Lagoon temperature and hydrodynamics of Reao Atoll (Tuamotu Archipelago, French Polynesia) before and during the 2024 El-Niño Marine Heatwave

Abstract. In French Polynesia, semi-closed atolls of the eastern Tuamotu Archipelago are home to fisheries and mariculture activities based on giant clam resources. These activities are increasingly vulnerable to global warming, as evidenced by the mass bleaching event observed in Reao atoll in March 2024, triggered by the most intense and prolonged Marine Heatwave (MHW) ever recorded in this lagoon. The GAIA project (manaGement strAtegy evaluatIon for small-scale fisheries in Atoll lagoons) was launched to address this issue and aimed to explore the sustainability of giant clam fisheries under climate change, with a particular focus on processes driving thermal exposure during MHWs. To achieve this, spatial and temporal temperature fluctuations, lagoon circulation, and water exchange dynamics between ocean and lagoon across the atoll rim were monitored using moored autonomous oceanographic sensors. A total of 6 monitoring periods took place between December 2016 and May 2025, with the most extensive instrumentation covering the entire February–March 2024 MHW event. The strength of these observations lies not only in their coverage of a unique and poorly sampled ecosystem, but also in their comprehensive documentation of an exceptional MHW event, from its onset and peak to its dissipation, providing an unprecedented record of the full life cycle of a major thermal anomaly in a tropical semi-closed atoll lagoon. This dataset also supports future modelling efforts to simulate temperature dynamics and identify thermal refugia in atoll lagoons under changing climate scenarios. All data were post-processed, quality-controlled, and formatted into interoperable NetCDF files. The full dataset is openly accessible through the SEANOE marine data platform: https://doi.org/10.17882/105885 (Le Gendre et al. 2025a).