Survival of seaweed forests in the modern era may depend on warming and marine heatwave patterns

Marine heatwaves (MHWs) are highly detrimental for the fucoid Phyllospora comosa and affect the physiological performance of seaweeds under different thermal profiles.

• Four thermal scenarios were tested: contemporary summer temperature (22°C), low persistent warming (24°C), a discrete MHW (22-27°C), and temperature variability followed by a MHW (22-24°C, 22-27°C).
• Physiological performance of seaweeds varied among species and was strongly influenced by the thermal profile.
• The highest temperature did not always correlate with the strongest negative effects on physiological performance.
• Ecklonia radiata exhibited sensitivity to prolonged thermal stress and showed a cumulative temperature threshold.
• Sargassum linearifolium demonstrated resilience but showed signs of decline, particularly under stable control and warming conditions.
• Responses of forest-forming seaweeds to ocean warming are complex and depend on specific thermal profiles and species vulnerability.

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