Flowering of Posidonia oceanica in the Italian Adriatic Sea following the 2024 marine heatwave

Mass flowering events of the seagrass Posidonia oceanica are rare reproductive phenomena that may reflect environmental stress responses and are pivotal for its survival over the coming decades within the global changes scenario. In December 2024, a widespread and synchronous flowering was observed across four meadows along the Southern Adriatic coast of Italy (Apulia Region), following a marine heatwave. Four meadows were surveyed by SCUBA diving, with shoot density (bundles m−2) and inflorescence counts to assess flowering intensity (FI, %) investigated at 10 stations per meadow. Sea Surface Temperatures (SSTs) and sunspot data from 1990 to 2024 were also analysed to explore the role of global warming in triggering the flowering event, as observed for many meadows in the past. All meadows exhibited inflorescences in 100 % of the sampling stations, with an overall mean FI of 33.57 ± 11.71 %, and peaks up to 66 %. Shoot density and FI were positively correlated: the denser the meadow, the higher the FI. SST data for the Southern Adriatic Sea revealed that 2024 was the warmest year in the last three decades, with record-breaking SSTs and a record 72 days exceeding 26 °C. This supports the hypothesis that temperature anomalies and prolonged thermal stress may be key triggers of sexual reproduction in P. oceanica. This event, one of the most intense ever recorded, poses further questions about the role of flowering events in the resilience and regeneration of P. oceanica meadows, as well as implications for the conservation of this fundamental coastal ecosystem.