Beyond single stressors: non-additive interactions between marine heat waves and light limitation in a keystone seagrass species

Coastal Mediterranean ecosystems are increasingly affected by multiple stressors, including marine heatwaves (MHWs) and reduced light availability, which threaten foundation species such as the seagrass Posidonia oceanica. Using a full-factorial experimental design, P. oceanica plants were exposed to two simulated marine heatwaves of different intensities as well as control temperature, under ambient and reduced light conditions. Increased temperature significantly reduced Photosystem II efficiency (Fv/Fm and Fv'/Fm') and enhanced non-photochemical quenching (NPQ) at the first heatwave, indicating photoinhibition, while lipid peroxidation remained relatively stable, suggesting limited oxidative stress. Low-light conditions promoted photo-acclimation, preserving pigments (Chl a, Chl b, β-carotene) and maintaining higher PSII efficiency. Integration with structural and morphological traits revealed cumulative negative effects of repeated heat exposure, including reduced leaf growth rate and increased necrosis, whereas shading mitigated thermal damage, buffering leaf necrosis after the second MHW. Temporal trajectory analysis further supported the buffering role of low light under combined stress, encompassing PSII activity, sugar metabolism, and leaf surface, highlighting the complexity of seagrass responses to multiple stressors. These results underscore the importance of combining physiological, structural, and morphological assessments and considering responses across plants' traits when assessing the effect of interacting stressors to predict the impacts of climate change on Mediterranean seagrass meadows.