Photon Fluence Rate and Temperature Effects on Temperate Atlantic Kelp Species

The Portuguese coast forms a key biogeographic transition zone where co-occurring kelp species show limited vertical overlap. This study aimed to understand whether temperature and light responses help explain the vertical niche differentiation of Laminaria ochroleuca, Saccorhiza polyschides, and Phyllariopsis brevipes. Results revealed that P. brevipes, despite occupying the southernmost range, showed low thermal tolerance: 27°C significantly increased respiration rates, indicating metabolic stress, and exposition at 30°C caused physiological stress. In contrast, L. ochroleuca and S. polyschides exhibited greater thermal resilience but displayed high light requirements, with evident stress at 30°C. These results suggest that light availability plays a key role in shaping vertical zonation, with species adapted to low light occupying deeper subtidal zones. S. polyschides, a high light-requiring species, dominates the shallow subtidal, while L. ochroleuca, also high light-requiring and temperature-tolerant, is abundant in both intertidal pools and shallow subtidal habitats. These findings raise new hypotheses regarding future distribution patterns under climate change: while L. ochroleuca may continue expanding polewards and potentially replace other Laminaria spp. at shallow depths, low-light-adapted, cold-water species may retain a competitive advantage in deeper zones.