The semi-enclosed nature of the Mediterranean Sea makes it particularly susceptible to rising temperatures, making it a recognised hotspot of climate change. A direct consequence is the increased frequency of prolonged periods of elevated temperatures, called ‘heatwaves’, which have been associated with an increase in mass mortality events among vulnerable marine species, as well as the spread of invasive species.

This work presents the five-year (2019-2024) outcomes from the ‘Mare Caldo’ project, in collaboration with Greenpeace, aimed at assessing the effects of water warming on rocky reef communities along the Italian coast, including eleven Marine Protected Areas (MPAs) and one unprotected site at Elba Island.

Seawater temperatures were monitored following the standardised protocol provided by T-MEDNet (a pan-Mediterranean network), which involves deploying temperature data loggers along reefs profiles from the surface down to 40 m depth. Underwater visual surveys were conducted to assess the ecological status of benthic communities and to collect data on the presence of warm-water species and on mortality events of the target species.

Data revealed abnormally high temperatures, with multiple heatwaves over the past 5 years. Mortality events on target species were observed across all areas. Data on warm-water species show a latitudinal distribution aligned with temperature gradient; while these species are naturally abundant in southernmost regions, their presence is increasing also in colder and northern areas.

The decline of native species and the spread of warm-water ones resulted in profound shifts in the structure and functioning of benthic communities. These findings highlight the ongoing changes of Mediterranean marine ecosystems under the influence of climate change. While MPAs provide valuable local conservation benefits, they alone are insufficient to mitigate the widespread impacts of climate change, emphasizing the urgent need for coordinated global efforts to protect marine biodiversity.