In recent years, an increasing number of studies have reported impacts of ocean warming and marine heatwaves on canopy-forming macroalgae, leading to physiological stress, reduced population size, and local extinctions. Warming events are projected to increase in intensity and frequency in the Mediterranean Sea, threatening the survival of these habitat-forming species. Therefore, understanding their thermal tolerance, which may vary across species, populations, and life stages, is essential to predict their persistence under future climate change.

This study aims to evaluate the thermal sensitivity of two Gongolaria elegans populations originating from contrasting environments. Controlled laboratory experiments were conducted using G. elegans adults and recruits from a shallow population in a surface rockpool (0.5 m depth) at Port de la Selva and a deeper population (13 m depth) at the Medes Islands.

Adults and recruits from both populations were exposed to five temperature treatments (21, 26, 27, 28 and 29 °C) for up to 35 days. Recruit survival and growth were monitored throughout the experiment, while adult thermal tolerance was assessed through changes in biomass.

Preliminary results indicate differences in thermal sensitivity between populations. Adults from the deeper population were more vulnerable to elevated temperatures, showing biomass losses exceeding 60% after 28 days at 26 and 27 °C, and after only 14 days at 28 and 29 °C. In contrast, adults from the shallow population exhibited greater thermal tolerance, with biomass losses remaining below 60% under all temperature treatments. Recruits were considerably more sensitive than adults, exhibiting high mortality at temperatures above 26 °C.

These results demonstrate intraspecific variation in thermal tolerance associated with population origin and life stage. Accounting for such variability is essential to assess the future persistence of canopy-forming macroalgae under ongoing climate warming.