Light-mediated temperature susceptibility of kelp species (, ) in an Arctic summer heatwave scenario.

Kelps (Phaeophyceae, Laminariales) are ecosystem engineers along Arctic rocky shores. With ongoing climate change, the frequency and intensity of marine heatwaves are increasing. Further, extensive meltwater plumes darken Arctic fjords. Assessing the effect of a sudden temperature increase at the cold-distribution limit of cold-temperate kelp species, we compared the responses of two kelp species (, ) to realistic Arctic summer heatwave scenarios (4-10°C) under low- and high-light conditions (3; 120 μmol photons m s) for 12 days. We found high-light causing physiological stress in both species (e.g., lower photosynthetic efficiency of photosystem II), which was enhanced by cold and mitigated by warm temperatures. Under low-light conditions, we found no temperature response, probably due to light limitation. Both species acclimated to light variations by adjusting their chlorophyll concentration, meeting cellular energy requirements. had ~150% higher phlorotannin concentrations than , possibly acting as herbivore-deterrent. Our findings suggest competitive advantages of kelps on different Arctic coasts with ongoing warming: has advantages in future areas, with low-light intensities, and possibly high grazing pressure and in areas with high-light intensities and low grazing pressure. Species composition changes might have cascading consequences on ecosystem functioning.