The warming trend of the Mediterranean Sea is a long-term process. It has resulted in a northwards and westwards range expansion and abundance increase of thermophilic species, both native and non-indigenous, and in a shrinking of the range of cold-affinity species. Marine heatwaves (MHWs) are relatively short-term extreme episodes that are responsible for spectacular mortality events in some species and have been extensively reported in the literature. In contrast, the species that benefit from MHWs (the ‘winners’) have been much less studied. A record-breaking MHW occurred in 2022 in the north-western Mediterranean Sea. We focus on three ‘winner’ species, the thermophilic green macroalgae Penicillus capitatus and Microdictyon umbilicatum and the endemic seagrass Posidonia oceanica. Penicillus capitatus, which is mainly present in the area as an inconspicuous turf of entangled filaments (espera stage), produced the erect paintbrush-like stage where sexual reproduction takes place. Microdictyon umbilicatum, usually uncommon, bloomed to the point of clogging fishing nets. Finally, a mass flowering of P. oceanica occurred in late August–September, followed the following year (April–May 2023) by the extensive production and dissemination of fruits and seeds. Both processes, the long-term warming trend and one-off heatwaves, both ‘losers’ and ‘winners’, shape the change in structure and functioning of Mediterranean ecosystems.

Following the unprecedented marine heatwaves of summer 2022, extensive flowering of the endemic seagrass Posidonia oceanica was witnessed across the western Mediterranean. To unravel the causes of this event, we conducted a pan-Mediterranean analysis across 442 sites spanning all Mediterranean ecoregions, with 76% exhibiting flowering. Flowering differed regionally, with both highest flowering prevalence (over 90%) and flowering intensity (up to 0.75) recorded in the Liguro-Provençal and Balearic Seas. We demonstrate that regions experiencing large summer sea surface temperature anomaly with high marine heatwave cumulative intensity displayed the highest flowering intensity. The probability of flowering was very likely when marine heatwave cumulative intensity exceeded ⁓120°C days. These findings suggest that marine heatwaves trigger flowering in P. oceanica and, given the high energetic cost of sexual reproduction, continued ocean warming may shift energy allocation toward flowering. This shift could profoundly affect the species’ reproductive strategies, posing major challenges and uncertainties for its long-term evolution.