a dozen estuaries set geographically among them, of a smaller scale. The physico-chemical quality of estuarine waters is a necessary support element for biological life and determines the distribution of species, on which many ecosystem services (e.g. professional or recreational fishing) depend. With rising temperatures and water levels, declining precipitation and population growth projected for the New Aquitaine region by 2030, the question of how the quality and ecological status of estuarine waters will evolve becomes increasingly critical. The MAGEST (Mesures Automatisées pour l’observation et la Gestion des ESTuaires nord aquitains) high-frequency monitoring of key physico-chemical parameters was first developed in the Gironde estuary in 2004 ; the Seudre and Charente estuaries were instrumented late 2020. First based on real-time automated systems, MAGEST is now equipped by autonomous multiparameter sensors. Depending of the stations, an optode is also deployed to secure dissolved oxygen measurement. By the end of 2020, MAGEST had 12 instrumented sites. Portets is a measuring station located in the upper Gironde estuary (Garonne subestuary, about 20 km upstream of the Bordeaux metropolis.