Ecophysiological responses of boreal-temperate Atlantic kelp at their equatorial range limit.

Portuguese kelp forests are the southernmost stronghold of boreal-temperate species in the Eastern North Atlantic, forming a key biogeographic boundary under rapid ocean warming. Within these forests, species such as Laminaria hyperborea and Saccharina latissima occupy distinct depth ranges, yet the physiological mechanisms underlying this zonation remain unclear. We compared their responses to temperature and light to test whether adult ecophysiology contributes to their vertical distribution and may also influence their latitudinal range limits. Both species displayed high tolerance to short-term thermal stress, maintaining positive net photosynthesis up to 30°C. Saccharina latissima showed moderate reductions in photosynthetic efficiency and productivity at this temperature, whereas L. hyperborea remained largely unaffected. Both species exhibited low light requirements, consistent with their persistence in deeper habitats. These results suggest that adult thermal and light tolerance alone do not constrain current distributions. However, processes acting at other life stages, such as reproduction, recruitment, habitat availability and other ecological drivers, are likely more influential in setting biogeographical and vertical range limits. These insights suggest that future changes in species dominance and community composition in southern kelp forests will be driven by shifts in ecological interactions and life-cycle dynamics, rather than by direct physiological constraints.