The role of sea temperature in Ostreopsis cf. ovata blooms in temperate areas—14 years of data gathered along the Northern Mediterranean coast

• Ostreopsis maximum abundances up to 10⁷ cells g⁻¹ FW and cells L -1 • Despite rising temperature, Ostreopsis blooms stabilized at most Mediterranean sites • Only 23% of stations showed a significant temperature–abundance correlation • Results show a complex, site-specific response driven by local factors • In some stations, an earlier occurrence of the Ostreopsis bloom was observed Ostreopsis cf. ovata is a benthic dinoflagellate able to produce toxins of the palytoxin group. It forms harmful blooms affecting marine ecosystems, coastal economies, and human health. Initially reported in tropical areas, this species is now found throughout the Mediterranean coast where, in certain sites, it reaches high cell abundances during the summer and/or fall periods. We analyzed data from long-term monitoring conducted between 2010 and 2023 in the Northwestern Mediterranean along the coasts of Spain, France, Principality of Monaco, and Italy, within the RAMOGE Agreement. Results revealed that O . cf. ovata is widely distributed and remains stable in historically colonized sites, both as epiphyte on benthic macroalgae and in the plankton. Sea surface temperature recorded during the periods with O . cf. ovata presence ranged from 15.5 to 34.8°C. Most sites did not exhibit clear temperature–abundance relationships, with significant correlations found at only 23% of the monitored stations (mostly showing higher values at warmer conditions). A few stations displayed significant temporal trends, either positive or negative, in abundances, bloom onset, or duration, with patterns varying by location. Overall, the results suggested that, while global warming may influence the dynamics of O . cf. ovata (in terms of abundances, time of bloom onset and duration), the response of the species is more complex and site-specific, shaped by local environmental factors. Besides the unquestionable increasing trend of seawater temperature in the Mediterranean Sea, this study highlighted a stabilization of the O. cf. ovata bloom phenomenon at most stations in the Mediterranean.